An LRR-only protein regulates abscisic acid-mediated abiotic stress responses during Arabidopsis seed germination

LRRop-1, induced by DOF6 transcription factor, negatively regulates abiotic stress responses during Arabidopsis seed germination. The lrrop-1 mutant has reduced ABA signaling, which is part of the underlying stress-remediation mechanism. The large family of leucine-rich repeat (LRR) proteins plays a role in plant immune responses. Most LRR proteins have multiple functional domains, but a subfamily is known to possess only the LRR domain. The roles of these LRR-only proteins in Arabidopsis remain largely uncharacterized. In the present study, we have identified 44 LRR-only proteins in Arabidopsis and phylogenetically classified them into nine sub-groups. We characterized the function of LRRop-1, belonging to sub-group V. LRRop-1 encodes a predominantly ER-localized LRR domain-containing protein that is highly expressed in seeds and rosette leaves. Promoter motif analysis revealed an enrichment in binding sites for several GA-responsive and stress-responsive transcription factors. The lrrop-1 mutant seeds showed enhanced seed germination on medium containing abscisic acid (ABA), paclobutrazol and NaCl compared to the wild type (WT), demonstrating higher abiotic stress tolerance. Also, the lrrop-1 mutant seeds have lower levels of endogenous ABA, but higher levels of gibberellic acid (GA) and jasmonic acid-Ile (JA-Ile) compared to the WT. Furthermore, lrrop-1 mutant seeds imbibed with ABA exhibited reduced expression of ABA-responsive genes compared to similarly treated WT seeds, suggesting suppressed ABA signaling events in the mutant. Furthermore, chromatin immunoprecipitation (ChIP) data showed that DNA BINDING1 ZINC FINGER6 (DOF6), a negative regulator of seed germination, could directly bind to the LRRop-1 promoter and up-regulate its expression. Thus, our results show that LRRop-1 regulates ABA-mediated abiotic stress responses during Arabidopsis seed germination.

Functional roles of the pepper leucine-rich repeat protein and its interactions with pathogenesis-related and hypersensitive-induced proteins in plant cell death and immunity

Pepper leucine-rich repeat protein (CaLRR1) interacts with defense response proteins to regulate plant cell death and immunity. This review highlights the current understanding of the molecular functions of CaLRR1 and its interactor proteins. Plant cell death and immune responses to microbial pathogens are controlled by complex and tightly regulated molecular signaling networks. Xanthomonas campestris pv. vesicatoria (Xcv)-inducible pepper (Capsicum annuum) leucine-rich repeat protein 1 (CaLRR1) serves as a molecular marker for plant cell death and immunity signaling. In this review, we discuss recent advances in elucidating the functional roles of CaLRR1 and its interacting plant proteins, and understanding how they are involved in the cell death and defense responses. CaLRR1 physically interacts with pepper pathogenesis-related proteins (CaPR10 and CaPR4b) and hypersensitive-induced reaction protein (CaHIR1) to regulate plant cell death and defense responses. CaLRR1 is produced in the cytoplasm and trafficked to the extracellular matrix. CaLRR1 binds to CaPR10 in the cytoplasm and CaPR4b and CaHIR1 at the plasma membrane. CaLRR1 synergistically accelerates CaPR10-triggered hypersensitive cell death, but negatively regulates CaPR4b- and CaHIR1-triggered cell death. CaHIR1 interacts with Xcv filamentous hemagglutinin (Fha1) to trigger disease-associated cell death. The subcellular localization and cellular function of these CaLRR1 interactors during plant cell death and defense responses were elucidated by Agrobacterium-mediated transient expression, virus-induced gene silencing, and transgenic overexpression studies. CaPR10, CaPR4b, and CaHIR1 positively regulate defense signaling mediated by salicylic acid and reactive oxygen species, thereby activating hypersensitive cell death and disease resistance. A comprehensive understanding of the molecular functions of CaLRR1 and its interacting protein partners in cell death and defense responses will provide valuable information for the molecular genetics of plant disease resistance, which could be exploited as a sustainable disease management strategy.