Plants are sessile organisms forced to adjust to their surrounding environment. In a single plant the photoautotrophic shoot is exposed to pronounced environmental variations recurring in a day–night 24 h (diel) cycle, whereas the heterotrophic root grows in a temporally less fluctuating environment. The contrasting habitats of shoots and roots are reflected in different diel growth patterns and their responsiveness to environmental stimuli. Differences between diel leaf growth patterns of mono- and dicotyledonous plants correspond to their different organization and placement of growth zones. In monocots, heterotrophic growth zones are organized linearly and protected from the environment by sheaths of older leaves. In contrast, photosynthetically active growth zones of dicot leaves are exposed directly to the environment and show characteristic, species-specific diel growth patterns. It is hypothesized that the different exposure to environmental constraints and simultaneously the sink/source status of the growing organs may have induced distinct endogenous control of diel growth patterns in roots and leaves of monocot and dicot plants. Confronted by strong temporal fluctuations in environment, the circadian clock may facilitate robust intrinsic control of leaf growth in dicot plants.
The question of how leaf and root growth are controlled forms the basis of many attempts to generate more successful plants for various purposes and under a range of boundary conditions. In order to achieve this goal, it is crucial to understand the control mechanisms of short-term growth responses on a scale that is relevant to important and/or recurring environmental variations. Furthermore, functional plant modelling, vegetation analysis in the context of global climate change, and modern plant breeding also require an improved understanding of dynamic growth processes, including those observed under day–night 24 h (diel) fluctuations of the environment.
Plant growth is a result of the interplay between environment and physiological processes controlled by endogenous regulatory mechanisms. The environment provides conditions and resources for growth, while the internal regulatory machinery integrates and translates the information coming from various environmental cues and orchestrates the output processes in the form of growth, defence, and reproduction to optimize resource acquisition and utilization, which eventually enhances fitness of the plant (Dodd et al., 2005; Graf et al., 2010; Kerwin et al., 2011). Like most other organisms on Earth, plants use the circadian clock to anticipate daily and seasonal fluctuations in their environments [higher plants (Harmer, 2009), mammalia (Ukai and Ueda, 2010), and algae (Matsuo and Ishiura, 2010)].
Leaves and roots, though part of the same organism, are exposed to completely different environments; the atmosphere and the pedosphere, respectively. These environments differ from each other in chemical composition and physical properties, and have distinct spatial and temporal heterogeneities. The atmosphere is characterized by strong and often predictable diel and seasonal variations in temperature, light intensity, and daylength. Other environmental factors such as wind or air humidity can also affect plant growth (de Langre, 2008), yet these changes are less predictable and do not follow regular cycles. In contrast to the atmosphere, the pedosphere is mainly characterized by spatial heterogeneity. The physicochemical properties of the soil substrate determine the soil capacity for water and mineral retention. Furthermore, soil compactness restrains root expansion (Bengough et al., 2006). Temporal changes in temperature do occur in the pedosphere as well, but they are delayed and dampened in amplitude compared with those in the atmosphere. For example, in a typical diel temperature cycle the atmospheric temperature varies by 16 °C and reaches a maximum at ∼14:00 h, whereas soil temperature at 10 cm depth varies by merely 3 °C and reaches a maximum 2 h later at 16:00 h (Walter et al., 2009).
The anatomical differences between leaves of mono- and dicotyledonous plants, especially the position of the growth zone in which cell division and elongation take place, predispose their leaf growth to distinct perception and sensitivity to atmospheric environments (Fig. 1). In dicots the leaf growth zone is more directly exposed to environmental changes, whereas that of monocots is in a more protected microclimate shielded by sheaths of older leaves (Davidson and Milthorpe, 1966). In addition, growing leaf tissue is already engaged in photosynthesis in many dicot species, while growth zones of monocot leaves remain heterotrophic for longer. In 1987, Rozema et al. already suggested that differences in diel leaf elongation of halophytic monocotyledonous and dicotyledonous plants are due to differences in the spatial arrangement of tissues and in hydraulic control mechanisms (Rozema et al., 1987). Similar to the monocot leaf, root growth occurs unidirectionally and in root-defined, linearly organized growth zones. Unlike the monocot leaf, however, the root growth zones are exposed more directly to the rhizosphere or pedosphere environment.
In this review, an overview of leaf and root growth patterns in monocot and dicot plants on a diel scale is first given. Then the control of diel growth is discussed, focusing on the effects of external cycles of environmental stimuli (e.g. light and temperature) and the role of the endogenous circadian clock and carbohydrate metabolism.
