Cellular export of sugars and amino acids: role in feeding other cells and organisms

Abstract Sucrose, hexoses, and raffinose play key roles in the plant metabolism. Sucrose and raffinose, produced by photosynthesis, are translocated from leaves to flowers, developing seeds and roots. Translocation occurs in the sieve elements or sieve tubes of angiosperms. But how is sucrose loaded into and unloaded from the sieve elements? There seem to be two principal routes: one through plasmodesmata and one via the apoplasm. The best-studied transporters are the H+/SUCROSE TRANSPORTERs (SUTs) in the sieve element-companion cell complex. Sucrose is delivered to SUTs by SWEET sugar uniporters that release these key metabolites into the apoplasmic space. The H+/amino acid permeases and the UmamiT amino acid transporters are hypothesized to play analogous roles as the SUT-SWEET pair to transport amino acids. SWEETs and UmamiTs also act in many other important processes—for example, seed filling, nectar secretion, and pollen nutrition. We present information on cell type-specific enrichment of SWEET and UmamiT family members and propose several members to play redundant roles in the efflux of sucrose and amino acids across different cell types in the leaf. Pathogens hijack SWEETs and thus represent a major susceptibility of the plant. Here, we provide an update on the status of research on intercellular and long-distance translocation of key metabolites such as sucrose and amino acids, communication of the plants with the root microbiota via root exudates, discuss the existence of transporters for other important metabolites and provide potential perspectives that may direct future research activities.

Cells can secrete specific compounds for various functions, for example, disposal, protection from 62 osmotic damage, feeding of other cells -either a neighboring cell in the same organism or cells from 63 other organisms -or solute distribution in multicellular organisms, and defense. A well-studied 64 example is Corynebacterium glutamicum, which effectively secretes glutamate and is therefore used 65 for the industrial production of glutamate (Nakayama et al., 2018). C. glutamicum secretes glutamate 66 via a mechanosensitive efflux transporter. Many bacteria secrete valine into their biofilms where it 67 serves as an antibiotic (Valle et al., 2008). This review focuses on processes in which major 68 metabolites, in particular sugars and amino acids, are secreted from plant cells. Key physiological 69 aspects discussed here relate to the distribution of assimilates in plants, as well as to the exchange of 70 metabolites with other organisms, in particular nectar secretion and feeding of beneficial and 71 pathogenic microbes. This review highlights families of transporters for metabolitessugars and 72 amino acidsand their role in carbon and nitrogen allocation: SWEETs and UmamiTs, as well as 73 additional transporters for other metabolites and their roles in physiology, pathogenesis, and 74 symbiosis. 75

SWEET and UmamiT transporters: evolution and structure 76
Members of the SemiSWEET-SWEET sugar transporter superfamily had originally been described as 77 homologs of Medicago truncatula NODULIN 3 (MtN3), based on the observation that its transcript 78 levels increased during nodulation (Gamas et al., 1996). SWEETs belong to an ancient family with 79 members present already in Archaea. Plant genomes typically contain approximately 20 SWEETs with 80 two conserved PQ-loop repeats (  (Arabidopsis thaliana) also seems to code for 'half'or SemiUmamiTs. For instance, UmamiT43 is 107 predicted with five transmembrane segments (http://aramemnon.uni-koeln.de), but has not yet been 108 functionally characterized. 109

SWEET and UmamiT Substrates 110
The activity of SWEETs was identified through a screen of polytopic membrane proteins with 111 unknown functions co-expressed with genetically encoded glucose or sucrose sensors in human 112 human cells, which are cultured in media with a neutral pH, and which lack plasma membrane H + -114 ATPases, may provide favorable conditions for identifying sucrose efflux transporters that might 115 function as uniporters or sucrose/H + antiporters (Fieuw and Patrick, 1993). SWEETs, similar as 116 SemiSWEETs, can transport hexoses and/or the disaccharide sucrose ( SWEET members fall into four clades in which clade 3 members preferably mediate sucrose transport 119 (Supplemental Fig. S1). In addition to sugar transport, several SWEETs are capable of transporting 120 gibberellic acid (GA), which at first sight, neither resembles glucose nor sucrose (Kanno et al., 2016; 121 Morii et al., 2020) (Table 2). Notably, GA biosynthesis pathway genes are enriched in phloem cell 122 types where AtSWEET11-13 and several other GA transporters, such as AtNPF4.6, are enriched ( Fig.  123 2) (Kim et al., 2021). While all characterized clade III SWEETs are plasma membrane-localized, other 124 members (i.e., clade II, IV) were also detected in vacuolar and ER membranes ( Table 2, Supplemental 125 Fig. S1). 126 Given the comparatively high number and diverse chemical properties of amino acids (i.e., charge, 127 polarity, aromaticity), the substrate recognition and transport mechanism of UmamiTs is likely more 128 complex compared to that of SWEETs. While some prokaryotic DMTs, like YddG, seem to 129 specifically transport aromatic amino acids, YdeD exports cysteine, asparagine, and glutamine, RhtA . However, the same transporters were also able to transport up to 13 additional 136 proteinogenic amino acids and structurally related metabolites (

