Abstract
BackgroundPlant cell walls are the main physical barrier encountered by pathogens colonizing plant tissues. Alteration of cell wall integrity (CWI) can activate specific defenses by impairing proteins involved in cell wall biosynthesis, degradation and remodeling, or cell wall damage due to biotic or abiotic stress. Polygalacturonase (PG) depolymerize pectin by hydrolysis, thereby altering pectin composition and structures and activating cell wall defense. Although many studies of CWI have been reported, the mechanism of how PGs regulate cell wall immune response is not well understood.ResultsNecrosis appeared in leaf tips at the tillering stage, finally resulting in 3–5 cm of dark brown necrotic tissue. ltn-212 showed obvious cell death and accumulation of H2O2 in leaf tips. The defense responses were activated in ltn-212 to resist bacterial blight pathogen of rice. Map based cloning revealed that a single base substitution (G-A) in the first intron caused incorrect splicing of OsPG1, resulting in a necrotic phenotype. OsPG1 is constitutively expressed in all organs, and the wild-type phenotype was restored in complementation individuals and knockout of wild-type lines resulted in necrosis as in ltn-212. Transmission electron microscopy showed that thicknesses of cell walls were significantly reduced and cell size and shape were significantly diminished in ltn-212.ConclusionThese results demonstrate that OsPG1 encodes a PG in response to the leaf tip necrosis phenotype of ltn-212. Loss-of-function mutation of ltn-212 destroyed CWI, resulting in spontaneous cell death and an auto-activated defense response including reactive oxygen species (ROS) burst and pathogenesis-related (PR) gene expression, as well as enhanced resistance to Xanthomonas oryzae pv. oryzae (Xoo). These findings promote our understanding of the CWI mediated defense response.
Highlights
Rice (Oryza sativa L.) is the world’s most important food crop for more than half of the world’s population, and a model species for genetic, cytological, and agricultural studies (Itoh et al 2005)
Under natural summer field conditions, the leaf tip of ltn-212 had a small necrosis region and dark brown necrotic spots were formed at the tillering stage (Additional file 1: Figure S1A-B)
Initial necrosis of ltn-212 only occurred at all leaf tips at the tillering stage and developed into 3–5 cm of dark brown necrotic tissue until ripening stage (Fig. 1a-c)
Summary
Rice (Oryza sativa L.) is the world’s most important food crop for more than half of the world’s population, and a model species for genetic, cytological, and agricultural studies (Itoh et al 2005). Understanding the molecular mechanisms of leaf development and identifying genes causing premature withering are of great interest to both plant biologists and plant breeders. The necrotic tips may be a specific regulation of the plant response to changes in the external environment or gene expression in vivo. The rice leaf tip necrosis 1 (ltn 1) mutant results in excessive accumulation of phosphate in shoots and causes plants to develop a leaf tip necrotic phenotype (Hu et al 2011). OsPT8 is involved in phosphate homeostasis in rice, and overexpressing OsPT8 resulted in excessive phosphate in shoots and chlorosis and necrosis in leaf tips under high phosphate (0.3 mM) supply (Jia et al 2011). Many studies of CWI have been reported, the mechanism of how PGs regulate cell wall immune response is not well understood
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