Since the invention of microscope the intricate and seemingly-bizarre patterning of pollen walls has fascinated plant scientists and natural historians. However, apart from helping protect the male sex cells in their journey from flower to flower, the function of the patterning of these very strong walls remains a mystery. Furthermore, how a pattern of this complexity, made from the highly-resistant polymer sporopollenin, is assembled in isolation on the surface of the young pollen cells in the anther continues to perplex plant cell biologists. The discovery of a new gene in rice – EXINE PATTERN DESIGNER 1 (EPAD1) – by Prof Wanqi Liang and colleagues in Shanghai Jiao Tong University, recently published in Plant Cell, promises to be an important step in understanding how pollen wall patterns are synthesised.
Stepping back into the last century, studies using transmission electron microscopy and genetics revealed two surprising facts about pollen walls. The first is that while the architecture of the wall is ‘designed’ by the young sex cell, most of the materials for making the walls come from the surrounding blanket of nurse cells (the tapetum – from the Latin for carpet). The second fact is that the pollen wall consists of 2 components, each made by a seemingly different mechanism; one, the basic ‘ground pattern’ of ridges and spines, and secondly, a number of ‘apertures’ through which the pollen germ tube can emerge on pollination.
As plant biology has entered the molecular age, an increasing number of genes have been discovered affecting pollen wall development, many by the Shanghai group. However, the majority of these act in the taptum cells and affect the composition and flow of wall components – rather than the pattern itself. EPAD1 is different – it is active only in the germ cells (young sex cells) and affects the wall patterning itself. It turns out that the gene encodes a protein ‘addressed’ to the cell membrane and three ‘(GPI)-anchors’ which may help it bind to membrane lipids. In their paper the Shanghai group suggests that EPAD1 may be involved in recruiting and arranging other regulatory proteins at the cell surface.
The timing of expression of the EPAD1 gene is particularly exciting in that it occurs very early in male ‘germline’ development – even before the final cell divisions that form the very young pollen cells. However it takes its effect far later; as Prof Liang says “EPAD1 takes action much later …… I believe that the primexine (early pollen wall) is not a rigid structure but somewhat flexible and may undergo phase transition …”. This suggests that a ‘fluid imprint’ of the pattern is set up in the cell membrane very early, and later may be modified by other patterning mechanisms, such as that marking the apertures – a view supported by Prof Liang who states that “………EPAD1 does not contribute to the aperture pattern formation. It seems that the aperture …. and the (basic) pattern(s) are controlled by two systems, though the initial signal may be the same…..”
There is clearly still a long way to go before we know the full story of pollen wall development, however the discovery of EPAD1 represents an important step on the journey. Two important immediate objectives are to discover how the EPAD1 protein contributes to the pattern, and how the aperture pattern is later overlaid on the original ‘ground patterning’. Of course, the EPAD data apply only to rice; as Prof Liang says, “Further investigations on EPAD1 orthologs (equivalents) in other grass species will help to unravel the mechanisms by which..(pollen wall)…patterning is imprinted onto the plasma (cell) membrane.”