A Stress-Responsive Caleosin-Like Protein, AtCLO4, Acts as a Negative Regulator of ABA Responses in Arabidopsis

Abstract

Caleosins or related sequences have been found in a wide range of higher plants. In Arabidopsis, seed-specific caleosins are viewed as oil-body (OB)-associated proteins that possess Ca(2+)-dependent peroxygenase activity and are involved in processes of lipid degradation. Recent experimental evidence suggests that one of the Arabidopsis non-seed caleosins, AtCLO3, is involved in controlling stomatal aperture during the drought response; the roles of the other caleosin-like proteins in Arabidopsis remain largely uncharacterized. We have demonstrated that a novel stress-responsive and OB-associated Ca(2+)-binding caleosin-like protein, AtCLO4, is expressed in non-seed tissues of Arabidopsis, including guard cells, and down-regulated following exposure to exogenous ABA and salt stress. At the seed germination stage, a loss-of-function mutant (atclo4) was hypersensitive to ABA, salt and mannitol stresses, whereas AtCLO4-overexpressing (Ox) lines were more hyposensitive to those stresses than the wild type. In adult stage, atclo4 mutant and AtCLO4-Ox plants showed enhanced and decreased drought tolerance, respectively. Following exposure to exogenous ABA, the expression of key ABA-dependent regulatory genes, such as ABF3 and ABF4, was up-regulated in the atclo4 mutant, while it was down-regulated in AtCLO4-Ox lines. Based on these results, we propose that the OB-associated Ca(2+)-binding AtCLO4 protein acts as a negative regulator of ABA responses in Arabidopsis.

Leaf Oil Body Functions as a Subcellular Factory for the Production of a Phytoalexin in Arabidopsis

“…Arabidopsis has eight caleosin homologs, including CLO3 (Hanano et al, 2006;Partridge and Murphy, 2009). In addition, one of the closely related homologs, CLO4, is localized at oil bodies in Arabidopsis leaves (Aubert et al, 2010;Kim et al, 2011). Hence, the other homologs might compensate for the lack of CLO3, resulting in no reduction of the 2-HOT content in the clo3 mutant.…”

“…Seed caleosin has been shown to possess peroxygenase activity (Hanano et al, 2006). Recent studies show that caleosin homologs are expressed in vegetative tissues (Aubert et al, 2010;Kim et al, 2011). However, the other proteins on leaf oil bodies are not known.…”

A non‐canonical caleosin from Arabidopsis efficiently epoxidizes physiological unsaturated fatty acids with complete stereoselectivity

“…AtClO5 (At1g23240) was detected in pollen coat and according to data from the public microarray database its gene is exclusively expressed in flowers, suggesting a role for this caleosin in flowering [35]. In sharp contrast, AtClO4, though present in flowers, is also expressed in roots, leaves and stems, suggesting that AtClO4 might be functionally active not only in reproductive systems but also in germination and in vegetative processes [36]. That class II caleosins are peroxygenases, or even enzymes, remains, however, to be demonstrated.…”

The Reductase Activity of the Arabidopsis Caleosin RESPONSIVE TO DESSICATION20 Mediates Gibberellin-Dependent Flowering Time, Abscisic Acid Sensitivity, and Tolerance to Oxidative Stress

“…Caleosins have now been endowed with new roles since they have also been detected in vegetative tissues, where they are associated with the endoplasmic reticulum (Hernandez-Pinzon et al, 2001), the vacuole (Carter et al, 2004), and the envelope of chloroplasts (Partridge and Murphy, 2009). In Arabidopsis (Arabidopsis thaliana), Arabidopsis thaliana Seed1 (ATS1; At4g26740; Nuccio and Thomas, 1999) appears to participate in the degradation of lipid reserves in seeds (Poxleitner et al, 2006), while the non-seed-specific Caleosin4 (CLO4; At1g70670; Naested et al, 2000) and RESPONSIVE TO DESSICATION20 (RD20; At2g33380; Yamaguchi-Shinozaki et al, 1992) are negative and positive regulators of abscisic acid (ABA) responses during germination and dehydration processes, respectively (Aubert et al, 2010;Kim et al, 2011). However, the molecular mechanisms by which distinct caleosins exert their functions remained unknown.…”

“…However, the molecular mechanisms by which distinct caleosins exert their functions remained unknown. The recent identification of ATS1 and CLO4 as peroxygenases (Hanano et al, 2006;Blée et al, 2012) has prompted the suggestion that caleosins might fulfill their physiological functions via their enzymatic production of oxylipins (Partridge and Murphy, 2009;Aubert et al, 2010;Kim et al, 2011). Among the primary oxylipins formed in vitro by peroxygenases are the unsaturated fatty acid hydroxides (FAOHs; Fig.…”