Co-evolution and molecular adaptation of the MLO-EXO70 functional module in the course of plant terrestrialization

点击翻译按钮获取中文摘要

MLO proteins are integral membrane proteins that operate as calcium channels and that are specific to photosynthetic eukaryotes. In land plants, they are typically present in multiple sequence-diversified isoforms per species and contribute to essential physiological processes such as root thigmomorphogenesis and gravitropism, fertilization, as well as pathogen defense. Exocyst is an evolutionarily conserved multisubunit protein complex found in all eukaryotes. Different from other taxa, in land plants the EXO70 exocyst subunit is present in multiple sequence-diversified isoforms. We recently discovered that Arabidopsis thaliana MLOs and EXO70s form a functional module that involves physical association of these proteins and that likely governs focal secretion, e.g. of cell wall-related cargo. We hypothesize that key features of the MLO-EXO70 functional module evolved in the course of plant terrestrialization. We, therefore, propose analyzing the co-evolution and molecular adaptation of the MLO-EXO70 module in Chara braunii (Charophyceae) and Mesotaenium endlicherianum (Zygnematophyceae), which as an extant representatives of the so-called ZCC grade/clade mark the organismal transition from water to land. The C. braunii and M. endlicherianum genomes code for MLO-like proteins (seven and three, respectively) that in part markedly differ from their canonical land plant counterparts regarding size and membrane topology. Further, these genomes encode two prototypical EXO70 subunits each. Using this set of proteins derived from sister species of land plants, we will assess the recently discovered calcium channel activity of MLOs, their well-established ability to associate with the cytoplasmic calcium sensor calmodulin, and their isoform-preferential interaction with specific EXO70 family members. We further intend to explore the expression patterns of MLO and EXO70 genes in the various parts of the complex C. braunii thallus in different conditions and to examine the natural genetic variation of these genes within the species C. braunii and, more broadly, within the Charophyceae. Together, these studies will reveal a global picture regarding the presumed co-evolution and molecular adaptation of the MLO-EXO70 functional module in the course of transition of life from water to land.