Diel fluctuations of root growth
The first diel measurements of root elongation rate date back to 1965 (Head, 1965). Time-lapse movies with an interval of 4 h were made for several days to study cherry (Prunus avium) root growth. The author reported a diel root elongation rate pattern with the highest growth rate at night and the lowest growth rate during the day. Unfortunately, no comments were made on the environmental conditions of the experiment (daylength, temperature, soil properties, or water availability), making it difficult to interpret these diel root growth patterns.
More recent studies with higher temporal resolutions (minutes instead of hours) revealed that root growth is highly responsive to temporal changes in environmental conditions. Root growth of Zea mays and Nicotiana tabacum quickly adjusted to new temperature regimes within a few minutes (Walter et al., 2002). In particular, root elongation growth seems to follow alterations in temperature almost linearly within a physiological temperature range between 20 °C and 30 °C (Fig. 1E) (Walter et al., 2002; Hummel et al., 2007). The root elongation rate is also sensitive to changes in nutrient availability (Walter et al., 2003), soil water potential (Sharp et al., 1988), and mechanical impedance of the soil (Bengough et al., 2006) (for a review, see Bengough et al., 2011).
When environmental conditions were kept constant, no change in diel root growth pattern was found in a number of species such as Arabidopsis thaliana (Chavarría-Krauser et al., 2008), Oryza sativa (Iijima et al., 1998), Sorghum bicolor (Iijima et al., 1998), Z. mays (Walter et al., 2002, 2003), and N. tabacum (Walter and Schurr, 2005; Nagel et al., 2006). These results are consistent with the strong effects of environment on root growth demonstrated under changing conditions as summarized above. Interestingly, marked diel oscillations of root tip growth have been reported recently in A. thaliana under constant conditions (Yazdanbakhsh and Fisahn, 2010); a growth maximum was recorded 1 h after dawn, followed by a steep decrease to reach a minimum at dusk and recuperation during the night.
An important difference between the experimental conditions of Yazdanbakhsh and Fisahn (2010) and those of Walter et al. (2002, 2005) or Iijima et al. (1998) is direct exposure of the entire root system, including the growing root tips, to light–dark cycles in the study by the former. As light is known to inhibit root growth (Pilet and Ney, 1978), the observed oscillation patterns in Arabidopsis root growth may be influenced by the inhibitory effect of root illumination (Schmidt and Walter, 2009). Also, complete enclosure of seedlings in a Petri dish—a widely used condition for root growth analysis—can affect growth processes through ethylene emission by leaves (Eliasson and Bollmark, 1988; Hummel et al., 2009).
Diel fluctuations of monocot leaf growth
Leaf elongation in monocots occurs in a defined growth zone at the basal part (Fig. 1A). The pattern of leaf elongation rates follows three phases: (i) exponential elongation rates before leaf appearance; (ii) stable elongation rates; and (iii) progressive decrease of elongation rates until the leaf reaches its final size (Parent et al., 2009). Leaf elongation rates (LERs) in several monocots, such as Hordeum vulgare, Z. mays and S. bicolor, have been shown to be stable and constant for a relatively long period (5–7 d) after the initial exponential growth (Bernstein et al., 1993; Munns et al., 2000; Sadok et al., 2007); one of the reasons for this is the spatial invariance of their elongation zone during this period (Muller et al., 2001). In O. sativa, on the other hand, this stable elongation period is very short, if it exists at all, and is followed by a long period of gradual elongation decrease (Parent et al., 2009).
Close analysis of diel growth patterns in monocot leaves has revealed either constant elongation or a slow decrease independent of time of day (Munns et al., 2000; Parent et al., 2009; Poiré et al., 2010b). Increasing evidence indicates that there is a strong correlation between the short-term changes of monocot LER and changes in temperature or water potential (Ben-Haj-Salah and Tardieu, 1995; Munns et al., 2000; Sadok et al., 2007; Poiré et al., 2010b). Below, the effects of temperature and water relations on LER in monocot plants are highlighted.
