Evolution of plastid FAX (fatty acid export) proteins and the plants' conquest of land - molecular and metabolic adaptation

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The colonization of land by descendants of freshwater algae was a watershed moment in earths’ history. Plants encountered many environmental challenges during the conquest of the land, such as desiccation, high intensity light, UV-radiation, lack of nutrient availability, mechanical damage, and pathogen infections. In consequence, a consecutive adaptation of metabolism and morphology took place during evolution of ancestors from early Streptophyte algae to todays seed plants. Among these versatile adjustments of land plant metabolism to terrestrial environments, changes in biosynthesis, distribution and homeostasis of lipids represent crucial steps to cope with stressful life. Fatty acid transport function of FAX (fatty acid export) proteins in the inner envelope membrane of chloroplasts has been shown to be crucial for cellular lipid homeostasis under normal and stress conditions such as cold acclimation in the model organisms Arabidopsis thaliana (seed plant) and Chlamydomonas reinhardtii (green micro alga). However, nothing is known about FAX proteins in the organisms relevant for first steps in plant terrestrialization. In order to close this gap and evaluate FAX protein evolution towards adjustment of metabolism to life on land, we aim to pinpoint molecular adaptation of plastid FAX proteins in bryophytes, i.e. in Physcomitrium patens and Marchantia polymorpha. With the emergence of the prominent apo-lipoprotein α-helix bundle at the N-terminus of plastid FAX that is conserved throughout evolution of monophyletic Phragmoplastophyta, we could already identify a crucial molecular element. Interestingly, the FAX apo domain is predicted to bridge the inner and outer envelope membrane of chloroplasts for fatty acid and lipid transport. Moreover, this domain is likely to mediate chloroplast-mitochondrium contacts for lipid exchange at phosphate starvation stress. Within the research project, we now aim to identify exact membrane topology, formation of hetero-oligomeric complexes as well as in planta function of apoFAX and FAX proteins in Physcomitrium patens and Marchantia polymorpha. In the spotlight is the role in adaptation of lipid metabolism in response to cold stress and mineral deficiency, namely phosphate and potassium starvation.