A SUBTILISIN-LIKE SERINE PROTEASE1 (OsSUBSrP1), plays an important role in the wax and cutin pathway, is essential for panicle development in rice

Panicle degeneration is a severe physiological defect and causes reduction in grain yield. In this study, we characterized and presented the functional analysis of our previously reported mutant apa1331 (apical panicle abortion1331) that showed apical spikelet degeneration. The anthers from the apical spikelets of apa1331 were degenerated, pollen-less and showed lack of cuticle formation. Transverse sections showed normal meiosis till stage 5-6, however, defects in post-meiotic microspore development were found at stage 8-9 in apa1331. Measurement of wax and cutin analysis showed a significant reduction in anthers of apa1331 compared to Wildtype (WT). Quantification of H2O2 and MDA has indicated the excessive ROS (reactive oxygen species) in apa1331. Trypan blue staining, and TUNEL assay revealed cell death and excessive DNA fragmentation in apa1331. Map-based cloning and Mutmap analysis identified a candidate gene (LOC_Os04g40720) that is a SUBTILISIN-LIKE SERINE PROTEASE (OsSUBSrP1) which harbored an SNP (A>G) in apa1331. CRISPR-mediated knock-out lines of OsSUBSrP1 displayed spikelet degeneration comparable to apa1331. Global gene expression analysis revealed a significant downregulation of wax and cutin biosynthesis genes e.g., OsWDA1, OsMS2 and OsCER4 in apa1331. Our study reports the novel role of SUBSrP1 in ROS-mediated cell death in panicle development.

Role of brassinosteroids in rice spikelet differentiation and degeneration under soil-drying during panicle development

Background Brassinosteroids (BRs) are a new group of plant hormones and play important roles in plant growth and development. However, little information is available if BRs could regulate spikelet development in rice (Oryza sativa L.) especially under soil-drying conditions. This study investigated whether and how BRs mediate the effect of soil-drying on spikelet differentiation and degeneration in rice. A rice cultivar was field-grown and exposed to three soil moisture treatments during panicle development, that is, well-watered (WW), moderate soil-drying (MD) and severe soil-drying (SD). Results Compared with the WW treatment, the MD treatment enhanced BRs biosynthesis in young panicles, increased spikelet differentiation and reduced spikelet degeneration. The SD treatment had the opposite effects. Changes in expression levels of key rice inflorescence development genes (OsAPO2 and OsTAW1), ascorbic acid (AsA) content, and activities of enzymes involved AsA synthesis and recycle, and amount of nonstructural carbohydrates (NSC) in young panicles were consistent with those in BRs levels, whereas hydrogen peroxide (H2O2) content showed opposite trend. Knockdown of the BRs synthesis gene OsD11 or application of a BRs biosynthesis inhibitor to young panicles markedly decreased OsAPO2 and OsTAW1 expression levels, BRs and AsA contents, activities of enzymes involved AsA synthesis and recycle, NSC amount in rice panicles and spikelet differentiation but increased the H2O2 content and spikelet degeneration compared to the control (the wide type or application of water). The opposite effects were observed when exogenous BRs were applied. Conclusions The results suggest that BRs mediate the effect of soil-drying on spikelet differentiation and degeneration, and elevated BRs levels in rice panicles promote spikelet development under MD by enhancing inflorescence meristem activity, AsA recycle and NSC partitioning to the growing panicles.