Nocturnal LER in monocots (Triticum aestivum, Z. mays, and O. sativa) has been shown to follow temperature alterations linearly in a range between 10 °C and 30 °C (Ben-Haj-Salah and Tardieu, 1995; Pietruszka and Lewicka, 2007; Poiré et al., 2010b). Variations of LER and temperature coincide almost perfectly throughout the diel cycle when evaporative demand is low (Poiré et al., 2010b). Thus, meristem temperature seems to be the dominant factor in controlling the rate of leaf elongation in monocots (Ben-Haj-Salah and Tardieu, 1995) even though plant-internal water relations, evapotranspiration, and alterations of environmental factors can modulate the diel leaf growth cycle. At constant temperature and low evaporative demand, no diel pattern of LER is observed (Parent et al., 2010; Poiré et al., 2010b) although transient effects of light–dark transitions may appear (Sadok et al., 2007).
Given the strong correlation with temperature, nocturnal (or diel) monocot LER (at low evaporative demand) can be described on the basis of thermal time (Fig. 1D) (Sadok et al., 2007). The classical concept of ‘thermal time’ is based on the linearity between the elongation rate and temperature:
In other words, the LER is a linear function of the thermal input the leaf receives over a certain time. It is important to notice that this formula can be used only when the relationship between elongation rate and temperature is linear, which holds true within a certain, species-specific temperature range (Bonhomme, 2000). For calculation of development rates in a temperature range extending to extremes, a recent formula (Parent et al., 2010) based on a combination of molecular reaction rates and the reversible inhibition of enzymes (Eyring, 1935; Johnson et al., 1942; Parent et al., 2010) with temperature changes can be considered. Finally, it should also be noted that care has to be taken when applying the thermal time concept for modelling diel leaf expansion rates in dicots as they are correlated with temperature changes to a much smaller extent (Poiré et al., 2010b).
Evaporative demand and water deficit
During the day period, evaporation and transpiration of leaf water can also affect LER. The LER has been shown to decrease with increasing evaporative demand (Munns et al., 2000; Reymond et al., 2003). Moreover, diurnal changes in LER are closely related to the transpiration rate and proportional to changes in evaporative demand even in the absence of a soil water deficit (Acevedo et al., 1979; Ben-Haj-Salah and Tardieu, 1996, 1997; Sadok et al., 2007). Hence, LER can be described as by Sadok et al. (2007):
Likewise, decreased water potential in the growing tissue due to diminished root water conductivity and the resulting increase in xylem tension can reduce LER during the afternoon (Tang and Boyer, 2008). The effects of soil water deficit on LER (in the absence of evaporative demand) have been shown to be proportional to pre-dawn water potential (Chenu et al., 2008).
The plant hormone abscisic acid (ABA) plays an important role in plant responses to water deficit (Bray, 1997). It has been proposed that ABA has three main effects on growth: (i) increasing the water conductance in the plant; (ii) buffering the negative effect of evaporative demand and related day–night alteration of LER; and (iii) a modest influence on non-hydraulic effects (Tardieu et al., 2010).
Diel dicot leaf growth patterns
In many dicot species, expanding cells of growing leaves are photosynthetically active (Stessman et al., 2002). Because gas exchange and growth processes take place in one and the same tissue, pronounced diel fluctuations of carbohydrate and water availability accompany growth processes of dicot leaves. Leaf veins of dicot plants are often arranged in a net-like structure, and leaf lamina expand in both width and length with specific genetic control (Tsukaya, 2006). Moreover, considerable cell division still occurs in elongating parts (Beemster et al., 2005). All of these make the situation more complex for dicot leaves than for monocot leaves.
A base–tip gradient in growth is often observed, with a maximum relative expansion rate at the base and a minimum at the tip region of the growing leaf (Schnyder and Nelson, 1988; Durand et al., 1995; Tardieu et al., 2000). This gradient is coordinated by earlier maturation of the tip part of the lamina compared with the basal part, and it decreases with time (Schmundt et al., 1998; Donnelly et al., 1999; Walter and Schurr, 2005). In N. tabacum, for example, there is an ∼4 d delay in maturation over the gradient (Walter and Schurr, 1999). However, this base–tip growth gradient is not a general feature of all dicot species. Species such as Glycine max, Populus deltoids, and Theobroma cacao show a homogenous growth distribution over the entire leaf (Ainsworth et al., 2005; Walter and Schurr, 2005; Matsubara et al., 2006; Czech et al., 2009).
