On the Inside

It is well known that nitrate is an important nutrient that supports plant growth and development. The application of nitrate also causes extensive changes in the expression of genes coding for proteins involved in nitrogen (N) metabolism. Indeed, genomic analyses have provided a comprehensive dataset of more than a thousand nitrate-responsive genes in Arabidopsis (Arabidopsis thaliana). Much less, however, is known about the signaling role of nitrite, the direct product of nitrate reduction. In this issue, Wang et al. (1735–1745) report that nitrite increases mRNA levels as quickly as nitrate in N-starved Arabidopsis roots. Both nitrite and nitrate inductions occur at concentrations as low as 100 nM. The response at low nitrite concentrations was not due to contaminating nitrate—a problem in several earlier studies. The speed of the response suggests that it is unlikely that reprovision of N is the cause of the nitrite response. Another possible mechanism is that nitrite is converted to nitric oxide, which elicits the response, but treatment with 250 mM nitrite revealed no increase in the fluorescence of roots stained with a nitric oxide-reactive dye. Transcriptome analysis using nitrate or nitrite showed that more than half of the nitrate-induced genes, which included genes involved in nitrate and ammonium assimilation, energy production, and carbon and N metabolism, responded equivalently to nitrite; however, the nitrite response was more robust and there were many genes that responded specifically to nitrite. Thus, it appears that nitrite can serve as a signal as well as if not better than nitrate.


Not all mobile mRNAs move the same
Long-distance mobile mRNAs play key roles in regulating various gene networks that control plant development and stress tolerance. Grafting experiments have confirmed that endogenous RNA molecules exist in phloem sap and mRNAs move in both the stock-to-scion and scion-tostock directions. The long-distance transport of RNAs is facilitated by specific proteins that prevent RNAs from being degraded during transport. Lv et al. (pp. 1587Lv et al. (pp. -1604 now shed light on the species-specific differences in the movement of mobile mRNAs. They use grafts involving different apple (Malus) genotypes to demonstrate that apple (Malus domestica) oligopeptide transporter3 (MdOPT3) mRNA can be transported over a long distance, from the leaf to the root, to regulate iron uptake. In marked contrast, the mRNA of Arabidopsis (Arabidopsis thaliana) oligopeptide transporter 3 (AtOPT3), the MdOPT3 homolog from A. thaliana, does not move from shoot to root. When, however, AtOPT3 was heterologously expressed in two woody species, Malus and Populus, it was able to move from the shoot to the root. In contrast, when the mobile homolog MdOPT3 was heterologously expressed in two herbaceous species, A. thaliana and tomato (Solanum lycopersicum), its ability to move from the shoot to the root was lost. Previous research has shown that more than 10% of total phloem sap proteins are RNA-binding proteins (RBPs), and it is believed that the binding of phloem-residing RNAs to RBPs may play important roles in loading, unloading, and long-distance transport of RNA in the phloem sieve system. The authors demonstrate that the different transmissibility of OPT3 in A. thaliana and Malus might be caused by divergence in RBPs between herbaceous and woody plants. This study provides insights into mechanisms underlying differences in mRNA mobility in plants.

How the cucumber fruit gets its warts
The "wartiness" of cucumber (Cucumis sativus) fruits is an important quality trait that greatly affects fruit appearance and market value. Consumer preferences for warts or not depends on both culinary considerations and cultural traditions. A cucumber wart consists of a spine (a fruit trichome) in combination with an underlying tubercule, an arched structure derived from several layers of surface cells. The existence of spines is an epistatic prerequisite for tubercule formation. Although several regulators have been reported to mediate spine or tubercule formation, the direct link between spine and tubercule development remains unknown. Wang et al. (pp. 1619Wang et al. (pp. -1635 now show that the gene HECATE2 (CsHEC2) is highly expressed in cucumber fruit peels including spines and tubercules. Knockout of CsHEC2 by the CRISPR/Cas9 system resulted in reduced wart density and decreased cytokinin accumulation in the fruit peel. Conversely, overexpression of CsHEC2 led to elevated wart density and cytokinin levels. Previous genetic studies showed that the warty phenotype is dominant over the non-warty fruit trait, and that fruit tubercule trait is controlled by a single dominant gene (CsTu), which encodes a transcription factor. CsTu apparently functions in cytokinin biosynthesis by indirectly promoting the expression of two cytokinin hydroxylase-like (CHL) genes, thereby stimulating cell division and ultimately causing the initiation of the fruit tubercule. Therefore, it is of interest that the present authors provide evidence that CsHEC2 directly binds to the promoter of the cytokinin hydroxylase-like1 gene (CsCHL1), thereby activating its expression. Evidence is also presented that CsHEC2 physically interacts with GLABROUS3 (CsGL3, a key spine regulator). These data suggested that CsHEC2 promotes wart formation by acting as an important cofactor for CsGL3 and CsTu to stimulate cytokinin biosynthesis in cucumber.

