More Denton: plant fertilization (Introduction)

by David Turell , Thursday, March 24, 2016, 00:34 (2957 days ago) @ David Turell

How the pollen reaches the ovule is amazing. It has to grow a tubule!:-http://www.evolutionnews.org/2016/03/baroque_botany102711.html-"The origin of angiosperms is frequently called "Darwin's abominable mystery" because the rise and diversification of flowering plants in the fossil record is so sudden.-***-"The mystery goes much deeper than the lack of ancestors, however. On pages 151-156, Denton pulls back the curtain on one of the most elaborate reproductive cycles in nature: the pollination of flowers. In particular, the "arduous" journey of the pollen tube to its target synergids in the ovule, leading to double fertilization -- unique to angiosperms -- is part of a pathway so complex it "simply beggars belief." There's no way, Denton contends, this "baroque" arrangement could have arisen by natural selection.-"The curtain pulled open a little more with the publication of two papers in Nature about pollen tube guidance. Pollen grains are as fine as dust, from 6 to 100 micrometers across depending on the species. Yet when they land on the pistil of a flower, an amazing transformation begins. The pollen grain grows a tube down the long style on the pistil toward the ovary, where ovules await fertilization. The tip of the tube contains two sperm cells needed to fertilize the egg and build the endosperm, a food source that the future seedling will consume upon sprouting.-"How does the pollen tube find its target in the dark? That's what the Nature papers discuss. The process requires detailed signaling between pollen and ovule. "In flowering plants, the female gametophyte secretes chemoattractant peptides to guide pollen tube growth so that it delivers the immobile sperm to the ovule-enclosed female gametophyte," the Editor's summary states.-***-"How pollen tubes find their target has long puzzled biologists. The female gametophyte is known to produce chemoattractant molecules, such as cysteine-rich peptides called LUREs, but the identity of their receptors on pollen tubes has been unclear. Two papers in this issue identify several molecules on the cell membrane that are involved in sensing one such attractant -- AtLURE1 -- in the model plant Arabidopsis thaliana. These discoveries underscore the molecular complexity of this male-female communication process,-***-
"It is well established that pollen-specific receptor-like kinase (RLK) proteins can regulate the growth of pollen tubes. These proteins typically have three domains: an ectodomain that interacts with extracellular signal molecules; a membrane-spanning domain; and a cytoplasmic domain that attaches phosphate groups to target molecules, inducing cellular responses to incoming signals (Fig. 1b). Using different genetic strategies and starting from an overlapping list of almost 30 pollen-expressed RLKs, the two groups searched for proteins that support ovule targeting by pollen tubes.-"The receptors, in other words, are complex three times over. They need to have docking ports for the incoming signal molecules (called "male discoverer" 1 and 2, as well as a pair of RLKs that regulate the tube growth). They need to span the membrane of the pollen tube. And they need to respond by switching on genes in the nucleus. Another team found a second pair of pollen-specific receptor kinases that also are involved. -***-"The number of players in just this one aspect of angiosperm fertilization has grown, and the scientists are still not sure they have found them all. This underscores Denton's point: these are all novelties that defy Darwinian selection. If all that is needed is getting sperm and egg together, there are simpler ways to do it than building a long pistil that requires an arduous journey by a pollen tube, multiple signaling molecules and receptors, and the elaborate dance of double fertilization. -***-"The arsenal of signalling molecules in plants -- in particular peptide signal molecules and RLKs -- is immense. It will not be surprising if more attractant-receptor pairs are discovered."-Comment: Darwin style evolution cannot create this complexity stepwise! Only saltation fits.