Microtubule motor proteins and the organization of the pollen tube cytoplasm

Molecular motors are molecules that drive a wide range of activities (for example, organelle movement, chromosome segregation, and flagellar movement) in cells. Thus, they play essential roles in diverse cellular functions. Understanding their structures, mechanisms of action and different roles is therefore of great practical importance. The role of microtubules during pollen tube growth is presently not identified, nor are basic properties. We do not know, for example, where microtubules are organized, the extent of microtubule dynamics, and the polarity of microtubules in the pollen tube. Roles of microtubules and related motors in organelle trafficking are not clear. Regardless of scarce information, microtubule-based motors of both the kinesin and dynein families have been identified in the pollen tube. Most of these microtubule motors have also been found in association with membrane-bounded organelles, which suggest that these proteins could translocate organelles or vesicles along microtubules. The biochemical features of these proteins are typical of the motor protein class. Immunofluorescence microscopy of pollen tubes probed with antibodies that cross-react with microtubule motors indicate that these proteins are localized in different regions of the pollen tube; therefore, they could have different roles. Although a number of microtubule motors have been identified in the pollen tube, the role of these proteins during pollen tube germination and growth or organelle movement is not yet recognized, as tube elongation and organelle movement in the pollen tube depend mostly on actin filaments. In the effort to understand the specific role that microtubules and related motors have in the pollen tube, it is therefore necessary to identify the molecular machinery that interacts with microtubules. Furthermore, it is crucial to clearly establish the types of interaction between organelles and microtubules. This review summarizes the current state of the art on microtubule motors in the pollen tube, mainly surrounding the putative roles of microtubule motors in organelle movement and cytoplasmic organization. Some hypotheses and speculations are also presented.