Sucrose Unloading in Grains Modulates the Disparity in Grain Filling of Superior and Inferior Spikelets in Rice

The poor filling of inferior spikelets (IS) compared with superior spikelets (SS) constitutes a major constraint on rice yield potential, yet the underlying physiological mechanisms remain inadequately understood. In this study, field experiments were conducted to examine phloem unloading patterns in SS and IS. We combined microscopic observation of rachis vascular bundles, symplastic tracer (carboxyfluorescein) transport assays, gene expression of key sucrose-transport and starch-synthesis components, and protein expression profiling of sucrose transporters (SUTs). Results indicated that SS exhibited superior grain filling performance relative to IS, which corresponded to its more developed phloem structure, larger vascular bundle and phloem areas, and a higher density of plasmodesmata at the intercellular interfaces within the dorsal vascular bundles. Carboxyfluorescein (CF) fluorescence signals were more intense in the vascular bundles of SS, suggesting an enhanced capacity for symplastic transport. SS exhibited higher transcript levels of genes encoding sugars will eventually be exported transporters (SWEET), SUTs, and cell wall invertase (CWI). Correspondingly, the protein abundance of SUT and CWI, along with CWI activity, was elevated in SS. These findings collectively indicate that SS possesses an enhanced capacity for phloem unloading through apoplastic and symplastic pathways. RNA-Seq analysis revealed that, compared to IS, SS exhibited upregulated expression of most genes involved in starch and abscisic acid biosynthesis, while genes for ethylene synthesis were downregulated. These results underscore the critical roles of phloem-unloading and phytohormonal regulation in determining grain filling, thereby providing an integrative explanation for the differential filling patterns between SS and IS in rice.