On a diel time scale, the rates of leaf expansion in dicot plants do not follow temperature and other environmental variations in the same way as observed in leaves of monocot plants. In Helianthus annuus (Boyer, 1968) and Acer pseudoplatanus (Taylor and Davies, 1985), maximal leaf growth rates were reported at night, in Phaseolus vulgaris (Davies and Van Volkenburgh, 1983) and Vitis vinifera (Shackel et al., 1987) maximal growth was reported during the day, and in Solanum lycopersicum the highest growth rates were found at the day–night transition (Price et al., 2001). Nevertheless, as the leaf expands, temperature, evaporative demand, and water deficit are factors that can influence the amplitude of the observed pattern, but the pattern itself remains stable (Poiré et al., 2010a; Pantin et al., 2011).
In previous studies, dicot leaf growth was mostly analysed by using linear variable displacement transducers that measure leaf elongation along the midvein, not taking expansion in width into account. Moreover, many earlier studies distinguished only between diurnal and nocturnal leaf growth by recording leaf dimensions at the beginning of the day and night, respectively. Time-lapse imaging-based methods became available about a decade ago to enable analysis of two-dimensional expansion dynamics in different dicot plants under a range of environmental conditions (Schmundt et al., 1998). For all species investigated so far, growing leaves exhibit repetitive diel leaf growth patterns without clear correlation to the diel atmospheric temperature regime (for a review of these analyses, see Walter et al., 2009). The observed diel leaf growth patterns can be categorized into two main types called Type 1 and Type 2 (Fig. 1B, C) (Walter et al., 2009). External environmental parameters, such as temperature and light intensity, can influence the amplitude but not the basic character of the observed pattern (Poiré et al., 2010b). Type 1 plants, such as N. tabacum (Walter and Schurr, 2005) and A. thaliana (Wiese et al., 2007), show a sinusoidal growth pattern with maximal growth rates around dawn and directly after onset of light (Wiese et al., 2007). The diel growth pattern of Type 2 plants, such as G. max (Ainsworth et al., 2005), P. deltoides (Matsubara et al., 2006), and T. cacao (Czech et al., 2009), is also sinusoidal but has a maximum at the end of the day. These three Type 2 plants have a homogenous growth distribution over the entire leaf. However, Populus trichocarpa, another Type 2 plant, shows a base–tip growth gradient across the lamina (SM, unpublished data), indicating that the formation of spatial and temporal (diel) growth patterns is not necessarily coupled. The contribution of different growth processes (cell division and expansion) cannot explain variations in diel leaf growth patterns either; the specific diel growth patterns, albeit with decreasing amplitude, are maintained over the base–tip gradient of Arabidopsis leaves (Wiese et al., 2007) and during transition from the predominantly cell division to cell expansion phase in growing leaves of T. cacao (Czech et al., 2009).
The origin of the different diel growth strategies of dicot plants is yet to be elucidated. Nevertheless, a study comparing the behaviour of several dicot and monocot species on transfer from day–night conditions to continuous light conditions has indicated that the circadian clock is an important driver of the observed, repetitive diel growth patterns in leaves of dicot species (Poiré et al., 2010b). Whereas an ∼24 h leaf growth rhythm continued in dicot plants after transfer to constant light and temperature regimes, the same treatment caused diel leaf growth variations to vanish in monocot species. In addition to these findings, the presence of the Type 1 growth pattern in isolated leaf discs floating on nutrient solution without any contact with the sink–source system of the intact plant (Biskup et al., 2009) clearly indicates that the circadian clock within the growing leaf itself plays an important role in regulation of the diel dicot leaf growth pattern. For A. thaliana, the diel pattern of hypocotyl elongation growth is also controlled by the circadian clock (Nozue et al., 2007); there, the observed Type 1-like growth pattern depends on the diel cycling of phytochrome-interacting factor 4 (PIF4) and PIF5.
Diel control of growth
The Earth’s rotation brings all organisms into changing but reccurring environmental conditions. Therefore, living organisms adjust their physiology and behaviour with the help of internal oscillators called the ‘circadian clock’ to anticipate these recurring events. For autotrophic plants, there is the need to fine-tune their photosynthesis, carbohydrate metabolism, and carbohydrate storage during the entire diel cycle (Lu et al., 2005; Gibon et al., 2009; Graf et al., 2010). As described above, leaf growth of monocot and dicot plants as well as growth of other plant organs follows different diel rhythms requiring an adapted rhythmic control. Diel gene expression of Arabidopsis is largely controlled by the circadian clock as well as by the diel changes in carbohydrates (Bläsing et al., 2005). The effects of light, nitrogen, and leaf water deficit show a smaller impact on the diel expression of genes in Arabidopsis rosettes (Bläsing et al., 2005). The diel control by the circadian clock, carbohydrates, and their impact on growth will be discussed in the following sections.
