Pollen Mutants

Pollen tube growth

Pollen hydration is usually tightly regulated and in vivo occurs only when desiccated pollen grains acquire water from the female, thus enabling pollen tube growth. In fact, in species exhibiting sporophytic self-incompatibility, the control of water uptake provides a mechanism for preventing self-pollinations. Despite this normally tight regulation of hydration, it is nonetheless true that pollen will also hydrate and grow a pollen tube if placed in a simple medium containing sucrose, boric acid and calcium.

In Arabidopsis thaliana, a self-compatible plant, once a pollen grain contacts a stigma papillar cell, the pollen coat, composed of lipids and proteins, softens into a gelatinous mixture and flows onto the papillar surface. This bridge between the pollen grain and papillar cell, sometimes termed a foot, establishes the route of water flow into the dessicated pollen grain. Pollen hydration is complete when the oblong pollen grain becomes round. As the pollen grain hydrates, Ca2+ flows into the pollen grain; this inflow, together with the shape change of the pollen grain, triggers cytoplasmic reorganization and the pollen grain is then activated for germination. This reorganization results in the formation of a cytoplasmic gradient of Ca2+. When fully hydrated, the pollen grain deposits callose, a ß 1,3-glucan, at one of the three pores, where the pollen tube will emerge. The pollen tube wall is an extension of the inner, or intine, pollen wall and is composed largely of callose. The pollen tube extends by tip growth through the pistil, in order to deliver the sperm to the embryo sac.