[Show abstract][Hide abstract] ABSTRACT: Chitin, an N-acetyl-D-glucosamine polymer, is a component of fungal cell walls and a microbe/pathogen-associated molecular pattern that elicits plant defense responses. As polymeric chitin is difficult to handle due to its insolubility in water, many studies on chitin-induced immune responses have used water-soluble low-molecular weight chitin instead. Thus, it is unclear if polymeric chitin can induce resistance. Here, we examined the elicitor activity of chitin nanofiber (CNF) of submicron thickness prepared from polymeric chitin. CNF showed a high dispersing ability in water and induced both reactive oxygen species (ROS) production and chitin-induced defense-related gene expression in Arabidopsis thaliana seedlings. The Arabidopsis chitin elicitor receptor kinase 1 (Atcerk1) mutant, which is impaired in chitin perception, also failed to respond to CNF. CNF exposure triggered ROS generation in suspension-cultured cells from Oryza sativa. Furthermore, pre-treatment of Arabidopsis leaves with CNF effectively reduced pathogen infection by both the fungus Alternaria brassicicola and the bacterium Pseudomonas syringae pv. tomato DC3000. These results demonstrate that CNF has elicitor activity and will help define the role of polymeric chitin in plant immune responses.
Preview · Article · Dec 2015 · Frontiers in Plant Science
[Show abstract][Hide abstract] ABSTRACT: OsPti1a (Pto-interacting protein 1a) has important roles in the regulation of immune responses in rice. Phosphorylation of a conserved threonine in OsPti1a is necessary to activate defense responses; however, the regulatory mechanism of OsPti1a-mediated immune responses is still obscure. Recently, we revealed that OsPti1a forms protein complex(es) at the plasma membrane and this localization is required for its function. Here, we show that membrane-localized OsPti1a was selectively phosphorylated. Additionally, phosphorylation was not required for the localization of OsPti1a at the membrane. These results suggest that OsPti1a protein is selectively regulated by its phosphorylation after OsPti1a localizes to the plasma membrane.
No preview · Article · Mar 2015 · Plant signaling & behavior
[Show abstract][Hide abstract] ABSTRACT: OsPti1a, an ortholog of tomato SlPti1, functions as a negative regulator of innate immunity in rice (Oryza sativa L.). In ospti1a mutants, the activation of immune responses including HR-like cell death is caused by the loss of OsPti1a protein; however, it is as yet unclear how OsPti1a suppresses immune responses. Here, we report that OsPti1a localizes to detergent-resistant membrane (DRM) fractions of the plasma membrane through lipid modification of the protein's N-terminus, which is highly conserved among Pti1 orthologs in several plant species. Importantly, mis-localization of OsPti1a after deletion of its N-terminus reduced its ability to complement the mutant phenotypes including HR-like cell death. Further, complex formation of OsPti1a depends on its N terminus-mediated membrane localization. LC-MS/MS analysis of OsPti1a complex-interacting proteins identified several defense-related proteins. Collectively, these findings indicate that appropriate complex formation by OsPti1a at the plasma membrane is required for negative regulation of plant immune responses in rice.
[Show abstract][Hide abstract] ABSTRACT: The AGC kinase OsOxi1, which has been isolated as an interactor with OsPti1a, positively regulates basal disease resistance
in rice. In eukaryotes, AGC kinase family proteins are regulated by 3-phosphoinositide-dependent protein kinase 1 (Pdk1).
In Arabidopsis, AtPdk1 directly interacts with phosphatidic acid, which functions as a second messenger in both biotic and
abiotic stress responses. However, the functions of Pdk1 are poorly understood in plants. We show here that OsPdk1 acts upstream
of the OsOxi1–OsPti1a signal cascade in disease resistance in rice. OsPdk1 interacts with OsOxi1 and phosphorylates the Ser283
residue of OsOxi1 in vitro. To investigate whether OsPdk1 is involved in immunity that is triggered by microbial-associated
molecular patterns, we analyzed the phosphorylation status of OsPdk1 in response to chitin elicitor. Like OsOxi1, OsPdk1 is
rapidly phosphorylated in response to chitin elicitor, suggesting that OsPdk1 participates in signal transduction through
pathogen recognition. The overexpression of OsPdk1 enhanced basal resistance against a blast fungus, Magnaporthe oryzae, and a bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). Taken together, these results suggest that OsPdk1 positively regulates basal disease resistance through the OsOxi1–OsPti1a
phosphorylation cascade in rice.
Full-text · Article · Nov 2010 · Plant and Cell Physiology
[Show abstract][Hide abstract] ABSTRACT: OsPti1a, a functional ortholog of tomato SlPti1, negatively regulates both basal resistance and R-gene-mediated resistance in rice. To investigate the molecular function of OsPti1a in defense responses, we searched for components interacting with OsPti1a using a yeast two-hybrid system. One of the interacting proteins is a Ser/Thr kinase that directly phosphorylates OsPti1a in vitro. This protein belongs to the AGC kinase family and is highly similar to AtOxi1, which is induced in response to a wide range of reactive oxygen species (ROS)-generating stimuli in Arabidopsis. Thus, it was designated OsOxi1. OsOxi1 was transiently phosphorylated in response to ROS and chitin elicitor. Both OsOxi1-overexpressing transgenic lines and the ospti1a mutant were highly sensitive to ROS treatment, indicating that OsOxi1 and OsPti1a are involved in ROS-mediated signaling in opposing ways. OsOxi1 is specifically expressed at infection sites where ROS are produced after inoculation with a blast fungus, Magnaporthe oryzae. Overexpression of OsOxi1 enhanced basal resistance to the blast fungus, indicating that OsOxi1 positively regulates disease resistance. OsOxi1 phosphorylates Thr-233 of OsPti1a and a point mutation of Thr-233 enhanced disease susceptibility to a bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo), suggesting that the phosphorylation of OsPti1a by OsOxi1 is essential for basal resistance to Xoo. Taken together, our data suggest that OsOxi1 positively regulates defense responses through the phosphorylation of OsPti1a, causing the release from an OsPti1a-dependent inhibition of the responses.
No preview · Article · Oct 2010 · Plant and Cell Physiology