Mechanisms Of XIAO' Function On Organ Size In Rice

One of the key basic questions in biology is how cells division and coordinate with plant morphogenesis, specially, how the cell cycle is integrated with plant development. This is an important question in plant development and equally central to human society due to its connected to yield. Rice is a staple food crop and has become a reference monocot plant for functional genomic research. The development of technical and resource platforms for rice functional genomic research will significantly accelerate our understanding of the genetic and molecular basis of organ morphogenesis. By mining the expression data of covering the entire life cycle of the rice plant and screening T-DNA insertion mutant library, a LRR-kinase termed XIAO involved in the control of organ size by contributing to the regulation of signaling and homeostasis of brassinosteroids and cell cycling in rice. The main results are as follows:1. We searched the rice gene expression database CREP (Collections of Rice Expression Profiling, http://crep.ncpgr.cn), which contains a large amount of microarray data covering the entire life cycle of the rice plant. It was shown that XIAO was expressed in most of the tissues. Young and developing tissues including immature leaf, root and panicle had notably higher expression levels than in mature tissues. Its expression was almost undetectable in the mature organs like flag leaf at heading stage.2. We searched the flanking sequence database RMD of our lab (http://rmd.ncpgr.cn), and found a T-DNA insertion in the single exon of LOC_Os04g48760in xiao mutant. The xiao mutants displayed reduced plant size, erect leaf and reduced setting rate. We found that the T-DNA insertion co-segregated with the phenotype in T1generation. The results of complementation experiments demonstrated that XIAO was responsible for this phenotype.3. For elucidating the cellular basis of the smaller organs, we compared the cell number and size between anther and leaf blade and the results show that reduced organ sizes was due to decreased cell numbers. The growth curve of leaf indicated that reduced cell division rate rather than the duration of cell division was the cause for the decrease of cell numbers of xiao mutant.4. Promoter fusion and in situ hybridization results suggested that XIAO was expressed in the mitotic tissues and function in the early stage of the organ development. Correlation coefficient analysis showed that a number of genes involved in cell cycling coexpressed with XIAO. In addition, the expression of CYCIaZm, CYCB2and MCM3were reduced in xiao mutant. 5. The xiao mutant displayed enhanced skotomorphogenesis and typical of BR related phenotype, physiological and molecular analysis show that response of xiao mutant to BR was tissue specific. GC-MS analysis showed that BR biosynthetic intermediates in both the early and late C-6oxidation pathway were decreased in xiao mutant, suggested that feed-back suppression system is activated.These results identified a regulator of BR signaling and homeostasis, suggested XIAO involving in cell cycle, revealed a bridge between the intrinsic developmental signals BR and the cell cycle machinery.