Early-Response to Dehydration 6-Like 14 is a vacuolar sugar importer required for acclimation to fluctuating light

Light intensities in nature fluctuate; however, our understanding of how plants acclimate to changing photon fluxes remains limited. Various chloroplast-located proteins contribute to the ability to tolerate fluctuating light (FL). Here, we show that an Arabidopsis (Arabidopsis thaliana) protein outside chloroplasts is involved in acclimation to FL. Early-Response to Dehydration 6-Like 14 (ERDL14) is a tonoplast-located sugar/H+ antiporter able to pump glucose and sucrose into the vacuole. The ERDL14 promoter contains several elements that enable its activation under high light (HL) and FL conditions, in which standard light intensities (3 mins at 120 µE) are periodically interrupted by short high-light phases (1 min at 1.000 µE). erdl14 loss-of-function mutants do not show phenotypic peculiarities under HL, which coincides with a marked down-regulation of ERDL14 gene expression after 2 days at HL. In contrast, FL treatment impairs erdl14 mutant development, most likely caused by down-regulation of the abundance of proteins involved in primary photosynthetic processes, leading to reduced photosynthetic quantum yield. This decreased abundance of proteins involved in primary photosynthetic processes is connected to increased cytosolic sugar levels in erdl14, indicating a "high-sugar repression", which is prevented in the wild type by the pumping activity of ERDL14. Knock-out mutants lacking other vacuolar sugar importers, i.e. tst1 and tst2, are not as strongly impaired in their ability to cope with FL, indicating a specific function of ERDL14.