Functional Analysis Of Arabidopsis FAX1 (Fatty Acid Export 1) In Male Reproductive Development

Fatty acid is one of the basic components of organisms,it plays important roles in many aspects of plant growth and development.It is not only an important source of energy,providing energy for the metabolic process of the organism,but also a precursor of some signal molecules,participating in the regulation of the organism.In addition,fatty acid involves in the biosynthesis of seed glycerides and membrane lipids,providing precursors for the biosynthesis of plant cuticular lipids,such as wax and cutin,root and stem suberin,and pollen extine,rendering plants resistant to unfavorable growth and developmental environment.The de novo biosynthesis of fatty acids in plants occurs mainly in the plastids and is catalysized by fatty acid synthase complex to generate C16:0,C18:0 and C18:1 fatty acids.and the produced fatty acids are then exported to the endoplasmic reticulum for further modification and assembly.The transport of lipids is regulated by a large number of lipid transport proteins and related transcription factors,therefore,fatty acid transport proteins are crucial in maintaining lipid balance and plant growth and development in plants.Previous studies on lipid transport focused mainly on the cuticular lipids.For example,LTP(Lipid transport protein)and ABCG(ATP binding cassette from the proteins-G)transporters,two types of known lipid transport proteins localize mainly on the cell membrane or the outside the cell membrane,and involve in the export of fatty acids from the cells.A recent study revealed that Arabidopsis FAX1(Fatty acid export 1),a plastid inner envelope localized protein,exports de novo synthesized fatty acids out of plastid,and loss of function of FAX1 reduce both biomass and male fertility significantly,affecting the vegetative and the reproductive growth of plants.FAX1 mutant plants are obviously shorter than wild-type ones,producing unfertile siliques and abnormal pollens with defective exine.Although levels of several plastid-produced lipids decrease while those of several endoplasmatic reticulum(ER)-derived species increase,the underlying mechanism of FAX1 on male fertility remains obscure.In this study,we first performed morphological analysis on anther and pollen along the developmental processes,and results of pollen grain staining,reciprocal cross,scanning electron microscopy(SEM)and semi-thin section observation of anthers at different developmental stages revealed that FAX1 mutation delayed the tapetum degradation,impaired pollen wall formation,reduced pollen viability,and impaired pollination.Segregation analysis verified that the FAX1 mutation influenced the male development of the plant.In addition,GUS staining and q RT-PCR analyses found that FAX1 expression increased significantly along the early flower development with an expression peak at stage 12.Furthermore,chemical analysis and SEM observation indicated that FAX1 mutation affected the cuticle lipid composition and surface patterning of floral organs.We also performed bioinformatics anslysis on FAX1 homologous gene in rice,which indicated that function of FAX1 is conserved at least in rice and Arabidopsis.Altogether,our data suggest that FAX1 affects multiple aspects of male reproductive development,which provides an important theoretical basis and a reference as well for studies regarding to lipid metabolism,especially lipid transport,on plant reproductive development.