LYSOPHOSPHATIDIC ACID ACYLTRANSFERASES 4 and 5 are involved in glycerolipid metabolism and nitrogen starvation response in Arabidopsis

Nitrogen (N) deficiency triggers an accumulation of a storage lipid triacylglycerol (TAG) in seed plants and algae. Whereas the metabolic pathway and regulatory mechanism to synthesize TAG from diacylglycerol are well known, enzymes involved in the supply of diacylglycerol remain elusive under N starvation. Lysophosphatidic acid acyltransferase (LPAT) catalyzes an important step of the de novo phospholipid biosynthesis pathway and thus has a strong flux control in the biosynthesis of phospholipids and TAG. Five LPAT isoforms are known in Arabidopsis; however, the functions of LPAT4 and LPAT5 remain elusive. Here, we show that LPAT4 and LPAT5 are functional endoplasmic-reticulum-localized LPATs. Seedlings of the double knockout mutant lpat4-1 lpat5-1 showed reduced content of phospholipids and TAG under normal growth condition. Under N starvation, lpat4-1 lpat5-1 seedlings showed severer growth defect than the wild-type in shoot. The phenotype was similar to dgat1-4, which affects a major TAG biosynthesis pathway and showed similarly reduced TAG content as the lpat4-1 lpat5-1. We suggest that LPAT4 and LPAT5 may redundantly function in endoplasmic-reticulum-localized de novo glycerolipid biosynthesis for phospholipids and TAG, which is important for the N starvation response in Arabidopsis.