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  • Thus RhoA mediated inhibition of DGK is a well characterized


    Thus RhoA-mediated inhibition of DGKθ is a well-characterized pathway, conserved from C. elegans to mammalian tissues, which enables G-protein coupled receptors to regulate the extent and kinetics of DAG-mediated signaling by fine tuning DGK enzymatic activity.
    Introduction Cellular membranes are composed of numerous lipids, with most having structural functions while a few have direct signal-transducing properties (Testerink and Munnik, 2011). In eukaryotes, typical signaling lipids are the phosphatidylinositol lipids (polyphosphoinositides; PPIs), certain lyso-phospholipids, diacylglycerol (DAG), and phosphatidic NBI 27914 hydrochloride synthesis (PA) (Munnik and Testerink, 2009, Munnik and Vermeer, 2010). Several families of enzymes participate in PA production, and phospholipase D (PLD) and phospholipase C (PLC)/diacylglycerol kinase (DGK) have major roles in the stimulus-induced generation of signaling PA (Wang et al., 2014). In the PLC/DGK pathway, diacylglycerol (DAG) is rapidly phosphorylated by DGK to generate PA (Testerink and Munnik, 2005). In plants, multiple DGKs have been grouped into three clusters based on gene architecture, evolutionary relationships, and sequence identity (Arisz et al., 2009). Although animal DGKs have been extensively studied and models for their function have been developed (Topham and Prescott, 2002), functional analysis of plant DGK genes is still fragmentary. In Arabidopsis thaliana, seven candidate genes (AtDGK1 through 7) encode putative DGK isoforms. Katagiri et al. (1996) first cloned a cDNA (AtDGK1) that putatively encodes a DGK from Arabidopsis. Furthermore, AtDGK1 is expressed in the roots, leaves, and shoots but not in the flowers and siliques (Snedden and Blumwald, 2000). The candidate AtDGK2 has also been cloned. Transcripts of AtDGK2 have been detected in all parts of the plant except the stems. Expression can be induced by chilling at 4°C, pointing to a role in cold-signal transduction (Gomez-Merino et al., 2004). It can also be transiently induced by wounding. Activity of AtDGK7 is affected by pH, detergents, and the R59022 inhibitor. In Arabidopsis, expression can be detected throughout the plant but is stronger in flowers and young seedlings. R59022 at 80μM inhibits root elongation, the formation of lateral roots, and plant growth, indicating that DGKs play an important role in those developmental processes (Gomez-Merino et al., 2005). Malus prunifolia (Willd.) Borkh. is an excellent rootstock widely used for apple grafting in China. It is a wild species of Malus that is naturalized to the desert mountain regions of northwest China. It also displays strong disease resistance and stress tolerance. The phylogenetic relationships among different cultivars and wild species of Malus have been researched by Velasco et al. (2010), and the results indicate that M. prunifolia is one of the wild species taxonomically closest to Malus domestica. To date, genomic analysis of the DGK family has been done only in Arabidopsis and rice. Far less information is available about this family in woody plant species such as apple (M. domestica), one of the most widely cultivated fruit trees. However, the recent release of draft genome sequences for the diploid variety ‘Golden Delicious’ provides a good opportunity for studying gene functions in Malus species (Velasco et al., 2010). Here, we performed a genome-wide identification and analysis of DGK family members in M. prunifolia. We also investigated expression patterns in various tissues and under different stresses. The objective was to gain further insight into the roles of DGKs in the developmental processes and stress responses by M. prunifolia. Our goal was to obtain knowledge that might constitute a foundation for future utilization of genetic engineering in rootstock production.
    Materials and methods
    Discussion Phosphatidic acid plays a pivotal role in plant reactions to environmental signals. Many environment cues can trigger a rapid PA response by activating the PLD pathway, the PLC/DGK pathway, or both. Moreover, PA derived from the DGK pathway can be distinguished from PLD-derived PA, based on its fatty acid composition and differential 32Pi-labeling characteristics (Arisz et al., 2009). It remains unclear which pathway is followed under certain stresses. Osmotic stress can lead to fast and transient formation of PA (Arisz et al., 2009). Under such circumstances, both pathways are generally activated, with the notable exceptions of tobacco pollen tubes and rice leaves, where salinity stress in fact inhibits PLD activity (Zonia and Munnik, 2004, Darwish et al., 2009). Because of increased focus on PA functioning, many PI-PLC, NPC, DGK, and PLD enzymes have now been characterized and specific physiological functions have been assigned to individual isoforms. However, research is still scarce about the roles of DGK, and our report is the first to describe the expression patterns and functioning of this gene family in fruit trees.