Germination of Monocolpate Angiosperm Pollen: Evolution of the Actin Cytoskeleton and 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 endof the colpus. While the cellulosic microfibnls of the innerlayer of the intine are disposed circumferentially in the centreof the grain, the microfibrils in these terminal sites are shorterand randomly oriented Soon after the beginning of hydration,inclusions of the vegetative cell begin movement, firstly ina rotatory manner, and then in a pattern focused on one or bothgermination sites, where the intine bulges as hydration progresses.These changes are associated with the evolution of the actincytoskeleton. Actin is present in the unactivated grain in theform of fusiform bodies. During hydration these dissociate toform finer fibrils, initially randomly disposed. Then, correlatedwith the change of the pattern of movement in the vegetativecell, the actin fibril system becomes polarized towards thegermination sites, where shorter fibrils accumulate. Callose,absent from the ungerminated grain, is deposited within thecellulosic wall in these locations, forming a shallow dome whicheventually develops into an annulus subtending the inner calloselining of the emerging tube. The transition to cylindrical growthis associated firstly with the development of zonation in thecytoplasm of the vegetative cell, with the tip occupied by apopulation of wall precursor bodies (P-particles) and a denseaggregate of short actin fibrils; and then with the establishmentof the ‘inverse fountain’ pattern of movement characteristicof the apical part of the extending tube. Narcissus pseudonarcissus L, pollen activation, pollen germination, actin cytoskeleton, tip-growth system, pollen-tube wall development