There are two aspects of the pollen tube physiology where the function of microtubules is still elusive: cell shaping and cell signaling. The former is known to depend on the organization of cortical microtubules, which in turn influences the pattern of cellulose deposition and then how a cell expands (Wasteneys 2004). The stability of cortical microtubules is a function of their orientation, because divergent microtubules within the array depolymerize in few minutes. The dynamics of microtubules in the cortex is likely dependent on the activity of different proteins that regulate their stability, the formation of bundles, the interaction with other cell structures, the relative sliding to each other and their interaction with the cellulose-synthase complex (Sedbrook 2004). All these concepts are partially adaptable to the pollen tube. In fact, pollen tube microtubules are aligned along the growth axis of the tube, while they are usually disposed transversally to the expansion axis in somatic cells. This divergent way to organize microtubules weakens the similarity of their role between the pollen tube and somatic cells and thus questions if microtubules control the shape of the pollen tube. Cell morphogenesis is a biological puzzle known to depend on two important events: the accumulation of extracellular material and the mechanical deformation of the cell surface (see Chapter IX).
The second critical aspect is cell signaling. The pollen tube is a cell that constantly grows and changes its direction following the interactions with other cells; however, we ignore how external signals influence the organization of microtubules and if (or how) alterations in their structure may affect the movement of organelles and the organization of the cell wall. The cascade of intracellular signals following the interaction of the pollen tube surface with extracellular hints has effects on many of the cellular activities in the pollen tube. It is likely that the microtubule cytoskeleton of the pollen tube responds to this signaling cascade and organizes accordingly. MAPs are good candidates to mediate such re-organization activity. In this context, proteins analogous to phospholipase D are candidates to convert the extracellular signals into signals that microtubules can recognize (Chapter VI). Activation of phospholipase D affects the organization of plant microtubules by releasing them from the plasma membrane and by partial depolymerization (Dhonuk-she et al. 2003). Hypothetically, during the in vivo growth of the pollen tube, external signals (after conversion into intracellular messengers that control the organization of microtubules) may regulate the transport rate of the generative cell and vegetative nucleus and the "concentration" of organelles in specific domains of the pollen tube. A nicely and intriguing though speculative hypothesis is that microtubules and actin filaments may regulate and reorganize after receiving external information. The reorganization of the cy-toskeleton may then influence the way organelles distribute in the pollen tube cytoplasm and, in turn, influence the growth direction and the polarity of the pollen tube. This hypothesis assumes that signaling molecules bind to mi-crotubules and mediate the activity of different environmental triggers (such as cold, osmotic stress and pathogens) (Wasteneys 2004). The consequence of this theory is that actin filaments are the operative force that guides organelle movement in the pollen tube, while microtubules may appropriately contribute to dispose organelles at their right place.
Acknowledgements We are grateful to Professor Bo Liu (University of California at Davis, USA) for carefully reading the manuscript and provide important suggestions and criticisms.
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