In the natural environment, plants are often bombarded by a combination of abiotic (such as drought, salt, heat or cold) and biotic (necrotrophic and biotrophic pathogens) stresses simultaneously. It is critical to understand how the various response pathways to these stresses interact with one another within the plants, and where the points of crosstalk occur which switch the responses from one pathway to another. Calcium sensors are often regarded as the first line of response to external stimuli to trigger downstream signaling. Abscisic acid (ABA) is a major phytohormone regulating stress responses, and it interacts with the jasmonic acid (JA) and salicylic acid (SA) signaling pathways to channel resources into mitigating the effects of abiotic stresses versus defending against pathogens. The signal transduction in these pathways are often carried out via GTP-binding proteins (G-proteins) which comprise of a large group of proteins that are varied in structures and functions. Deciphering the combined actions of these different signaling pathways in plants would greatly enhance the ability of breeders to develop food crops that can thrive in deteriorating environmental conditions under climate change, and that can maintain or even increase crop yield.

YchF is a subfamily of the Obg family in the TRAFAC class of P-loop GTPases. The wide distribution of YchF homologues in both eukarya and bacteria suggests that they are descendents of an ancient protein, yet their physiological roles remain unclear. Using the OsYchF1-OsGAP1 pair from rice as the prototype, we provide evidence for the regulation of GTPase/ATPase activities and RNA binding capacity of a plant YchF (OsYchF1) by its regulatory protein (OsGAP1). The effects of OsGAP1 on the subcellular localization/cycling and physiological functions of OsYchF1 are also discussed. The finding that OsYchF1 and OsGAP1 are involved in plant defense response might shed light on the functional roles of YchF homologues in plants. This work suggests that during evolution, an ancestral P-loop GTPase/ ATPase may acquire new regulation and function(s) by the evolution of a lineage-specific regulatory protein.

Summary• G-proteins (guanine nucleotide-binding proteins that usually exhibit GTPase activities) and related signal transduction processes play important roles in mediating plant defense responses; here, a rice (Oryza sativa) cDNA clone, OsGAP1, encoding a GTPase-activating protein (GAP) that also contains a protein kinase C conserved region 2 (C2) domain is reported.• An interacting G-protein partner for the OsGAP1 protein was identified by yeast two-hybrid library screening and confirmed by co-immunoprecipitation; the GTPaseactivation activity of OsGAP1 on this interacting G-protein was demonstrated using in vitro assays.• OsGAP1 was induced by wounding in rice and the presence of the R locus Xa14 enhances such induction.• Gain-of-function tests in transgenic rice and Arabidopsis thaliana showed that constitutive expression of OsGAP1 led to increased resistance to bacterial pathogens in both monocots and dicots.

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Receptor-like protein kinases (RLKs) containing an extracellular leucine-rich repeat (eLRR) domain, a transmembrane domain and a cytoplasmic kinase domain play important roles in plant disease resistance. Simple eLRR domain proteins structurally resembling the extracellular portion of the RLKs may also participate in signalling transduction and plant defence response. Yet the molecular mechanisms and subcellular localization in regulating plant disease resistance of these simple eLRR domain proteins are still largely unclear. We provided the first experimental evidence to demonstrate the subcellular localization and trafficking of a novel simple eLRR domain protein (OsLRR1) in the endosomal pathway, using both confocal and electron microscopy. Yeast two-hybrid and in vitro pull-down assays show that OsLRR1 interacts with the rice hypersensitive-induced response protein 1 (OsHIR1) which is localized on plasma membrane. The interaction between LRR1 and HIR1 homologs was shown to be highly conserved among different plant species, suggesting a close functional relationship between the two proteins. The function of OsLRR1 in plant defence response was examined by gain-of-function tests using transgenic Arabidopsis thaliana. The protective effects of OsLRR1 against bacterial pathogen infection were shown by the alleviating of disease symptoms, lowering of pathogen titres and higher expression of defence marker genes.