In plant cells, fatty acid (FA) synthesis occurs in the plastid stroma and thus requires subsequent FA export for lipid assembly in the endoplasmic reticulum. In this context, the membrane-intrinsic protein FAX1 has been described to mediate FA-export across the plastid inner envelope (IE). In Arabidopsis, FAX1 function is crucial for pollen cell wall formation, male fertility, cellular lipid homeostasis and plant biomass. Based on conserved structural features and sequence motifs, we here define the plant FAX-protein family localized in plastids. Besides their membrane-intrinsic domain, the plastid-targeted FAX1-FAX3 contain distinct N-terminal stretches. Among them, the apolipoprotein-like α-helical bundle of FAX2 is the most prominent. Further, we could unequivocally localize FAX2 and FAX3 proteins together with FAX1 to the IE membrane of chloroplasts and develop a topology model for FAX1, FAX2, and FAX3. In yeast, all plastid FAX proteins - i.e. FAX1, FAX2, FAX3, FAX4 - can complement for FA-transport function. For FAX1 we show a new function in pollen tube growth as well as together with FAX3 in seed/embryo development and in rosette leaf growth. Since in comparison to fax1 single knockout mutants, fax1/fax3 double knockouts are seedling lethal and not able to develop mature rosette leaves, we conclude that FAX1 and FAX3 function together in vegetative leaf growth.
Plants are extremely plastic organisms. They continuously receive and integrate environmental information and adjust their growth and development to favor fitness and survival. When this integration of information affects subsequent life stages or the development of subsequent generations, it can be considered an environmental memory. Thus, plant memory is a relevant mechanism by which plants respond adaptively to different environments. If the cost of maintaining the response is offset by its benefits, it may influence evolutionary trajectories. As such, plant memory has a sophisticated underlying molecular mechanism with multiple components and layers. Nonetheless, when mathematical modelling is combined with the knowledge of ecological, physiological, and developmental effects as well as the molecular mechanisms as a tool for understanding plant memory, the combined potential becomes unfathomable for the management of plant communities in natural and agricultural ecosystems. In this review, we summarize recent advances in the understanding of plant memory, discuss the ecological requirements for its evolution, outline the multilayered molecular network and mechanisms required for accurate and fail-proof plant responses to variable environments, point out the direct involvement of the plant metabolism and discuss the tremendous potential of various types of models to further our understanding of the plant's environmental memory. Throughout, we emphasize the use of plant memory as a tool to unlock the secrets of the natural world.
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