A novel mechanism underlying programmed cell death in plant defense signaling

During plant-microbe interactions, hypersensitive response (HR) cell death is intimately associated with plant disease resistance. In this study, rice lesion mimic mutant spotted leaf11 (spl11) was used to investigate the molecular mechanism underlying PCD in plant disease resistance. spl11 plants expressed constitutive cell death and showed enhanced, non-race specific resistance to both M. grisea and Xanthomonas oryza pv. oryza (Xoo) pathogens. Genome-wide gene expression profiling indicated that the spl11 mutation caused significant changes in rice transcriptome. Of the genes that were more than 3.5-fold induced in spl11, over 50 percent were either defense- or oxidative stress/cell death-related, indicating that cell death and defense signaling is activated by the spl11 mutation.; spl11 was identified from an ethyl methanesulfonate (EMS)-mutagenized IR68 population and was shown to be inherited in a recessive monogenic fashion. The Spl11 gene was isolated via a map-based cloning strategy. The isolation of the Spl11 gene was facilitated by the identification of three additional spl11 alleles from an IR64 mutant collection. The predicted SPL11 protein contains a U-box domain and an armadillo (ARM) repeat domain, which were demonstrated in yeast and mammalian systems to be involved in ubiquitination and protein-protein interactions, respectively. A single base substitution was detected in spl11, which results in a premature stop codon in the SPL11 protein. Expression analysis indicated that Spl11 is induced in both incompatible and compatible rice-blast interactions. In vitro ubiquitination assays indicated that the SPL11 possesses E3 ubiquitin ligase activity that is dependent on an intact U-box domain, suggesting the ubiquitination system plays a role in the control of plant cell death and defense.; Eight SPL11-interacting proteins (SPINS) were identified in the yeast two-hybrid screenings. Two of the SPINs were putative pre-mRNA processing-related proteins, suggesting a connection between alternative splicing and Spl11-mediated cell death and defense signaling. Seventy-seven and sixty-three U-box containing proteins were identified from rice and Arabidopsis genomes, respectively. Atspl11, a T-DNA insertion mutant of the Arabidopsis Spl11 ortholog displayed cell death and stunted growth phenotype analogous to that of rice spl11 mutant, suggesting that the function of the Spl11 gene might be conserved between monocots and dicots.