Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere

Abstract

In plants, the root is a typical sink organ that relies exclusively on the import of sugar from the aerial parts. Sucrose is delivered by the phloem to the most distant root tips and, en route to the tip, is used by the different root tissues for metabolism and storage. Besides, a certain portion of this carbon is exuded in the rhizosphere, supplied to beneficial microorganisms and diverted by parasitic microbes. The transport of sugars toward these numerous sinks either occurs symplastically through cell connections (plasmodesmata) or is apoplastically mediated through membrane transporters (MST, mononsaccharide tranporters, SUT/SUC, H+/sucrose transporters and SWEET, Sugar will eventually be exported transporters) that control monosaccharide and sucrose fluxes. Here, we review recent progresses on carbon partitioning within and outside roots, discussing membrane transporters involved in plant responses to biotic and abiotic factors.

Possible Roles of Rhizospheric and Endophytic Microbes to Provide a Safe and Affordable Means of Crop Biofortification

“…Sugars constitute a significant proportion of exudates, and are a major source of carbon for microbes [159]. Sweet (sugars-will-eventually-be-exported transporters)-mutant plants showed higher sugar exports from roots than wild-type plants [160]. Until now, no sugar transporters directly involved in exporting sugars into the rhizosphere have been identified.…”

AM-Induced Alteration in the Expression of Genes, Encoding Phosphorus Transporters and Enzymes of Carbohydrate Metabolism in Medicago lupulina

“…In contrast, the transcript level of MlSTP13 increased significantly in the roots of M. lupulina-mycorrhized plants in the latter stages ( Figure 4). Such results obtained for M. lupulina may indicate an intensive flux of hexoses at the FI stage from the arbuscules of R. irregularis to the cytosol of plant cells, or hexose transport from the cell wall space to the arbuscule-containing cells [29]. P i transfer associated with carbohydrate metabolism is localized mainly in peri-arbuscular space [62].…”

“…The mycelium of the AM fungi probably receives carbohydrates from the apoplast of the root cortex [5] as glucose [46], and less as sucrose [2]. The exact location and mechanism of carbon transfer from host plants to AM fungi is not yet fully discovered, but it is the focus of recent studies of plant-microbial symbiotic interactions [17,18,[27][28][29]33].…”

“…The assumption is that AM affects the activity of symbiosis specific plant genes encoding metabolic enzymes and sugar transporters [27][28][29]. For example, in the case of symbiotic sucrose synthase MtSucS1 knockdown, the development of arbuscules in M. truncatula was inhibited [30].…”

“…These include: detection and analysis of sink cell hexose transporters MtMSTs in Medicago truncatula [27,35], the sucrose transporters SlSUTs in Solanum lycopersicum during mycorrhization (e.g., low expression of SlSUT2 leads to higher mycorrhization) [29,36], StSUT1 in S. tuberosum (overexpression of the gene leads to increased mycorrhization) [29,37], as well as SUT4 [29] and genes of the SWEET family, StSWEET2c, StSWEET7a, StSWEET12a in S. tuberosum (which have specific localization in arbuscular-containing root cells) [29,38] and LjSWEET3 in Lotus japonicus (increased expression in AM) [29,39]. Expression of the genes from the MtSTP family was revealed in arbuscule-containing cells of M. truncatula [29]. Thus, AM induces alterations in the representation of sugar transporters from different gene families; however, they are not well documented [40].…”