The caleosin RD20/CLO3 regulates lateral root development in response to abscisic acid and regulates flowering time in conjunction with the caleosin CLO7

The caleosins are encoded by multi-gene families in Arabidopsis thaliana and other plant species. This work investigates the role of two family members, RD20/CLO3 and CLO7, in flowering transition and in root development in response to ABA treatment. Gene expression of the caleosin RD20/CLO3 is induced by ABA in the root tissues and RD20/CLO3 has a negative affect on the total number of lateral roots as well as the length of the lateral roots in response to ABA treatment. The rd20/clo3 mutant has more and longer lateral roots in response to ABA treatment compared to the wild-type, showing that RD20/CLO3 plays a role in the ABA signaling pathway affecting this trait. In contrast, the caleosin CLO7 is not expressed in the roots and does not affect root architecture in response to ABA treatment. The disruption of both RD20/CLO3 and CLO7 together causes a dramatic early-flowering phenotype under long-day conditions, whereas single mutations in these genes do not affect flowering time under these conditions. Both yeast two-hybrid and bimolecular fluorescence complementation showed that both RD20/CLO3 and CLO7 interact with each other and can form homodimers and heterodimers. Taken together, these findings suggest that members of the caleosin gene family play both different and redundant roles in plant development.

The caleosin CLO7 and its role in the heterotrimeric G-protein signalling network

The investigation of the caleosin CLO7 in relation to heterotrimeric G-protein signalling in Arabidopsis showed that the gene plays a role in seed germination and embryo viability. The caleosin CLO7 belongs to a multi-gene family of calcium-binding proteins which are characterized by single EF-hand motifs. Other members of the caleosin gene family have been shown to affect transpiration and seed germination as well as play a role in both abiotic and biotic stress responses. The proteins are associated with lipid droplets/oil bodies and some members of the gene family have been shown to have peroxygenase activity. Members of the gene family have also been shown to interact with the α subunit of the heterotrimeric G protein complex. In this study, we further expand on the diversity of physiological responses in which members of this gene family play regulatory roles. Utilizing BiFC and Y2H protein-protein interaction assays, CLO7 is identified as an interactor of the heterotrimeric G protein α subunit, GPA1. The full-length CLO7 is shown to interact with both the wild-type GPA1 and its constitutively active form, GPA1^QL, at the plasma membrane. Point mutations to critical amino acids for calcium binding in the EF-hand of CLO7 indicate that the interaction with GPA1 is calcium-dependent and that the interaction with GPA1^QL is enhanced by calcium. Protein-protein interaction assays also show that CLO7 interacts with Pirin1, a member of the cupin gene superfamily and a known downstream effector of GPA1, and this interaction is calcium-dependent. The N-terminal portion of CLO7 is responsible for these interactions. GFP-tagged CLO7 protein localizes to the endoplasmic reticulum (ER) and to lipid bodies. Characterization of the clo7 mutant line has shown that CLO7 is implicated in the abscisic acid (ABA) and mannitol-mediated inhibition of seed germination, with the clo7 mutant displaying higher germination rates in response to osmotic stress and ABA hormone treatment. These results provide insight into the role of CLO7 in seed germination in response to abiotic stress as well as its interaction with GPA1 and Pirin1. CLO7 also plays a role in embryo viability with the clo7gpa1 double mutant displaying embryo lethality, and therefore the double mutant cannot be recovered.