Exploring the relationship between photosynthate dynamics, sugar transporters, and seed development in pigeonpea...

Plants synthesize carbohydrates in mature leaves that act as primary carbon sources for biosynthesis, osmotic regulation, and signalling. Sugar transporter gene families regulate carbon partitioning, phloem loading and unloading, and sink uptake, thereby influencing growth, and yield. In pigeonpea, efficient sugar allocation to developing embryo/seed is critical for determining seed number per pod (SNPP). Photosynthate dynamics were studied in five pigeonpea genotypes contrasting for SNPP using integrated biochemical, in-silico, gene expression analyses and FT-IR spectroscopy. Quantification of soluble sugars and starch across ovule to seed developmental stages, coupled with bioinformatic analysis, identified 8 SUTs, 3 MSTs, and 24 SWEETs. Structural and phylogenetic analyses revealed conserved domains, chromosomal distribution, and gene duplications events, with most transporters predicted to localize to the plasma membrane. Anthrone assays and HPLC profiling showed peak soluble sugar accumulation at open flower stage, followed by redistribution to seeds and pod walls. Genotypes with low seeds number accumulated higher sugars, reflecting lower sink demand while the reverse trend was observed in starch content. Spatio-temporal expression analysis revealed distinct regulatory roles of sugar transporters with CcSUT1, CcSUT3, and CcSUT7 were highly expressed during early reproductive stages, critical for ovule retention and seed set. CcSUT2 functioned as a negative regulator, potentially limiting sugar flux and contributing to low seed number. CcSWEET5, CcSWEET7, and CcSWEET20 showed higher expression in high-SNPP genotypes enhancing hexose transport into developing seeds and promoting seed number. This study identifies key sugar transporters regulating seed development and provides candidate genes for improving pigeonpea yield through molecular breeding.