Sugar transport in root: the dark side for plant adaptation to the environment

Roots are a very crucial organ as they anchor the plant to the soil and provide nutrients and water required for its full development. Plants are autotrophic and the excess of photosynthates (sugar) produced in the aerial part are transported to the sinks to allow their growth. Indeed, C partitioning is one of the major determinant in plant development and crop productivity. Roots are a specific sink organ which growth depends exclusively on the fine-tuned import of sugar through the phloem. In Arabidopsis thaliana plants, sucrose transport requires the specific activity of two types of membrane transporters involved in phloem loading: AtSUCs and AtSWEETs. Moreover, we have identified specific root expressed AtSUC and AtSWEET genes in two different culture systems: hydropony (Araponics©, Durand et al, 2018) and rhizobox (Durand et al, 2016, Plant Physiol, 170: 1460-1479). To precise their role in sucrose transport, the expression of these genes was followed under different water regimes and compared with [U-¹⁴C]-Suc and ¹⁴CO₂ pulse-chase experiments on whole plants. A rather good correlation between the expression of these transporters and the transport of assimilated C to the root was found. In parallel, the 2 most expressed genes: AtSUC1 and AtSUC2 were localized at the gene and protein levels, by using plants expressing GUS protein under the control of their respective promoters, antibodies and KO mutants.

We also demonstrated that water deficit affected the root architecture by allowing an almost equivalent surface of soil to be explored with fewer lateral roots. Morevover, the bacterial diversity of the rhizobiome of rhizobox plants was characterized for the first time and compared to the one of plants grown in pots. Therefore, understanding the C source-sink relationships in roots could be a major goal for breeders.