Sucrose transport and metabolism control carbon partitioning between stem and grain in rice

Abstract

SummaryThe source-sink relationship is key to the overall crop performance in terms of biomass and grain yield. Detailed understanding of the factors that determine source-sink dynamics, such as developmental and physiological features of source and sink tissues, photoassimilate partitioning from source to sink tissues, and metabolic status of the tissues, is imperative for optimization of crop performance. We investigated the differences in the source-sink relationship between a cultivated rice variety Oryza sativa cv. Nipponbare and a wild rice Oryza australiensis that show striking differences in biomass and grain yield traits. The wild rice, accumulating higher biomass, was not only photosynthetically efficient but also had efficient phloem loading and sucrose export from leaves. However, sucrose was not efficiently mobilized to panicle due to fewer number of smaller vascular bundles at the panicle base as well as lower expression of OsSWEETs and OsSUT1 in developing spikelets of O. australiensis compared to Nipponbare. High cleavage activity of Sucrose Synthase followed by higher expression of Cellulose Synthase genes in the wild rice stem efficiently utilized photosynthates for the synthesis of structural carbohydrates, resulting in high biomass. In contrast, the source-sink relationship favored high grain yield in Nipponbare via accumulation of transitory starch in the stem, due to higher expression of starch biosynthetic genes, to be mobilized to panicles with the onset of grain filling. Thus, vascular features and sucrose transporter functions together with functions of key sugar metabolic enzymes contributed to the variations in the source-sink relationship between the selected cultivated and wild rice.