Pollen-Pistil Interaction

Pollination places a pollen grain on a stigma, but landing there is not, by itself, enough to

guarantee fertilisation -- the pistil actively evaluates every grain of pollen it receives, and

only pollen recognised as compatible is permitted to proceed further. This whole sequence of

recognition and response, beginning the instant a pollen grain touches a stigma and continuing

through the growth of its pollen tube, is what is meant by pollen-pistil interaction.

The critical recognition step is carried out chiefly by the tissue of the stigma itself: proteins

present on the pollen grain's exine come into contact with, and are read by, proteins on the

stigma's own receptive surface. If the pollen is judged compatible -- meaning it belongs to the

correct species and is genetically acceptable to that particular pistil (crucially including

whether or not the pollen is 'self' in a self-incompatible species) -- the stigma supports its

further development: the pollen grain absorbs water and nutrients from the stigmatic surface,

swells, and germinates, putting out a slender pollen tube through one of its germ pores, the soft

points in the exine where sporopollenin is absent.

Once germinated, the pollen tube begins to grow, and it is guided as it does so, apparently by

chemical cues released by the tissue of the pistil (a phenomenon sometimes referred to broadly as

chemotropism), down through the transmitting tissue of the style. As the tube elongates, its

vegetative (tube) nucleus travels at the growing tip, ahead of the two non-motile male gametes,

which follow behind within the tube's cytoplasm; in species where the generative cell has not

already divided before pollen shed, this division into two male gametes takes place inside the

growing tube itself during this journey.

On reaching the base of the style, the pollen tube passes into the ovary and continues to grow

toward a particular ovule. The route by which it finally enters that ovule can vary, but by far the

commonest pathway in flowering plants is entry directly through the micropyle, the narrow opening

left in the ovule's integuments -- a route given the specific name porogamy. Having entered through

the micropyle, the pollen tube penetrates the nucellus and, guided in its final approach by the

filiform apparatus of a synergid cell, grows into and enters one of the two synergids of the

embryo sac. There, the tip of the pollen tube ruptures, releasing its two male gametes directly

into the cytoplasm of that synergid.

This precise, guided delivery -- from stigma, through the style, into the ovary, through the

micropyle, and finally into a synergid -- ensures that both male gametes are placed exactly where

they are needed, immediately adjacent to both the egg cell and the central cell of the embryo sac, …