Abstract:OsCEBiP, a chitin-binding protein, and OsCERK1, a receptor-like kinase, are plasma membrane (PM) proteins that form a receptor complex essential for fungal chitin-driven immune responses in rice. The signaling events immediately following chitin perception are unclear. Investigating the spatiotemporal regulation of the rice small GTPase OsRac1, we find that chitin induces rapid activation of OsRac1 at the PM. Searching for OsRac1 interactors, we identified OsRacGEF1 as a guanine nucleotide exchange factor for … Show more
“…In M. truncatula, overexpression of the N-terminal part of RopGEF2 results in short swollen root hairs, and RopGEF2 could interact with ROP10 in yeast cells, suggesting that RopGEF2 is an activator of ROP10 activity . Similarly, it was reported that the PM-associated chitin receptor CERK1 in rice (Oryza sativa) can phosphorylate RacGEF1 and that the phosphorylated RacGEF1 functions as an activator of Rac1, which represents an essential element of chitin-induced defense gene activation (Akamatsu et al, 2013). Therefore, we speculate that RopGEF2 activates ROP10 of M. truncatula in response to NFs in a similar manner to how RacGEF1 activates Rac1 in chitin-treated rice.…”
Rhizobia preferentially enter legume root hairs via infection threads, after which root hairs undergo tip swelling, branching, and curling. However, the mechanisms underlying such root hair deformation are poorly understood. Here, we showed that a type II small GTPase, ROP10, of Medicago truncatula is localized at the plasma membrane (PM) of root hair tips to regulate root hair tip growth. Overexpression of ROP10 and a constitutively active mutant (ROP10CA) generated depolarized growth of root hairs, whereas a dominant negative mutant (ROP10DN) inhibited root hair elongation. Inoculated with Sinorhizobium meliloti, the depolarized swollen and ballooning root hairs exhibited extensive root hair deformation and aberrant infection symptoms. Upon treatment with rhizobia-secreted nodulation factors (NFs), ROP10 was transiently upregulated in root hairs, and ROP10 fused to green fluorescent protein was ectopically localized at the PM of NF-induced outgrowths and curls around rhizobia. ROP10 interacted with the kinase domain of the NF receptor NFP in a GTP-dependent manner. Moreover, NF-induced expression of the early nodulin gene ENOD11 was enhanced by the overexpression of ROP10 and ROP10CA. These data suggest that NFs spatiotemporally regulate ROP10 localization and activity at the PM of root hair tips and that interactions between ROP10 and NF receptors are required for root hair deformation and continuous curling during rhizobial infection.
“…In M. truncatula, overexpression of the N-terminal part of RopGEF2 results in short swollen root hairs, and RopGEF2 could interact with ROP10 in yeast cells, suggesting that RopGEF2 is an activator of ROP10 activity . Similarly, it was reported that the PM-associated chitin receptor CERK1 in rice (Oryza sativa) can phosphorylate RacGEF1 and that the phosphorylated RacGEF1 functions as an activator of Rac1, which represents an essential element of chitin-induced defense gene activation (Akamatsu et al, 2013). Therefore, we speculate that RopGEF2 activates ROP10 of M. truncatula in response to NFs in a similar manner to how RacGEF1 activates Rac1 in chitin-treated rice.…”
Rhizobia preferentially enter legume root hairs via infection threads, after which root hairs undergo tip swelling, branching, and curling. However, the mechanisms underlying such root hair deformation are poorly understood. Here, we showed that a type II small GTPase, ROP10, of Medicago truncatula is localized at the plasma membrane (PM) of root hair tips to regulate root hair tip growth. Overexpression of ROP10 and a constitutively active mutant (ROP10CA) generated depolarized growth of root hairs, whereas a dominant negative mutant (ROP10DN) inhibited root hair elongation. Inoculated with Sinorhizobium meliloti, the depolarized swollen and ballooning root hairs exhibited extensive root hair deformation and aberrant infection symptoms. Upon treatment with rhizobia-secreted nodulation factors (NFs), ROP10 was transiently upregulated in root hairs, and ROP10 fused to green fluorescent protein was ectopically localized at the PM of NF-induced outgrowths and curls around rhizobia. ROP10 interacted with the kinase domain of the NF receptor NFP in a GTP-dependent manner. Moreover, NF-induced expression of the early nodulin gene ENOD11 was enhanced by the overexpression of ROP10 and ROP10CA. These data suggest that NFs spatiotemporally regulate ROP10 localization and activity at the PM of root hair tips and that interactions between ROP10 and NF receptors are required for root hair deformation and continuous curling during rhizobial infection.