Components and function of the circadian clock
The vast majority of recent molecular findings on the function of the circadian clock in plants have involved analyses of Arabidopsis shoots (James et al., 2008). The dicot leaf rosette of Arabidopsis needs to regulate the complexity of its metabolic constraints and environmental cues by tightly controlling the timing of many processes (Green et al., 2002; Dodd et al., 2005; Covington and Harmer, 2007). Hence, it is no surprise that leaf growth of dicot plants, the result of the integration of many metabolic pathways, is controlled to a strong extent by the circadian clock (Fig. 2). The clock is involved in many physiological events such as flowering time (Yanovsky and Kay, 2003; Imaizumi and Kay, 2006), elongation growth of the hypocotyl (Dowson-Day and Millar, 1999; Nozue et al., 2007) and stomatal responses (Gorton et al., 1993), auxin signalling and responses (Covington and Harmer, 2007), and starch degradation during the night (Graf et al., 2010), and it also plays a role in plant defence (Kerwin et al., 2011). A matching of the oscillation period of the circadian clock with daily rhythms in environmental changes is therefore essential and gives a fitness advantage to the plant (Green et al., 2002; Michael et al., 2003b; Dodd et al., 2005; Yerushalmi et al., 2011). In future studies, it will be important to reveal the temporal dynamics of how different elements of the circadian clock are controlling certain phases of the diel leaf growth cycle.
The circadian clock runs with a period close to 24 h even in the absence of environmental triggers, can be reset by environmental cues (e.g. light or temperature), and is temperature compensated. The oscillator is truly endogenous as rhythmicity is observed in etiolated seedlings that have never been exposed to changing environmental conditions (Salomé et al., 2008). The circadian clock is an endogenous control network consisting of transcriptional–translational feedback loops (Fig. 2). The core of the central loop consists of three components: CCA1 (CIRCADIAN CLOCK ASSCIATED 1), LHY (LATE ELONGATED HYPOCOTYL), and TOC1 (TIMING OF CAB EXPRESSION1) (Wang and Tobin, 1998; Alabadí et al., 2001). CCA1 and LHY are dawn-phased genes that inhibit the expression of the evening-phased gene TOC1 by binding directly to the TOC1 promoter (Alabadí et al., 2001; Mizoguchi et al., 2002). TOC1, in turn, reciprocally regulates the expression of CCA1 and LHY transcripts to form a feedback loop (Alabadí et al., 2001; Makino et al., 2002). The ‘morning’ loop is formed by the repressor activity of the PSEUDO-RESPONSE REGULATOR 9 (PRR), PRR7, and PRR5 on promoters of their activators CCA1 and LHY (Farré et al., 2005; Nakamichi et al., 2010). Lastly the ‘evening’ loop consists of TOC1 and an unknown component Y which reciprocally regulate each other (Locke et al., 2005; Zeilinger et al., 2006).
The components of the circadian oscillator are tissue specific (Thain et al., 2002; James et al., 2008). Roots, for instance, have a simplified version of the circadian clock consisting solely of a functional morning loop (James et al., 2008). The evening genes, although expressed, do not oscillate and—in general—a smaller number of genes shows diel gene expression in the root compared with genes expressed in the shoot (James et al., 2008). Also in shielded developing maize ears, diel gene expression is strongly reduced compared with the photosynthetically active leaf, where the expression of oscillator genes of all loops shows strongly reduced amplitudes (Hayes et al., 2010). Homologues for most of the genes of the central oscillator are found in other species, both monocot (Lemna gibba, Lemnapaucicostata, O. sativa, and Z. mays) and dicot (G. max, Ipomoea nil, and S. lycopersicum) (Izawa et al., 2003; Miwa et al., 2006; Hayama et al., 2007; Murakami et al., 2007; Facella et al., 2008; Serikawa et al., 2008; Hayes et al., 2010; Hudson, 2010). This suggests that the fundamental elements of the circadian clock seem to be conserved in monocots and dicots. However, differences in amplitude, phase, and contribution to oscillation were found between species (Miwa et al., 2006; Hayama et al., 2007; Serikawa et al., 2008; Hayes et al., 2010), and it is unclear which clock elements are active in which tissues of the shoot systems. These results suggest that the precise role for some clock genes has diverged in angiosperm evolution (for detailed reviews on the circadian clock, see Harmer, 2009; McClung and Gutierrez, 2010; Pruneda-Paz and Kay, 2010).