Transport mechanisms of SWEETs and UmamiTs 146
The study of sugar transport mechanism in plants started more than 40 years ago (Giaquinta, 1976 219  in cells from almost all cell types of the leaf, with an apparent preferential accumulation in cells from  220  the bundle sheath/xylem cells, overlapping with transcripts from other amino acid transporters (Fig.  221 3A-C). More cell type-specific expression was observed for AtUmamit5/WAT1 in epidermal cells, 222 whereas transcripts of AtUmamiT10, 27, and 31 were almost exclusive for guard cells (Fig. 3A), 223 pointing towards stomatal functions. As cell types in the leaf have distinct metabolic activities 224 reflected by the differential transcript level of metabolic pathway genes (Kim et al., 2021), it will be 225 interesting to assess the substrate specificity and the role of these UmamiTs in respect to the cell types 226 where they are enriched (Box 1). More detailed analyses were performed using reporters for a number 227 of UmamiT family members, yielding evidence for roles in phloem loading and seed filling. 228

Roles of UmamiTs in phloem loading 229
Amino acids are the main transport forms of organic nitrogen in the phloem of most plants. We  characterization of UmamiTs and cell specific metabolism will be useful to understand the regulation 246 of amino acid allocation. 247 Consequently, sugars and amino acids must be exported from one cell into the apoplasm before then 267

Roles for SWEETs and UmamiTs in phloem unloading
can be re-imported in the adjacent cell. 268 One of the most elegant systems for studying metabolite efflux is the "empty seed The evidence for SWEETs in feeding tissues in the developing seed is not limited to Arabidopsis, but 285 also exists in crops. In rice, OsSWEET11 and 15 are essential for transporting sugar through distinct 286 apoplasmic pathways ( domestication that was likely recruited by farmers and breeders who selected for large grains. 293 Although substantial progress has been made, the full path of sucrose in none of the species has been 294 unraveled. 295 Roles for UmamiTs in amino acid supply to seeds 296 The transport of amino acids and sucrose shares commonalities since both processes must undergo 297 similar symplasmic and apoplasmic steps. Uptake of amino acids into the embryo has been shown to 298 occur via H + /amino acid symporters such as the AAPs, while efflux processes are mediated by proton 299 gradient-independent, transporter-mediated mechanisms (

Roles for SWEETs in pollen nutrition 325
Pollen germination and tube growth initially rely on nutrient storage in the pollen grain. Pollen grains, 326 pollen tubes, and the anther tapetum are sink tissues that are symplasmically isolated, requiring an 327 unloading pathway through the apoplasmic space. In Arabidopsis, AtSWEET8 and AtSWEET13, also 328 known as RUPTURED POLLEN GRAIN (RPG) 1 and 2, respectively, were suggested to function in 329 the efflux of sugar in the tapetum and microspores for pollen cell wall synthesis ( infection. Since sweet2 mutants were susceptible to the oomycete, it has been predicted that 368 AtSWEET2 modulates sugar secretion to limit carbon loss to the rhizosphere. A detailed description of 369 the role of SWEETs is presented in summary of the role of SWEETs in symbiosis and microbiota-feeding is presented in Table 4 and  and performing a combined single-cell and spatial transcriptomics, will provide insight on host cell-487 pathogen interaction at the cell type and spatial resolution. Importantly, the development of genetically 488 encoded biosensors targeted to various cellular compartments will empower dissection of the 489 mechanisms for distribution and fluxes of different nutrients. We expect that these technologies will 490 allow us to better understand symbiosis establishment, plant-pathogen interaction, and enable us to 491 systematically engineer nutrient flux in plants to increase crop yield in the future.