The GPAT4/6/8 clade functions in Arabidopsis root suberization non-redundantly with the GPAT5/7 clade required for suberin lamellae

Abstract

SummaryLipid polymers such as cutin and suberin strengthen the diffusion barrier properties of the cell wall in specific cell types and are essential for water relations, mineral nutrition, and stress protection in plants. Land plant-specific glycerol-3-phosphate acyltransferases (GPATs) of different clades are central players in cutin and suberin monomer biosynthesis. Here, we show that the GPAT4/6/8 clade in Arabidopsis thaliana, which is known to mediate cutin formation, is also required for developmentally regulated root suberization, in addition to the established roles of GPAT5/7 in suberization. The GPAT5/7 clade is mainly required for abscisic acid-regulated suberization. In addition, the GPAT5/7 clade is crucial for the formation of the typical lamellated suberin ultrastructure observed by transmission electron microscopy, as distinct amorphous globular polyester structures were deposited in the apoplast of the gpat5 gpat7 double mutant, in contrast to the thinner but still lamellated suberin deposition in the gpat4 gpat6 gpat8 triple mutant. The intrinsic phosphatase activity of GPAT4, GPAT6, and GPAT8, which leads to monoacylglycerol biosynthesis, may be important for suberin biosynthesis. GPAT5/7 lack an active phosphatase domain. Notably, gpat5 gpat7 phenotypes were partially reverted by treatment with a phosphatase inhibitor or the expression of phosphatase-dead variants of GPAT4/6/8. Thus, GPATs that lack an active phosphatase domain, which are predicted to synthetize lysophosphatidic acids, might be crucial for the formation of the lamellated structure of suberin. GPATs with active and non-active phosphatase domains appear to have non-redundant functions and must cooperate to achieve the efficient biosynthesis of correctly structured suberin. Significance statementThe establishment of proper lamellated suberin in roots plays essential roles in regulating mineral nutrition and water relations in plants. The basis for the macromolecular arrangement determining the ultrastructure and properties of suberin lamellae is unknown. Here, we report that both the GPAT4/6/8 and GPAT5/7 clades of glycerol-3-phosphate acyltransferases (GPATs) contribute to suberin formation in Arabidopsis roots. In addition, we reveal that the GPAT5/7 clade is required for the formation of the lamellated suberin ultrastructure. Several lines of evidence suggest that the loss of phosphatase activity in GPATs might have played a crucial role in the formation of suberin lamellae during evolution.