Entrainment by the environment: light and temperature
Plants synchronize their clock by signal inputs from the environmental cycles in light and temperature. The red and blue light photoreceptors, phytochromes (PHYs) and cryptochromes (CRYs), respectively, mediate parts of the light signal input into the clock (Somers et al., 1998). The interactions between the PHY/CRY signalling pathways are synergistic and depend on light quality as well as fluence rate (Casal and Mazzella, 1998). Even though light signalling via PHYs and CRYs is important for normal clock function, these pathways are not essential for clock function and can be compensated by other input signals (Yanovsky et al., 2000).
ZEITLUPE (ZTL) and two ZTL homologues, FLAVIN BINDING KELCH F-BOX 1 (FKF1) and LOV KELCH PROTEIN2 (LKP2), act as photoreceptors by a photochemically active LOV domain to regulate TOC1 expression in the evening part of the oscillator (Mas et al., 2003; Kim et al., 2007; Sawa et al., 2007). Furthermore, LWD1/2 (Wu et al., 2008), SRR1 (Staiger et al., 2003), LIP1 (Kevei et al., 2007), XCT (Martin-Tryon and Harmer, 2008), ELF3 (McWatters et al., 2000), and ELF4 (Kikis et al., 2005) all affect light input into the oscillator, suggesting a complex regulation of input signalling by light.
The circadian clock is temperature compensated; this compensation is established by a dynamic balancing of morning components (LHY/CCA1) versus evening components (TOC1/GI) (Gould et al., 2006; Portoles and Mas, 2010). Nonetheless, temperature cycles are sufficient to keep the oscillator running (Salomé and McClung, 2005; Thines and Harmon, 2010). Moreover, temperature cycles alone can drive at least half of all transcripts critical for synchronizing internal processes, such as cell cycle and protein synthesis (Michael et al., 2008). The genes involved in temperature compensation, together with the other clock genes ELF3, PRR9, and PRR7, are known components of temperature input into the oscillator (Salomé and McClung, 2005; Thines and Harmon, 2010). Different loops of the oscillator appear to have different temperature sensitivity, as two output signals, CAT3 and CAB2 expression, have differential temperature sensitivity (Michael et al., 2003a).
The circadian oscillator takes inputs at several time points during the diel cycle to assess the environment and to regulate output accordingly (Sawa et al., 2007). The importance of light and temperature signalling integrated into the clock, in contrast to their direct effect on growth, depends on the dominance of the circadian clock on the organ and plant species. Hence, to derive the exact connection between circadian clock elements and the timing of leaf growth processes, a precise understanding of the role of the above-mentioned elements will be required. This may eventually contribute to clarification of differences between Type 1 and Type 2 species as well as to understanding the variable fine-tuning of diel leaf growth patterns in different species in response to environmental variations.
Clock-induced growth regulation
So far, the regulation of growth by the circadian clock has been best investigated for Arabidopsis hypocotyl elongation growth (Dowson-Day and Millar, 1999; Nozue et al., 2007). In short-day conditions, hypocotyl elongation rates were found to be rhythmic and to peak shortly after dawn (Nozue et al., 2007). However, in continuous light, the maximum elongation rate was shifted to the subjective night (Dowson-Day and Millar, 1999; Nozue et al., 2007). Normal expression of the diel hypocotyl growth pattern seems to require light input and circadian regulation, which serves to time the transcript and protein abundance of two positive hypocotyl growth regulators, PIF4 and PIF5, at the end of the night (Nozue et al., 2007). Recently the molecular basis of the circadian gating in hypocotyl growth has been unravelled. The circadian evening component ELF3 forms a complex with ELF4 and LUX (LUX ARRHYTHMO), two other clock components; then LUX targets the entire complex to the PIF4 and PIF5 promoter by directly binding to it (Nusinow et al., 2011). All three components have been shown to be required for the proper expression of PIF4 and PIF5 (Nusinow et al., 2011). Since dicot leaf growth in continuous light shows rhythmicity as the hypocotyl does (Poiré et al., 2010b), a similar control pattern of growth timing can be hypothesized.
