Characterization of the rice glup4 mutant suggests a role for the small GTPase Rab5 in the biosynthesis of carbon and nitrogen storage reserves in developing endosperm

Rice is unique among plants in that it accumulates major amounts of two types of seed storage proteins, prolamines and glutelins, which along with starch, serve as sources of carbon and nitrogen for the post-germinative seedling. Here, we investigate the role of the small GTPase Rab5 in the biosynthesis of storage proteins through characterization of the loss-of-function mutant glup4. In situ RT-PCR and fluorescence microscopy revealed the glup4 mutant is impaired in glutelin mRNA localization to the cortical endoplasmic reticulum in developing endosperm cells. Furthermore, microarray and two-dimensional difference in gel electrophoresis analysis of transcript and protein abundance, respectively, identified a number of genes whose expression is affected by the loss of Rab5, including starch biosynthetic enzymes. Our results indicate Rab5 is important for RNA and protein sorting in rice seed and supports previous observations that the biosynthesis of starch and storage proteins may be interrelated.