Microtubules and the positioning of the vegetative nucleus and generative cell in the angiosperm pollen tube: A quantitative study

In germinating pollen of the liliaceous angiosperm Endymion nonscriptus, the vegetative nucleus (VN) leaves the grain first, followed by the generative cell (GC), a sequence maintained during the linear period of pollen-tube growth. The VN and GC undergo some random repositioning within the protoplast, but during the interval of extension preceding GC division they move slowly, but consistently, towards the apex. In a colchicine concentration eliminating microtubules (MTs) from the vegetative cell (VC) germination is not affected, but the sequence of departure of the VC and the GC from the grain is reversed in most instances, and the two bodies are more widely dispersed in the tube thereafter. We interpret the results as implying that the positioning of VN and GC in the VC is governed by two forces: (i) that resulting from the balancing stresses placed on their myosin-bearing surfaces in interaction with the two polarities of actin fibrils in the cytoplasm; and (ii) a second tension imposed by an endoplasmic MT system which links the bodies with microtubule organizing centres (MTOCs) at the cell membrane just proximal to the growing point. The nature of the interaction of the MT system with the VN and GC remains to be established, but it may be significant that the presence of a dynein-like molecule in the angiosperm pollen tube has recently been reported.