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Plants Recognise Their Molecular Signature
WHEN a pollen grain lands on a flower, fertilisation does not always follow. The plant may accept some pollen grains, but reject others. Biologists are discovering, however, that this selection procedure is far from random. It relies on precise genetic mechanisms, which, in turn, depend on a single gene, known as the s gene.
The phenomenon tn which a plant rejects pollen is often known as self-incompatibility, because the pollen that ft rejects is usually its own. Plants are capable of fertilising, or self-pollinating, themselves because they possess both male parts—the anthers, which produce pollen—and female parts—the stigma and the style. Such in breeding is not desirable. though, so plants have evolved stringent mechanisms to stop "selT pollen fertilising them. New experimental work carried out by Adrienne Clarke and her colleagues at the University of Melbourne has shed light on these processes
When a pollen grain lands on the surface of the stigma of a plant, the grain puts out a tube that grows down into the flower through the style, eventually reaching the ovary. As the pollen tubes grow, they deposit a substance, known as callose, which researchers can see by using a fluorescent marker. .
Clarke and her colleagues have studied the wild tomato. Lycopersicon peruvlanum. In this plant, the callose forms long, fluorescent tracks which are ended by callose plugs Pollen that Is 'compatible* forms tracks that proceed unhindered and terminate at the ovary of the plant
Clarke has found, however, that some pollen grains grow in an abnormal way In some plant species, ’self pollen may die on the stigma. In other species, seif pollen gets as far
as the style, but its growth then becomes irregular and it often dies. Such abortive attempts at pollination are typical of pollen grains that are genetically identical to those that are produced by the anthers of the host flower, and so are ’Incompatible".
Recently, researchers have applied the techniques of molecular biology to the mechanisms of self-lncompat
other words, pollen grains carrying a certain allele cannot germinate on a style that also carries that allele. A pollen grain si. for example, cannot germinate on a style sls2 or sls3 but can germinate on a styfo s2s3.
in the 1950s. scientists adapted techniques that had been used to identify differences in human blood groups to show that different plants
ibility. The behaviour of pollen grains that are incompatible depends on the s gene.
This gene lias two forms, or alleles, in the adult plant, but only one allele in the pollen grain It has multiple alleles. ■II of which vary slightly Re searchers denote them si. s2. •3. s4. and so on These alleles code for •lightly different pro tclns in Individual plants of the same specks.
The situation Is compll cated because both pollen grain and style contain proteins produced from the a gene Somehow a pollen grain and style that carries the same • sHete repel each other In
contained different proteins, latter on. scientists liked these proteins, which are often abundant, to the pres ence of different s alleles. B. Nasrallah and colleagues at Cornell University. New York, first sequenced an s gene in a plant of the cabbage family. Brassica oleraceae Clarke and her colleagues have also de scribed the sequences of s genes from the ornamental to bacco, Nicotiana alata
The Melbourne team used genetic probes to locate mes senger RNAs for the s genes in the stigma, the style and the regions in which the pollen tubes would grow. They
showed that the sequences of s alleles were variations on a theme. Some parts of the sc quence remained very similar in different alleles. Other regions. however. varied considerably. This kind of genetic structure is reminiscent of some human genes, such as those of the major histocompatibility complex, which is involved In the recognition of self or foreign tissue.
There are so many different s alleles in a plant population that the chance of random encounter between pollen and style carrying the same allele Is very unlikely. In addition, forced inbreeding can result in new allele forms evolving. This Kmotes outbreeding, that Is.
r-ding with other plants.
The question biologists now want to answer is how the protein encoded by the s gene Inhibits the growth of similar pollen. Biologists know that plants secrete the s protein Into the extracellular matrix that fills the spaces between cells tn the style, but they do not know how it acts to stop the pollen dead In its tracks.
Possibly, the s gene undergoes cutting and rearrange ment that leads to two different messenger RNAs being produced, one specifying the protein in the pollen, the other the protein In the style.
In pollen, the s protein does not appear to be secreted outside the cells, a* It is in the style, suggesting that the two are different Alternatively, the same protein could be main) factored from the same rnes senger RNA at both sites, but modified In structure later, giving rise to two different proteins rather than on
—Sarah Guthrie.