SUC2 sucrose transporter is required for leaf apoplasmic sucrose levels. Consequences for phloem loading strategies

Summary * The SUC/SUT sucrose transporters belong to a family of active H+/sucrose symporters, with a role of SUC2 in active apoplasmic phloem loading to drive long-distance phloem transport of sucrose in Arabidopsis. However, the cooperation with the symplasmic pathway for phloem loading remains unclear. * In this study, we explored the consequences of reducing either apoplasmic or symplasmic pathways of phloem loading. We compared a series of lines with modified expression of SUC2 gene, and we analyzed the effects on plant growth, sugar accumulation in source and sink organs, phloem transport, and gene expression. * Our data revealed that a modified expression of SUC2 impacted apoplasmic sucrose levels in source leaves but did not impact phloem transport, as might be expected, while increasing foliar storage of carbohydrates. This response differed from lines in which symplasmic communications between phloem cells was disrupted by the over-expression of a plasmodesmata-associated protein, NHL26. * Altogether, our studies indicate an unexpected effect of SUC2 for apoplasmic sucrose levels in source leaves, together with SUC1, and suggest a feedback regulation on foliar storage. This data sheds new light on the interplay between symplasmic and apoplasmic pathways for sugar loading and the consequences on leaf water flows. Summary statementThe mechanisms that coordinate apoplasmic and symplasmic loading pathways, and their effects on foliar carbon storage, remain largely unexplored. Surprisingly, the sucrose transporter SUC2 plays a significant role in maintaining sucrose levels in the apoplasm, shedding light on how apoplasmic sugar levels and water flows can interact for phloem loading.