Scientists have made a fresh discovery about how flowers reproduce so prolifically. Researchers at Brown University, Rhode Island, in America, have investigated the way flowers control sperm distribution to female gametes.
They discovered that Arabidopsis flowers ensure gamete fusion has happened before additional pollen is repelled. This leads to successful fertilisation even if the initial pollen is faulty.
Senior author Mark Johnson, a Brown associate professor of biology, says. "There is a mechanism that prevents too many pollen tubes from delivering too many sperm. But the other cool thing is that there is also a way to salvage fertilisation if the first father is a dud."
During the process of pollination, hundreds of pollen grains carry sperm as they fix on a flower's stigma and grow a pollen tube along the style shaft approaching the ovules that house two female cells.
The process could be chaotic, but the researchers found that flowers have developed an efficient safeguard to ensure two fertile sperm reach the female cells and are successful.
When the gametes and sperm fuse, the signals that attract the tubes to ovule stops, discovered study author Kristin Beale, a Browns graduate student who works alongside Mark Johnson.
Previous studies believed that if one pollen tube was used, others were discouraged, but Brown researchers found the gamete fusion was the trigger, because until then, seed formation was not guaranteed.
Even though the reproduction of plants has been investigated for many years, it was only recently that new techniques using pollen mutant helped further advance research.
The team, including report author Alexander Leydon, carried out a number of experiments on Arabidopsis, the rockcress weed.
They found the pollen mutant hap2 creates a pollen tube to an ovule and then bursts on the release of sperm. But its sperm is unable to fuse with the gametes.
The researchers carried out a new technique that added fluorescent colours to the pollen tubes and its sperm. This allowed them to investigate how the tubes behaved with the ovules.
In one experiment they used healthy sperm carried by red and green coloured tubes in equal measure. Just 1% of ovules received multiple tubes (a process named polytubey by Kristin Beale).
The researchers then used sperm with 25% that was faulty. Polytubey rose 10-fold and one ovule attracted four tubes, which suggests polytubey is accepted until a sperm is successful at fertilisation.
They then used red tags for mutant sperm and green tags for wild pollen and discovered that polytubey only occurred with the mutant sperm.
They found that ovules targeted with mutant sperm could accept additional pollen, but if wild pollen sperm was attracted, any other pollen tubes were blocked.
The also discovered that one of the cells attracting pollen tubes keeps going in the ovule until the gamete fusion is successful. The team did not find out which signalling molecule blocks the process of polytubey after fusion, but it had to be strong and quick.
The study could help farmers overcome fertilisation problems in corn crops, Mark Johnson adds.
The research was financed by The National Science Foundation and the National Institutes of Health Brown University Initiative to Maximize Student Development.