Functional specialization within the vacuolar sorting receptor family: VSR1, VSR3 and VSR4 sort vacuolar storage cargo in seeds and vegetative tissues

Abstract

Two different gene families have been proposed to act as sorting receptors for vacuolar storage cargo in plants: the vacuolar sorting receptors (VSRs) and the receptor homology-transmembrane-RING H2 domain proteins (RMRs). However, functional data on these genes is scarce and the identity of the sorting receptor for storage proteins remains controversial. Through a genetic screen we have identified the mtv2 mutant, which is defective in vacuolar transport of the storage cargo VAC2 in shoot apices. Map-based cloning revealed that mtv2 is a loss of function allele of the VSR4 gene. We show that VSR1, VSR3 and VSR4, but not the remaining VSRs or RMRs, participate in vacuolar sorting of VAC2 in vegetative tissues, and 12S globulins and 2S albumins in seeds, an activity that is essential for seedling germination vigor. Finally, we demonstrate that the functional diversification in the VSR family results from divergent expression patterns and also from distinct sorting activities of the family members.