Short and Solid Culm/RFL/APO2 for culm development in rice.

The culm development of rice is characterized by elongation and medullary cavity (MC) formation, which are determined by node formation meristem and residual meristem, respectively. Although many factors have been shown to affect culm elongation, molecules involved in MC formation remained to be identified. In this study, we show that a point mutation in SHORT and SOLID CULM (SSC), the rice homologue of Arabidopsis LFY, resulted in plants with drastically reduced culm length and completely abolished MC formation. Analysis of transgenic plants with moderately enhanced SSC expression revealed significant decreases in plant height and MC size in contrast to slight changes in heading date, indicating that the culm developmental process is much more tightly monitored by the gene. Transcriptomic analysis revealed the differential expression of knotted-1 like homeobox (KNOX) protein genes and gibberellin (GA) metabolic genes in the ssc mutant background, and most of the genes contained well-conserved LFY-binding cis-elements that could be effectively recognized by SSC. Genetic analysis found that the reduced culm length of the mutant could be largely rescued by the GA-accumulating mutation eui, whereas MC formation remained unchanged in the double mutant plants. Taken together, our results suggest that SSC affects culm elongation mainly through maintaining GA homeostasis, while functions in MC formation by mediating residual meristem activity possibly via the KNOX pathway. The present study provides a potential strategy for improving the culm morphology and plant architecture in rice by manipulating SSC and/or its downstream components.