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.