ScienceWeek

PLANT BIOLOGY: ON MATE SELECTION IN FLOWERING PLANTS

The following points are made by Bruce Mcclure (Nature 2004 429:249):

1) Mate selection has a huge effect on a species. In general, it is good to avoid inbreeding and it is bad to mate with other species. Humans possess sophisticated sensory faculties that help make these selections. Flowering plants have the same mating imperatives, but they lack our sensory faculties, are largely immobile, and rely on the vagaries of factors such as wind or insects to assist mating. Their remarkable success is due, in part, to the evolution of sophisticated mating control systems that require communication between pollen (the male player in events) and pistil (the female organs, consisting of a stigma on which the pollen lands, the egg-containing ovary, and a style to provide passage from one to the other).

2) Plants alternate between a diploid generation, with two copies of each gene, and a haploid generation, the gametophyte (the generation that produces gametes, the sperm or egg), with only one copy of each gene. In flowering plants the diploid generation is visible and familiar to us; the male and female gametophytes are not because they are microscopic. But what mostly sets flowering plants apart from other plants is that their eggs are enclosed in the diploid tissues of the pistil. Pollen must germinate and produce a pollen tube to carry the haploid sperm cells through these tissues. This requirement provides an opportunity for a two-sided conversation in which a decision can be made about whether pollen should be accepted or rejected.(1,2)

3) Studies of self-incompatibility mechanisms have been the most effective way of listening in on these conversations. Self-incompatibility allows plants to avoid inbreeding by rejecting pollen from close relatives and, in some cases, it contributes to rejecting pollen from other species as well. The result is that genetic diversity and fitness are maintained. Compatibility is genetically controlled by an "S-locus" carrying distinct specificity genes, one expressed in the pollen and the other in the pistil. The S-locus is extremely polymorphic; even small populations may have dozens of different S-haplotypes (S1-, S2-, and so on). Gametophytic self-incompatibility is the most common type of genetic control. Haploid pollen is rejected when its S-haplotype is the same as either of the two S-haplotypes in the diploid pistil. Any other combination is compatible. The challenges are to identify the gene products that bring about S-specific pollen rejection and to determine how they function.

4) Thomas and Franklin-Tong(1) address function: they demonstrate that gametophytic self-incompatibility in the field poppy (Papaver rhoeas) involves programmed cell death. In poppies, the conversation opens on the pistil side with S-proteins produced on the stigma surface. Pollen that does not share an S-haplotype with the pistil is compatible. The conversation goes well for such pollen, and access to the ovary is permitted. For incompatible pollen (that is, an S-haplotype that matches the pistil), the conversation goes poorly. The S-proteins interact with an as-yet-unidentified pollen factor, resulting in immediate inhibition of pollen-tube growth and -- as Thomas and Franklin-Tong(1) now demonstrate -- initiation of events that are typical of the programmed-cell-death response (release of mitochondrial cytochrome c, generation of a caspase-like enzyme activity, and DNA fragmentation). These results reveal a whole new landscape of events on the pollen side, and show just how badly such a conversation can end for an undesirable mating partner.(3-5)

References:

1. Thomas, S. G. & Franklin-Tong, V. E. Nature 429, 305-309 (2004)

2. Sijacic, P. et al. Nature 429, 302-305 (2004)

3. Kao, T.-h. & Tsukamoto, T. Plant Cell advance online publication 9 March 2004 (doi:10.1105/tpc.016154).

4. Qiao, H. et al. Plant Cell 16, 582-595 (2004)

5. Luu, D. T. et al. Nature 407, 649-652 (2000)

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com