Germination of monocolpate angiosperm pollen: evolution of the actin cytoskeleton wall during hydration, activation and tube emergence

The monocolpate pollen grain of Narcissus pseudonarcissus L. has two preferred sites for tube emergence, one at each end of the colpus. While the cellulosic microfibrils of the inner layer of the intine are disposed circumferentially in the centre of the grain, the microfibrils in these terminal sites are shorter and randomly oriented. Soon after the beginning of hydration, inclusions of the vegetative cell begin movement, firstly in a rotatory manner, and then in a pattern focused on one or both germination sites, where the intine bulges as hydration progresses. These changes are associated with the evolution of the actin cytoskeleton. Actin is present in the unactivated grain in the form of fusiform bodies. During hydration these dissociate to form finer fibrils, initially randomly disposed. Then, correlated with the change of the pattern of movement in the vegetative cell, the actin fibril system becomes polarized towards the germination sites, where shorter fibrils accumulate. Callose, absent from the ungerminated grain, is deposited within the cellulosic wall in these locations, forming a shallow dome which eventually develops into an annulus subtending the inner callose lining of the emerging tube. The transition to cylindrical growth is associated firstly with the development of zonation in the cytoplasm of the vegetative cell, with the tip occupied by a population of wall precursor bodies (P-particles) and a dense aggregate of short actin fibrils; and then with the establishment of the 'inverse fountain' pattern of movement characteristic of the apical part of the extending tube.