The special role of carbohydrates in growth control
Carbohydrates are required for growth as building blocks to produce, for example, cell wall polymers, and as energy carriers to drive growth activities. To control the availability and quality of carbohydrates, plants and other organisms evolved a complex signalling system that allows the integration of carbohydrates as signalling molecules into the control of gene expression, metabolism, growth, and development (Rolland et al., 2006; Smeekens et al., 2010). Carbohydrates are the product of photosynthesis during the day, and a defined fraction of them is stored as starch. This fraction provides a nocturnal supply to source and sink cells, and its degradation is adjusted to the expected night period (Gibon et al., 2004; Lu et al., 2005; Graf et al., 2010). The availability and efficient degradation of this starch reservoir is required for nocturnal growth, as can be concluded from the negative correlation of biomass and the remaining starch reservoir at dawn (Cross et al., 2006). Diurnal growth on the other hand is limited by carbohydrate storage during photosynthesis, as suggested by the negative correlation of shoot biomass to starch content at dusk (Sulpice et al., 2009; Graf and Smith, 2011). Also, starch degradation, but not starch synthesis, was shown to correlate strongly and positively with the relative growth rate of Arabidopsis in varying daylengths (Gibon et al., 2009). Therefore, carbohydrate metabolism plays a very important role in the control of leaf growth. In later stages of leaf development, leaf growth is not only restricted by the metabolic component. The hydraulic status of the leaf then becomes an even more limiting factor (Pantin et al., 2011). It would exceed the scope of this review to elaborate putative control mechanisms of the hydraulic status of leaf growth such as the interaction of turgor with yielding of the expanding cell wall, stiffening of cell walls by components such as lignin, the activity of transmembrane proteins such as aquaporins, the generation of reactive oxygen species, or the generation of hydraulic gradients within the leaf by varying xylem element sizes. All of these dynamically altering regulatory mechanisms could potentially interact with the circadian clock. As one example of dynamic metabolic input into growth, carbohydrate metabolism is highlighted here.
Leaves of intact plants, excised leaf discs, and roots of Arabidopsis starch deficiency mutants do not grow or grow very slowly during the night due to a lack of available carbohydrates (Gibon et al., 2004; Wiese et al., 2007; Biskup et al., 2009; Yazdanbakhsh et al., 2011). Instead, in such mutants, a leaf growth increase at the end of the day correlates with an excess of soluble carbohydrates. The carbohydrate status apparently has a direct impact on the amplitude of the observed pattern, while still retaining the general phasing of the leaf growth cycle comparable with that of wild-type plants (Wiese et al., 2007). Overall the starch metabolism acts as an integrator of the metabolic response in a regulatory network that balances growth with the carbon supply (Sulpice et al., 2009; Graf et al., 2010; Graf and Smith, 2011). Furthermore, many sugar-responsive genes show marked diel expression changes and sugar levels have a profound impact on diel gene regulation (Bläsing et al., 2005).
The diel carbohydrate status of the dicot plant has a huge effect on the observed growth pattern of leaves and roots (Gibon et al., 2004; Nagel et al., 2006; Wiese et al., 2007; Yazdanbakhsh et al., 2011). Similarly, carbohydrate availability is an important driving force in the basal zone of monocot leaf growth (Davidson and Milthorpe, 1966; Schnyder and Nelson, 1987). The completely shielded growth zone of the monocot leaf is heterotrophic and provided with photosynthates from the already fully differentiated tip part of the leaf (Allard and Nelson, 1991), and is also supported by surrounding fully expanded leaves (Brégard and Allard, 1999). Thus, in monocot leaves, photosynthesis is spatially clearly separated from the growth zone (Fig. 1), comparable with the situation in roots. The diel export of carbohydrates from the distal, source part of a young maize leaf correlates positively with LER (Kalt-Torres and Huber, 1987). Yet the diversity in monocot carbohydrate storage forms (fructans, sucrose, and starch) and cellular compartmentalization of storage carbohydrates leads to a very complex situation there that is far from being elucidated.
In roots, carbohydrate availability strongly regulates dynamic growth activity (Aguirrezabal et al., 1994; Freixes et al., 2002). Even a change of light intensity—an environmental parameter to which the shoot is exposed—affects root growth via carbohydrate availability within <1 h (Nagel et al., 2006), demonstrating the important role of carbohydrates in short-term whole-plant growth control.