Auxin flow and endodermal development
The three specialized tissues of the roots of vascular plants, the epidermis, ground tissue (comprising the endodermis and cortex), and the vascular tissue, are derived from stem cell niches at the root apical meristem. The endodermis serves as a selective barrier that restricts the free diffusion of nutrients to the central vasculature. Seo et al. (pp. 1577Seo et al. (pp. -1586 now show that auxin is a key regulator determining the division of endodermal cells in Arabidopsis. Auxin induced the division of endodermal cells in wild-type plants, but not in the auxin signaling mutant auxin resistant3-1 (axr3-1). The authors also report that the endodermisspecific activation of auxin responses by the directed expression of truncated AUXIN RESPONSIVE FACTOR5 (DARF5), a more active form of the gene, in root endodermal cells also induced endodermal cell division. The auxin transport inhibitor 1-naphthylphthalamic acid (NPA) led to auxin accumulation in endodermal cells, which, in turn, induced endodermal cell division. In addition, knock-out of P-GLYCOPROTEIN1 (PGP1) and PGP19, which mediate centripetal auxin flow, promoted the division of endodermal cells. The findings reported reveal a tight link between the endodermal auxin response and endodermal cell division, suggesting that auxin is a key regulator controlling the division of root endodermal cells, and that PGP1 and PGP19 are involved in regulating endodermal cell division

Shoot herbivory impairs nematode root infection
Herbivore-induced defense responses are regulated by a network of interconnected signaling pathways in which plant hormones play a major regulatory role. Among them, the jasmonates, a family of oxylipins, have emerged as key signals in plant responses to chewing insect herbivores, such as beetles and caterpillars. Herbivore-induced defenses are typically expressed not only locally in the damaged tissue, but also systemically. The majority of studies on plant-mediated interactions between herbivores are constrained to aboveground tissues. A growing body of evidence, however, suggests that inducible systemic plant defenses can simultaneously influence the respective feeding rates of aboveground and belowground herbivores. However, the mechanisms driving these systemic effects and the longdistance signals involved remain poorly understood. Using tomato (S. lycopersicum) as a model plant, Martínez-Medina et al. (pp. 1762-1778 have explored the impact of leaf herbivory by the moth Manduca sexta on the performance of the root-knot nematode Meloidogyne incognita. They report that that leaf herbivory reduced M. incognita performance in the roots. By analyzing the root expression profile of a set of oxylipin-related marker genes and jasmonate root content, the authors demonstrate that leaf herbivory systemically activates the 13-Lipoxygenase (LOX) and 9-LOX branches of the oxylipin pathway in roots and counteracts the M. incognita-triggered repression of the 13-LOX branch. Further experiments revealed that having intact jasmonate perception in the shoot was essential for the inhibitory effects of moth shoot feeding on nematode root-feeding; intact jasmonate synthesis was not. These results highlight the impact of leaf herbivory on the ability of M. incognita to manipulate root defenses and point to an important role for jasmonate signaling in shoot-to-root signaling.

Inhibited chloroplast N assimilation under high CO 2
It is projected that global food production needs to be doubled by 2050 to meet demands. The improvement of photosynthetic efficiency is regarded as a major strategy for increasing crop yield potential. For C 3 crops, one possible strategy for achieving this goal might be to inhibit photorespiration by increasing the CO 2 concentration around Rubisco. Unfortunately, while increased atmospheric CO 2 concentrations typically stimulate photosynthesis and crop yield in the major C 3 cereal crops, they also decrease protein content. Insufficient leaf nitrogen (N) content can limit the development of sinks, such as grains, fruits, tubers, etc., which, in turn, causes feedback inhibition of photosynthetic gene expression. Insufficient N may also limit N investment into components of the photosynthetic apparatus, such as Rubisco and chlorophyll. To help make sense of this enormously complex problem, Zhao et al. (pp. 1812Zhao et al. (pp. -1833 have developed a dynamic systems model of plant primary metabolism, which includes the Calvin-Benson cycle, the photorespiration pathway, starch synthesis, glycolysisgluconeogenesis, the tricarboxylic acid cycle, and chloroplastic N assimilation. This model successfully captures responses of net photosynthetic CO 2 uptake rate, respiration rate, and nitrogen assimilation rate to different irradiance and CO 2 levels. The authors then applied this model to predict the inhibition of N assimilation under elevated CO 2 . Simulations suggested that enhancing the supply of a-ketoglutarate (2-OG) is a potential strategy to maintain high rates of N assimilation under elevated CO 2 . This model provides a basic framework for supporting the design and engineering of C 3 plant primary metabolism for enhanced photosynthetic efficiency and N assimilation in the looming high-CO 2 world.

A small-molecule antagonist of jasmonic acid perception and auxin responses
In chemical genomics, large chemical libraries are screened to identify molecules reversibly affecting the activity of a protein, protein families, or a biochemical or signal transduction pathway. Hormone antagonists are invaluable tools for exploring the role of the hormone in specific tissues and developmental stages, and for identifying regulatory processes of the signaling pathway. The phytohormone jasmonoyl-L-isoleucine (JA-Ile) regulates many stress responses and developmental processes in plants. Using two complementary chemical screens, Chini et al. (pp. 1399Chini et al. (pp. -1413 searched for different and cheaper antagonists of JA-Ile perception. In this issue, they identify and characterize three commercially available JA-Ile antagonists. Of the three, the most intriguing was a molecule called J4 that exhibited JA-antagonist effects in planta, preventing several JAmediated responses such as gene expression, growth inhibition, chlorophyll degradation, and anthocyanin accumulation in Arabidopsis, tomato (S. lycopersicum), and tobacco (Nicotiana benthamiana). In addition to antagonizing JA signaling, J4 also inhibited the closely related auxin pathway but no other hormonal pathways. Furthermore, the mode of action of this molecule is conserved in land plants, indicating its potential use in any plant species. This commercially available compound should prove to be a powerful tool for the pharmacological analysis and dissection of the JA and auxin signaling pathways.