Plasma membrane‐localized SEM1 protein mediates sugar movement to sink rice tissues. (6th December 2021)

• Record Type: Journal Article
• Authors: Wang, Yanwei; Sun, Jing; Deng, Chen; Teng, Shouzhen; Chen, Guoxin; Chen, Zhenhua; Cui, Xuean; Brutnell, Thomas P.; Han, Xiao; Zhang, Zhiguo; Lu, Tiegang
• Is Part Of: Plant journal. Volume 109:Number 3(2022)
• Page Start: 523
• Page End: 540
• Publication Date: 2021-12-06
• Subjects: callose biosynthesis -- carbohydrate partitioning -- starch accumulation -- sugar trafficking -- photosynthesis; Plant molecular biology -- Periodicals; Plant cells and tissues -- Periodicals; Botany -- Periodicals; 580
• DOI: 10.1111/tpj.15573
• Languages: English
• ISSNs: 0960-7412

Abstract

SUMMARY: The translocation of photosynthate carbohydrates, such as sucrose, is critical for plant growth and crop yield. Previous studies have revealed that sugar transporters, plasmodesmata and sieve plates act as important controllers in sucrose loading into and unloading from phloem in the vascular system. However, other pivotal steps for the regulation of sucrose movement remain largely elusive. In this study, characterization of two starch excesses in mesophyll ( sem ) mutants and dye and sucrose export assays were performed to provide insights into the regulatory networks that drive source–sink relations in rice. Map‐based cloning identified two allelic mutations in a gene encoding a GLUCAN SYNTHASE‐LIKE ( GSL ) protein, thus indicating a role for SEM1 in callose biosynthesis. Subcellular localization in rice showed that SEM1 localized to the plasma membrane. In situ expression analysis and GUS staining showed that SEM1 was mainly expressed in vascular phloem cells. Reduced sucrose transport was found in the sem1‐1/1‐2 mutant, which led to excessive starch accumulation in source leaves and inhibited photosynthesis. Paraffin section and transmission electron microscopy experiments revealed that less‐developed vascular cells (VCs) in sem1‐1/1‐2 potentially disturbed sugar movement. Moreover, dye and sugar trafficking experiments revealed that aberrant VC development was the main reason for the pleiotropic phenotype of sem1‐1/1‐2 . In total, efficient sucrose loading into the phloem benefits from an optional number of VCs with a large vacuole that could act as a buffer holding tank for sucrose passing from the vascular bundle sheath. Significance Statement: The mutants identified (starch excess in mesophyll1) hyperaccumulated starch in leaf tissues despite strong sink demand. Cloning and characterization of the gene revealed that it likely plays a primary role in the formation of callose in vascular tissues of leaf and stem. We verified that export of sucrose from leaf tissues in mutants was impaired. Importantly, we initially hypothesized that phloem benefits from an optional number of VCs with a large vacuole that could act as a buffer holding tank for sucrose passing from the vascular bundle. This study could provide new genetic tools to probe source–sink relations in rice, and is thus of interest to photosynthesis research community.