Green Sex Rules
Sex is exhausting but biologically rewarding. After all, the complicated gene exchange brings variety and flexibility. It's just that effort without benefits isn't that rewarding - which is why sex without a gene-swapping partner should better remain an exception. Plants have apparently internalized this.
Valentine's Day over, chocolate gifts nibbled, florist sales increased - time to put the matter of romance, love and feelings aside for ten minutes. But if we stay with the masses of flowers that have just been given away and their relatives, we can state soberly that plants obviously have it easier when dealing with the opposite sex. Plants know no partner choice, no partner charm and therefore, to put it quite emotionally, no appointment gifts. Also, no need to worry about your own looks to impress potential herbal mating partners. Instead, just anonymous sex – with whom, the wind determines. Or maybe another busy nectar collector.
Where it blows, drags or shoots the male seeds of plants, and on whose female pistils the pollen then lands to produce new plant life - in the green realm it is all often left to chance. However, despite all the straightforwardness, it is precisely here that a danger lurks: loss of control. Not only is the mating partner anonymous, i.e. unknown and unchosen – what also prevents plants from accidentally fertilizing themselves with their own pollen, i.e. inbreeding? In the case of loneliness and the desire to have children, self-fertilization may make sense – but sexual exchange and the mixing of genes that goes with it was not invented in vain in the course of evolution.
Since the undesired path of their own pollen to their own pistils can be quite short, plants immediately invented a number of anti-inbreeding rules. The simplest: Have either only male or female flowers, so be a dioecious species. Or be monoecious - that is, with female and male flowers on one plant individual - but produce the sexual characteristics at different times of the year.
However, many plants have flowers that combine both female pistils and male pollen deposits. They mostly rely on a biochemical barrier to self-fertilization and use molecular mechanisms to differentiate between desired, "compatible" foreign pollen and undesired, "incompatible" self-pollin. Exactly how the mechanism of "self-incompatibility" works has occupied botanists for a good hundred years. Around two decades ago, plant inbreeding researchers presented a prime suspect: the S-RNase family of enzymes.
S-RNases, once released, destroy all RNA of a pollen tube recognized as undesirable. The cells of the female pistils do not produce S-RNases just in time, but continuously and constantly pour them out - a compatible, but not an incompatible pollen should then somehow be able to keep the poisonous guard at bay. A role is played here by a so-called SLF recognition substance produced by the pollen, which is characterized by S- locus- F -box gene is encoded: If the pollen SLF signals the toxic pistil S-RNases "No inbreeding suspected", the RNA-degrading destroyers are neutralized before they penetrate the pollen tube, and fertilization can begin.
So much for the long-established theory. It is not entirely complete, say Ariel Goldraij of the State University of Cordoba in Argentina and his colleagues. In fact, the S-RNases of the pistil tissue always get into the pollen tube, which grows greedy for fertilization. There, in the pollen cells, the voracious S-RNases remain locked away in a membrane-enclosed compartment. Only now does the origin of the pollen decide its fate - either the compartments burst open within about 36 hours and release the S-RNase pack, or the compartment and ultimately the pollen remain intact, which means that nothing stands in the way of fertilization. An experiment with fluorescent markers that bind to S-RNase made these different scenarios clearly visible to the researchers led by Goldraij.
The researchers were also able to locate the wrecking ball, which is effective in incompatible pollen and destroys the protective compartment envelope around the toxic S-RNases: It is called HT-B and is a small protein that, like the S-RNases in the pistil of the flower is produced. Compatible pollen with a legitimate desire to be fertilized must therefore contend with two opponents: the dangerous but closable S-RNases and their potential liberators, the HT-B proteins. And in this two-front defensive battle, old acquaintances help the pollen, explain Goldraij and colleagues: the SLF gene products.
Compatible pollen fight their way out because the SLF search parties patrolling in them do not see any signs of a closer relationship to the pistils - for example in the biochemical control of the compartments filled with pistil proteins. If this proof of relationship is missing - but only then - they start to break down the stamp protein HT-B. This can then no longer destroy the compartment membrane, which is why the pollen-degrading S-RNases are never released. It works the other way around with incompatible pollen, where HT-B is allowed free rein due to an inbreeding situation recognized by SLF – the end of the pollen.
How all this works "exactly" still has to be clarified, the researchers say happily. We may first draw an interim conclusion here: Plants are not entirely uncomplicated either. If we recapitulate the complex self-fertilization prevention pathway from S-RNAse via HT-B and SLF, then the simple human variant of dioeciousness regains enormous charm. Especially at the sight of blooming Valentine's Day gifts. And ten minutes without romance, love and feelings are enough.
