Glycerol-3-Phosphate Acyltransferase 3 (OsGPAT3) is required for anther development and male fertility in rice

Journal of Experimental Botany, Vol. 68, No. 3 pp. 513–526, 2017 doi:10.1093/jxb/erw445 Advance Access publication 12 January 2017 This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details) RESEARCH PAPER Glycerol-3-Phosphate Acyltransferase 3 (OsGPAT3) is required for anther development and male fertility in rice Xiao Men1, Jianxin Shi1, Wanqi Liang1, Qianfei Zhang1, Gaibin Lian1, Sheng Quan1, Lu Zhu1, Zhijing Luo1, Mingjiao Chen1, and Dabing Zhang1,2,* 1 * Correspondence: Received 10 August 2016; Editorial decision 8 November 2016; Accepted 9 November 2016 Editor: Daphne Goring, University of Toronto Abstract Lipid molecules are key structural components of plant male reproductive organs, such as the anther and pollen. Although advances have been made in the understanding of acyl lipids in plant reproduction, the metabolic pathways of other lipid compounds, particularly glycerolipids, are not fully understood. Here we report that an endoplasmic reticulum-localized enzyme, Glycerol-3-Phosphate Acyltransferase 3 (OsGPAT3), plays an indispensable role in anther development and pollen formation in rice. OsGPAT3 is preferentially expressed in the tapetum and microspores of the anther. Compared with wild-type plants, the osgpat3 mutant displays smaller, pale yellow anthers with defective anther cuticle, degenerated pollen with defective exine, and abnormal tapetum development and degeneration. Anthers of the osgpat3 mutant have dramatic reductions of all aliphatic lipid contents. The defective cuticle and pollen phenotype coincide well with the down-regulation of sets of genes involved in lipid metabolism and regulation of anther development. Taking these findings together, this work reveals the indispensable role of a monocot-specific glycerol-3-phosphate acyltransferase in male reproduction in rice. Key words: Anther development, glycerol-3-phosphate acyltransferase, lipid metabolism, male sterility, microgametophyte, rice, tapetum. Introduction Male reproductive development in higher plants is a complicated biological process that includes the development of the anther and the generation of pollen (Liu and Qu, 2008; Ma, 2005; Sanders et al., 1999; Zhang et al., 2011; Zhang and Wilson, 2009). The developed anther wall has four somatic layers: the epidermis, the endothecium, the middle layer, and the tapetum (Goldberg et al., 1993). The innermost cell layer of the anther wall, the tapetum, which encompasses the meiotic cells (microsporocytes) at the center, plays a crucial role in regulating programmed anther development and microspore/pollen formation (Li et al., 2006; Parish and Li, 2010; Zhang and Yang, 2014). Tapetal cell differentiation and tapetum development are critical for the early events in male reproduction, including meiosis, while tapetal degeneration is vital for formation of viable pollen during late pollen development (Ma, 2005; Wilson and Zhang, 2009; Zhang and Liang, 2016; Zhang et al., 2011). Tapetal cells are characterized by the presence of abundant organelles and vigorous metabolic activities, secreting various monomers or precursors for the synthesis of the anther cuticle, pollen wall, and © The Author 2017. Published by Oxford University Press on behalf of the Society for Experimental Biology. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. State Key Laboratory of Hybrid Rice, Shanghai Jiao Tong University and University of Adelaide Joint Centre for Agriculture and Health, School of Life Sciences and Biotechnology, Shanghai Jiao Tong University, Shanghai 200240, China 2 School of Agriculture, Food and Wine, University of Adelaide, Urrbrae, South Australia 5064, Australia 514 | Men et al. of the abovementioned genes and transcription factors is involved in the metabolism of glycerolipids, an important cutin component in plants (Graça et al., 2002; Li-Beisson et al., 2013), let alone the genetic, biochemical, and molecular mechanisms underlying the involvement of glycerolipid metabolism in anther and pollen development. The glycerolipid triacylglycerol (TAG) and its derivatives are important storage and membrane lipids and indispensable components of biological polymers including cutin and suberin in plants (Pollard et al., 2008). TAG is generated by connecting fatty acids to a glycerol backbone (Coleman and Lee, 2004). Glycerol-3-phosphate acyltransferases (GPATs) catalyze the first step of TAG biosynthesis by acylating glycerol 3-phosphate at the sn-1 or sn-2 hydroxyl with an acyl donor, acyl-CoA or acyl-ACP, and generating lysophosphatidic acids (LPAs) that can act as signaling molecules in regulating cell growth (Moolenaar et al., 1997; Sheng et al., 2015; Takeuchi and Reue, 2009). Because GPAT displays the lowest specific activity toward a very broad group of substrates, it has been considered to be the rate-limiting enzyme (Wendel et al., 2009; Zheng and Zou, 2001). In animals, GPATs usually acylate glycerol-3-phosphate at the sn-1 position, and are required for membrane lipid synthesis and energy storage. In contrast, in land plants, most GPATs are sn-2 GPATs, which catalyze the reaction in which glycerol is an anchor point for the linear or cross link with fatty acids, playing important roles in the assembly of cutin or suberin in plants (Pollard et al., 2008; Yang et al., 2012). Arabidopsis has eight sn-2 GPATs with different functions (Chen et al., 2011b). GPAT4 and GPAT8 have high sequence similarity and are suggested to be functionally redundant duplicated genes. Neither the gpat4 nor the gpat8 single mutant showed any obvious cuticle defect, whereas the gpat4gpat8 double mutant exhibited a marked decrease in cutin content in leaves and stems (Li et al., 2007). GPAT6 is highly expressed in flowers (Zheng et al., 2003). Its mutant displayed defective nanoridges on petal surfaces and a significant reduction of cutin monomers in flowers (Li-Beisson et al., 2009). Further biochemical analyses demonstrated that GPAT4, GPAT6, and GPAT8 prefer C16:0 and C18:1 ω-oxidized substrates and have additional phosphatase activity, resulting in the conversion of sn-2 LPA to sn-2 MAG, which is also an important intermediate for polyester assembly (Yang et al., 2012). GPAT5 is required for the synthesis of suberin in seed coat and root, and the gpat5 mutant exhibited strong reduction of very long chain (C22–C24) fatty acid monomers and their derivatives (Beisson et al., 2007). GPAT7, which is phylogenetically most closely related to GPAT5, takes part in suberin synthesis in the wounding response (Yang et al., 2012). GPAT5 and GPAT7 accommodate a broad chain length range of both ω-oxidized and unsubstituted substrates, but they do not possess phosph (...truncated)