“…OsRacGEF1, the GEF for OsRac1, is a part of the plant-specific ROP-nucleotide exchanger (PRONE)-type GEF family. The cytoplasmic domain of OsCERK1 interacts with the OsRacGEF1 and phosphorylates it directly (Akamatsu et al 2013). Therefore, another type chitin signalling pathway consisting of OsCERK1 – OsRacGEF1 – OsRac1 is also presented in rice.…”
Section: Resistance and Signalling In Plantsmentioning
confidence: 99%
“…Therefore, another type chitin signalling pathway consisting of OsCERK1 – OsRacGEF1 – OsRac1 is also presented in rice. An OsCERK1-mediated immunity branching at OsRLCK185 and OsRacGEF1 was demonstrated (Akamatsu et al 2013). 10.1080/21501203.2018.1473299-F0002Figure 2.Proposed working model of the signal transduction in rice.…”
Section: Resistance and Signalling In Plantsmentioning
Chitin is the second abundant polysaccharide in the world after cellulose. It is a vital structural component of the fungal cell wall but not for plants. In plants, fungi are recognised through the perception of conserved microbe-associated molecular patterns (MAMPs) to induce MAMP-triggered immunity (MTI). Chitin polymers and their modified form, chitosan, induce host defence responses in both monocotyledons and dicotyledons. The plants’ response to chitin, chitosan, and derived oligosaccharides depends on the acetylation degree of these compounds which indicates possible biocontrol regulation of plant immune system. There has also been a considerable amount of recent research aimed at elucidating the roles of chitin hydrolases in fungi and plants as chitinase production in plants is not considered solely as an antifungal resistance mechanism. We discuss the importance of chitin forms and chitinases in the plant–fungal interactions and their role in persistent and possible biocontrol.Abbreviations ET, ethylene; GAP, GTPase-activating protein; GEF, GDP/GTP exchange factor; JA, jasmonic acid; LysM, lysin motif; MAMP, microbe-associated molecular pattern; MTI, MAMP-triggered immunity; NBS, nucleotide-binding site; NBS-LRR, nucleotide-binding site leucine-rich repeats; PM, powdery mildew; PR, pathogenesis-related; RBOH, respiratory burst oxidase homolog; RLK, receptor-like kinase; RLP, receptor-like protein; SA, salicylic acid; TF, transcription factor.
“…Os-Rac1 plays a critical role in defense responses induced by various MAMPs, including chitin elicitor Kawano et al, 2014a). After chitin is recognized by Os-CEBiP, which localizes to the extracellular side of the PM, Os-Rac1 is converted from the GDP to the GTP form by the guanine nucleotide exchange factor Os-RacGEF1, which is phosphorylated by Os-CERK1, a coreceptor of chitin (Kaku et al, 2006;Shimizu et al, 2010;Akamatsu et al, 2013). In this study, we provided evidence that endogenous Os-Rac1 is transiently localized to PM microdomains at an early stage of the chitin response.…”
Section: Role Of Pm Microdomains In the Rac1-rbohb/h-mediated Mti Patmentioning
confidence: 99%
“…In addition, mammalian Rac1 is palmitoylated at C178, a modification that is essential for targeting to rafts and actin cytoskeleton remodeling (NavarroLérida et al, 2012), and present in not only rafts but also non-rafts, where Rac1 is predominantly inactivated (Moissoglu et al, 2014). Activation of Os-Rac1 occurs within 3 min after chitin treatment (Akamatsu et al, 2013), suggesting that GTP-bound Os-Rac1 is localized to PM microdomains, since Os-Rac1 had accumulated in the DRM fraction by 10 min of chitin treatment. We therefore propose that the localization of Os-Rac1 to PM microdomains, possibly through palmitoylation, is necessary for its function, similar to other small GTPases.…”
Section: Role Of Pm Microdomains In the Rac1-rbohb/h-mediated Mti Patmentioning
Numerous plant defense-related proteins are thought to congregate in plasma membrane microdomains, which consist mainly of sphingolipids and sterols. However, the extent to which microdomains contribute to defense responses in plants is unclear. To elucidate the relationship between microdomains and innate immunity in rice (Oryza sativa), we established lines in which the levels of sphingolipids containing 2-hydroxy fatty acids were decreased by knocking down two genes encoding fatty acid 2-hydroxylases (FAH1 and FAH2) and demonstrated that microdomains were less abundant in these lines. By testing these lines in a pathogen infection assay, we revealed that microdomains play an important role in the resistance to rice blast fungus infection. To illuminate the mechanism by which microdomains regulate immunity, we evaluated changes in protein composition, revealing that microdomains are required for the dynamics of the Rac/ROP small GTPase Rac1 and respiratory burst oxidase homologs (Rbohs) in response to chitin elicitor. Furthermore, FAHs are essential for the production of reactive oxygen species (ROS) after chitin treatment. Together with the observation that RbohB, a defense-related NADPH oxidase that interacts with Rac1, is localized in microdomains, our data indicate that microdomains are required for chitin-induced immunity through ROS signaling mediated by the Rac1-RbohB pathway.
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