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  • The members in apple are

    2019-10-16

    The 8 members in apple are similar in number to the 7 in Arabidopsis and the 8 in rice, but fewer than the 10 found in mammalian species. GenBank numbers have also been reported for DGK homologues from tomato (AW035995), maize (AY106320), wheat (BT009326), a Populus cross (BU828590), grape (CB981130), and apricot (Prunus armeniaca; CB821694) (Gomez-Merino et al., 2004). This demonstrates their wide distribution in eukaryotes. Whereas we could divide MdDGKs into three distinct clusters, members of DGK families in mammals fall into five subtypes, I through V (Topham and Prescott, 1999). We noted that the genes in Cluster I, which comprises the most complex DGKs in plants, most closely resembled the DGKs within Type III, which are the most basic DGKs in mammalian sglt (Arisz et al., 2009). This seems to indicate that the mammalian DGK genes are more advanced and have diversified functions. In plants, genes in Cluster I contain the conserved catalytic DGK kinase domain, two protein kinase C conserved region 1 (C1-type) domains that are cysteine-rich and thought to be responsible for binding DAG, sglt plus a predicted trans-membrane helix that targets these DGKs to the membrane (Vaultier et al., 2008). Genes in Clusters II and III lack the latter two domains and have only the conserved kinase domain (Arisz et al., 2009). Some DGKs in Cluster III are distinguished by alternative splicing variants that contain a calmodulin-binding domain (CBD) (Snedden and Blumwald, 2000, Gomez-Merino et al., 2004). Our Cluster-I genes carried two copies of the DAG/PE-binding domain, the first flanked by an upstream basic region while an extCRD-like sequence followed the second. Each of those copies contained a C6/H2-type core with slightly different arrangements of amino acids. By comparison, the DGKs in Clusters II and III had simple organizations that lacked the domains described above but had only the DGKc and DGKa domains. Multiple alignments of all apple DGK genes revealed a low level of conservation, indicating that this family had a high degree of variation during the process of evolution. However, within each cluster, the members were highly conserved. Each cluster was characterized by exon/intron organizations and motifs that were very similar in number, size, and distribution, further demonstrating strong intra-cluster conservatism. Our phylogenetic tree showed that Clusters II and III formed a larger clade, which implied that those genes had originated via duplication from a common ancestor. Functional studies have primarily been conducted through transcriptome analysis and knock-out mutants. For example, AtDGK1 and AtDGK2 were found in the Arabidopsis cold-responsive transcriptome (Lee et al., 2005). Working with a knock-out mutant, van Wees et al. (2008) reported that AtDGK5 is required for conferring basal resistance against different virulent pathogens (van Wees et al., 2008). More than that, apple fruitlet abscission might be a result of DGK expression based on findings that a member of that family shared 62% identity with AtDGK5 in the seed transcriptome (Botton et al., 2011). We monitored patterns of expression for 6 MdDGK genes and found that all except MdDGK7 were highly expressed in the stems. In contrast, few if any transcripts of AtDGK7 and AtdGK2 have been reported as detectable in stems. Therefore, we suggest that expression in that tissue type is perhaps unique to woody plants. We also monitored stress-induced expression of MdDGKs. Along with MdDGK1, MdDGK2, MdDGK5 and MdDGK7, Clusters I and II contain two well-known stress-related genes from Arabidopsis, AtDGK2 and AtDGK7. However, no functional research has previously been reported for genes in Cluster III, two of which are MdDGK4 and MdDGK8. In fact, we found that both were responsive to drought and salt treatments. Therefore, our results with those two genes provide a foundation for future investigations of functioning by Cluster-III members.