How Does Pollen Reach the Ovule? It Grows a Tube and Finds the Way

Pollen delivers the male cells and it lands on the outside of the flower, while the ovule waiting to be fertilised sits enclosed within tissue at the base of the style, frequently several centimetres away. Nothing can swim there, since flowering plants dispensed with the water that earlier plant groups required for fertilisation, so the pollen grain solves the problem by growing. It germinates on a receptive surface, produces a tube that extends down between the cells of the style, and carries the male cells along inside it, delivering them directly into the ovule at the end. That adaptation, removing the need for water, is a substantial part of why flowering plants succeeded so comprehensively.

The tube extends in an unusual way that has made it a standard subject of study:

• •Growth occurs only at the extreme tip rather than along the length
• •New wall material is delivered to that tip in vesicles and fused into place
• •Rates reach several micrometres per second, which is exceptionally fast for a plant
• •Internal pressure drives the extension against the surrounding tissue
• •The tip carries a high concentration of calcium that oscillates as it grows
• •Growth frequently pulses rather than proceeding smoothly

Navigation over a considerable distance through solid tissue is guided rather than random, and identifying the signals took a long time. The tissue of the style supplies a gradient of nutrients and signalling molecules that orient growth downwards. Close to the target, cells within the ovule itself release small proteins that attract the tip directly, and those attractants were identified in the 2000s after decades of searching, with experiments showing tubes turning towards a source of the isolated substance. The signalling is species-specific in many plants, which contributes to keeping species distinct, since a tube from the wrong species is not attracted and frequently is not supported by the tissue at all.

A stigma receiving pollen typically receives a great deal of it from several sources, and what happens next is competitive rather than orderly. Grains germinate at different times and tubes grow at different rates, and since the ovules are limited, the tubes arriving first do the fertilising. That competition is genuine selection, since growth rate depends partly on the genes carried in the pollen itself, so pollen competition favours particular combinations before fertilisation occurs at all. Experiments applying more pollen than necessary produce measurably more vigorous offspring than applying the minimum, which is a result with implications for plant breeding and for understanding why plants produce pollen in such extravagant quantities.

A great many flowering plants prevent self-fertilisation, and the mechanism operates at exactly this stage. Recognition systems compare markers on the pollen against those of the plant it landed on, and where they match, meaning the pollen came from the same individual or a close relative, the tube is prevented from germinating or is arrested partway down the style. Several distinct molecular systems achieve that and they evolved independently. The effect is to enforce outcrossing, which maintains genetic variation, and it is why some fruit trees require a different variety planted nearby to set fruit at all, and why a single tree of certain species can flower abundantly and produce nothing.

Flowering plants dispensed with the water earlier groups needed for fertilisation, so pollen grows a tube through the style and delivers the male cells directly to the ovule. Growth occurs only at the extreme tip, driven by internal pressure, at rates of several micrometres per second. Attractant proteins released by the ovule guide it, and recognition systems arrest tubes from the same individual, which is why some fruit trees need a pollination partner.