AMHERST, Mass. — One of the great mysteries in plant biology is how, given the clouds of pollen released by dozens of plants species all at the same time, an individual plant can recognize which particular species' pollen grains will induce fertility and which to reject. We are now one step closer to solving the mystery thanks to research recently published in Science by an international team from the University of Massachusetts Amherst and China's Shandong Agricultural University.
Many flowering plants have evolved what's known as "self-incompatibility," or the inability to mate with itself and close relatives. In this way, a plant can avoid the pitfalls of inbreeding. But what about the pollen from species that are more distantly related, yet within the same family?
UMass Amherst's Alice Cheung , Distinguished Professor of Biochemistry and Molecular Biology at UMass Amherst and one of the paper's senior authors and a key member of the team that used the Brassicaceae family of plants—which includes cabbages, broccoli, kale, turnips, the oil crop canola and other common vegetables—to study the poorly understood mechanism of "interspecific incompatibility," or ISI, which is what keeps pollen from broccoli from fertilizing kale and producing a hybrid species—kale-occoli. The problem is that breeding between distantly related relatives to generate new species with and improved or a wider range of traits is beneficial to agricultural crops and thus food security.
The molecular workings of ISI unfortunately remain "very much a black box, compared with what we know about self-incompatibility systems and their mechanisms," says Cheung.
Cheung and her colleagues have made great strides in understanding how ISI works in the current study and even introduced a strategy for crossing distantly related species within Brassicaceae.
The team's breakthrough involves how plants of different species communicate during the pollination process, either accepting or rejecting pollen grains. A protein called SRK, the key protein known to control self-incompatibility in the Brassica stigma—the tip of the pistil which constitutes the pollen-reception surface of the female reproductive organ—recognizes a specific chemical signal, called SIPS, on pollen from a different Brassica species, such as from the model Brassica Arabidopsis, that it wants to reject. But this is only half the story.
The SIPS-SRK pair then recruits another enzyme, FERONIA, which both Cheung and co-senior author Qiaohong Duan, from Shandong Agricultural University, have a long history in studying . The FERONIA/SIPS-SRK interaction then creates a highly reactive chemical known as ROS, which essentially blocks the pollen from entering the pistil. Furthermore, Cheung and her colleagues' suggest a breeding strategy to overcome Brassicae pollen's incompatibility, which could hasten success in crosses between distantly related species in the same family.
