Phospholipase C in abiotic stress-triggered lipid signaling in plants

Abstract

The plant cell membrane plays a crucial role in abiotic stress tolerance by acting as a physical barrier, separating the internal cellular milieu from the external surroundings. Upon perception of stimulus at the cell membrane an array of steps, such as generation of secondary messengers, activation of effector protein and modifcation of the cellular metabolism, takes place (Das et al., 2017). The membrane often undergoes a remodeling process in which the membrane lipid composition changes due to the action of various regulatory membrane proteins, to adapt to the changing environmental conditions. Recent advances in plant sciences have shown that lipids regulate various cellular processes, e.g. lipid remodeling, stress tolerance, hormonal response, etc. (Heilmann, tory lipids that are involved in membrane restructuring during stress in plants (Das et al., 2017). Phospholipids are crucial for structure development of the plant cell membrane and the synthesis of secondary messengers. Phospholipases are the enzymes that act on the phospholipids to generate secondary messengers in plants. Increasing research has shown that phospholipases are involved in a wide variety of processes in plants like growth, development and regulating abiotic and biotic stress tolerance (Heilmann and Heilmann., 2015, Singh et al., 2015). Among different classes of plant phospholipases (PLA, PLC and PLD), phospholipases C (PLCs) are the important enzymes which catalyze the hydrolysis of the phospholipids. On the basis of substrate specifcity and cellular functions, PLCs in plants have been categorized as phosphoinositide phospholipase C (PI-PLC) and phosphatidylcholine-PLC (PC-PLC). PI-PLC hydrolyzes the phosphoinositides, particularly PI (4,5) P2, while PC-PLC prefers phosphatidylcholine (PC) but can also hydrolyze other lipids including phosphatidylethanolamine (PE) and phosphatidylserine (PS); therefore PC-PLC is also known as non-specifc PLC (NPC) (Aloulou et al., 2018). PI-PLC hydrolyzes the glycerophosphate ester linkage on the glycerol side of phospholipid to produce diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). In animals, DAG remains bound to the membrane and activates protein kinase C (PKC) while IP3 moves to the cytoplasm where it binds to the ligand gated calcium channels and releases calcium from the intracellular reserves (Vossen et al., 2010). However, plants lack IP3 receptors and PKC, and the role of secondary messengers is played by the phosphorylated products of DAG and IP3 i.e. phosphatidic acid (PA), diacylglycerol pyrophosphate (DGPP) and hexakisphosphate (IP6). In addition to PI-PLC, DAG can also be produced by the hydrolysis activity of the NPCs that hydrolyze the phosphatidylcholine and phosphatidylethanolamine. The DAG produced by NPCs also mediates lipid signaling as a secondary messenger; thus the importance of the roles of NPCs is demonstrated in plant metabolism, plant growth and development and hormone and abiotic stress signaling (Hong et al., 2016). PI-PLCs have been reported in a wide array of plant species including nine members of Arabidopsis thaliana (Zhang et al., 2012), four of Oryza sativa (rice) (Singh et al., 2013), six of Solanum lycopersicon (tomato) (Vossen et al., 2010), three of Solanum tuberosum (potato) (Kopka et al., 1998), 12 of Glycine max (soybean) (Wang et al., 2015), one of Pisum sativum (pea) (Liu et al., 2006), three of Vigna radiata (mung bean) (Kim et al., 2004), one of Avena sativa (oat) (Huang and Crain, 2009), two of lily (Pan et al., 2005) and two of Physcomitrella (Repp et al., 2004). Also, NPCs have been reported in many plants. In-depth analysis revealed that six NPCs are encoded in the Arabidopsis genome (Hong et al., 2016), nine NPCs in soybean (Huang et al., 2010), fve NPCs in rice (Singh et al., 2013) and 11 NPCs in Gossypium hirsutum (cotton) (Song et al., 2017). In this chapter, we present an overview of PLCs in plants and discuss their various important aspects, such as domain structure, regulation of activity and signaling mechanism, along with the recent updates on the role of PLCs in abiotic stress signaling and responses in plants.