Abstract
The orosomucoids (ORM) are ER-resisdent polypeptides encoded by ORM and ORMDL (ORM-like) genes. In humans, ORMDL3 was reported as genetic risk factor associated to asthma. In yeast, ORM proteins act as negative regulators of sphingolipid synthesis. Sphingolipids are important molecules regulating several processes including stress responses and apoptosis. However, the function of ORM/ORMDL genes in plants has not yet been reported. Previously, we found that temperature sensitive genetic male sterility (TGMS) rice lines controlled by tms2 contain a deletion of about 70 kb in chromosome 7. We identified four genes expressed in panicles, including an ORMDL ortholog, as candidates for tms2. In this report, we quantified expression of the only two candidate genes normally expressed in anthers of wild type plants grown in controlled growth rooms for fertile and sterile conditions. We found that only the ORMDL gene (LOC_Os07g26940) showed differential expression under these conditions. To better understand the function of rice ORMDL genes, we generated RNAi transgenic rice plants suppressing either LOC_Os07g26940, or all three ORMDL genes present in rice. We found that the RNAi transgenic plants with low expression of either LOC_Os07g26940 alone or all three ORMDL genes were sterile, having abnormal pollen morphology and staining. In addition, we found that both sphingolipid metabolism and expression of genes involved in sphingolipid synthesis were perturbed in the tms2 mutant, analogous to the role of ORMs in yeast. Our results indicated that plant ORMDL proteins influence sphingolipid homeostasis, and deletion of this gene affected fertility resulting from abnormal pollen development.
Highlights
Orosomucoid (ORM) family proteins are ER proteins encoded by ORM and ORMDL (ORM-like) genes
ORMDL3 is involved in pro-inflammatory diseases by binding and inhibiting sarco-endoplasmic reticulum Ca2+ pump (SERCA), which reduce ER Ca2+ concentration and increase unfolded-protein response (UPR), a process believed to induce inflammation [6]
The temperature-sensitive genetic male sterility (TGMS) system provides a powerful tool for the production of hybrid rice, in turn leading to greater yields of this vital crop
Summary
Orosomucoid (ORM) family proteins are ER proteins encoded by ORM and ORMDL (ORM-like) genes These proteins are highly conserved from yeasts to plants to human. Genetic interaction profiles of orm2D deletion mutants showed inverse correlation with the interaction patterns of yeast mutants with reduced LCB1 and LCB2 expression. Over-expression of ORM1 or ORM2 resulted in reduced LCB levels, while cells with deleted ORM1/2 had highly elevated levels of LCBs [9]. Yeast cells with ORM1/2 deletions increased flux throughout the sphingolipid pathway, resulting in growth defects [9] though the precise metabolic impact of loss of ORM function on sphingolipid synthesis in yeast is currently ambiguous, since orm1D/orm2D mutants have been reported to have reduced [8] or elevated [9] levels of ceramides. Alteration in ORM gene expression or mutations to their phosphorylation sites perturbed sphingolipid metabolism, leading to the hypothesis that ORMs play a central role in lipid homeostasis [9,10]
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