Interaction of carbohydrates and the clock—a link between resource utilization and integration of environmental changes
In Z. mays, 10% of the transcriptome is under direct circadian regulation, and in A. thaliana this number even reaches ∼30% (Covington et al., 2008; Khan et al., 2010). When different conditions are taken into account, the total sum of transcripts that can show diel rhythmicity is estimated to be close to 90% (Michael et al., 2008). Many clock-controlled genes are modulated in a diurnal/nocturnal regime concomitant with the changing carbohydrate metabolism (Bläsing et al., 2005), and sucrose modulates the clock oscillation in amplitude and phase in Arabidopsis shoot and root tissue (Dalchau et al., 2011). The observed difference of clock oscillation or clock output in developing ears of Z. mays and also in seeds of G. max (Hudson, 2010) compared with leaves might be caused by the sugar-importing sink status of these organs (Hayes et al., 2010). A significant proportion of genes under circadian regulation are involved in metabolism or hormone signalling in plants (Michael et al., 2008; Khan et al., 2010). The circadian clock influences plant metabolism and hormone signalling including auxin gating, the tricarboxylic acid (TCA) cycle, and carbohydrate metabolism and storage (Lu et al., 2005; Covington and Harmer, 2007; Fukushima et al., 2009; Graf et al., 2010). Increased starch accumulation has been shown for mono- and dicotyledonous plants lacking the oscillator gene GI (Eimert et al., 1995; Izawa et al., 2011).
The circadian clock ensures carbohydrate availability throughout the night (Graf et al., 2010), exerting an indirect control of the nocturnal growth potential (Graf and Smith, 2011). Therefore, carbohydrate flux to or accumulation in sink organs is a crucial mediator and modulator of clock oscillations and might be key to the mechanistic understanding of the interaction between the circadian clock and growth processes.
Interaction of root and shoot growth
Roots and leaves live in completely different settings and have adapted to these in unique ways. Diel growth patterns often differ between above- and belowground organs. However these organs are integral parts of a single plant system and they are highly dependent on each other for growth and survival. Optimal resource use efficiency demands highly coordinated fluxes of carbohydrates, water, and mineral nutrients that are acquired by one organ and delivered to the other. Hence, organ growth patterns that have evolved under certain environmental constraints can be considered to reflect the optimal reaction pattern with which an organ can grow in its environmental context. How this optimized resource use efficiency is realized with respect to shoot–root signalling is outside the scope of this review. Clearly the signalling between shoot and root growth is modulated by phytohormones (Sharp and LeNoble, 2002; Ghanem et al., 2011). In addition, carbohydrate- and water-related effects of dynamic organ growth control are known to be linked to shoot–root signalling (Nagel et al., 2006). The diminished transport of water by cooled roots, for example, has been shown to diminish diurnal but not nocturnal leaf growth (Poiré et al., 2010a). A photosynthesis signal, possibly sucrose or a derivative, is proposed to synchronize the circadian oscillators of shoot and root (James et al., 2008). Furthermore, expression of genes coding for transport of water, ions, metabolic solutes such as sucrose, micronutrients, and signalling molecules, including Ca2+, shows diel rhythmicity and might contribute to the control of fluxes between root and shoot, optimizing the overall plant performance (Haydon et al., 2011).
Monocots and dicots, as well as different plant organs, cope in different ways with their surrounding environment, and this is reflected in the growth patterns (Fig. 1). Differing organ and plant architectures conceivably contribute to the evolution of differing growth strategies. Through the course of evolution, the leaves of dicots started adjusting their growth to a greater extent to the circadian clock to avoid growth at unfavourable times during the diel cycle as the growth zone is vulnerably exposed to strong fluctuations in the environment. In contrast, the monocot leaf growth zone and root growth zones are less exposed to environmental fluctuations in the diel cycle and probably therefore do not require such a stringent diel control by the circadian clock. Thereby they invested more in the optimization of their growth performance to direct environmental conditions (Fig. 2).
Interestingly, the growing dicot leaf can sustain a lot of its growth activities from its own photosynthesis, whereas the heterotrophic monocot growth zone and the root growth zones are true sink tissues. The intensity of the sink strength of these organs might be another reason as to why monocot leaf growth and root growth are less responsive to the circadian clock: Circadian gene expression might simply be overridden by the high flux of imported carbohydrates or by more intense metabolic feedback loops there.
CHLOROPHYLL A/B-BINDING PROTEIN 2
CIRACADIAN CLOCK ASSOCIATED 1
FLAVIN BINDING KELCH F-BOX 1
leaf elongation rate
LATE ELONGATED HYPOCOTYL
LIGHT INSENSITIVE PERIOD 1
LOV KELCH PROTEIN2
light, oxygen and voltage
sensitivity to red light reduced
TIMING OF CAB 1
XAP5 CIRCADIAN TIMEKEEPER