The Study Of MAP18Regulating The Direction Of Pollen Tube Growth In Arabidopsis

Pollen tube growth is a rapid and polarized process that occurs exclusively at the tube tip. The elongation of pollen tubes within the female reproductive tissues is crucial for delivering sperm cells to ovules for fertilization. In addition, the growth of pollen tubes is a model system for studying polarized cell expansion or tip growth.Previously report has demonstrated that Arabidopsis thaliana Microtubule-Associated Protein18(MAP18) belongs to a DREPP family and regulates cortical microtubule organization by destabilizing microtubules. Bioinformation analysis indicates that MAP18is expressed highly in pollen and pollen tubes. However, there is no report about whether MAP18is involved in pollen tube growth yet. The investigation was therefore carried out in this thesis.Observations showed no obvious differences in germination or pollen tube elongation rate between wild type and mapl8mutant, suggesting that MAP18might not function in these processes. However, mapl8pollen tubes displayed abnormal growth patterns, with swollen tips and bending growth in vitro. In vivo observations also indicated that both mapl8and MAP18OX pollen tubes displayed entangled, sporadic growth patterns in the transmitting tracts, sometimes failing to reach the ovules and resulted in reduced micropyle targeting. These observations demonstrated that MAP18is involved in the regulation of the direction of pollen tube growth in Arabidopsis.To demonstrate the mechanism how MAP18regulates the direction of pollen tube growth, the cytoskeleton of map18and MAP18OX pollen tubes was observed. Althought no obvious changes of the microtubule cytoskeleton was observed in map18pollen tubes, the organization and dynamics of F-actin in the apical and subapical regions were siginificantly altered in growing map18and MAP18OX pollen tubes. These results suggest that MAP18functions in F-actin organization and dynamics in the apical and subapical regions. Biochemical analysis indicated that MAP18bound to and severed actin filaments in a Ca2+-dependent manner in vitro. Expressing the point mutation proteins of MAP18in map18pollen tubes demonstrated that the F-actin-severing activity of MAP18is critical for appropriate growth patterns of pollen tubes.In addition, observations showed that MAP18localized to the plasma membrane in the flank region but not in the the apical and subapical regions of growing pollen tubes. However, MAP18localized to whole plasma membrane in non-growing pollen tubes, but disappeared from the apical plasma membrane when tube growth was restarted. These observations indicated that the subcellular localization of MAP18is important for the function of MAP18.These results revealed that MAP18functions as a key factor involved in controlling the direction of pollen tube growth by severing F-actin, and suggesting that regulations of the organization and dynamic of F-actin is important not only for the pollen tube growth but also for the control of the direction of pollen tube growth. This study demonstrates a new function of MAP18that involved in controlling the direction of pollen tube growth and provides insights into the mechanism that controls the direction of pollen tube growth based on the F-actin-severing activity of MAP18.