Plant automatic response to shade stress (Introduction)

by David Turell , Monday, January 04, 2016, 14:48 (3027 days ago)

Plants have competition for sunlight. If a neighbor grows too tall it can shade out a neighbor. Plants have sensors for this event and respond automatically:-http://www.sciencedaily.com/releases/2015/12/151225141555.htm-"To escape this deadly shade, plants have light sensors that can set off an internal alarm when threatened by the shade of other plants. Their sensors can detect depletion of red and blue light (wavelengths absorbed by vegetation) to distinguish between an aggressive nearby plant from a passing cloud.-"Scientists at the Salk Institute have discovered a way by which plants assess the quality of shade to outgrow menacing neighbors, a finding that could be used to improve the productivity of crops. The new work, published Dec. 24, 2015 in Cell, shows how the depletion of blue light detected by molecular sensors in plants triggers accelerated growth to overcome a competing plant.-***-"The new work upends previously held notions in the field. It was known that plants respond to diminished red light by activating a growth hormone called auxin to outpace its neighbors. However, this is the first time researchers have shown that shade avoidance can happen through an entirely different mechanism: instead of changing the levels of auxin, a cellular sensor called cryptochrome responds to diminished blue light by turning on genes that promote cell growth.-***-"The focus of the team's research efforts was cryptochromes, blue light-sensitive sensors that are responsible for telling a plant when to grow and when to flower. Cryptochromes were first identified in plants and later found in animals, and in both organisms they are associated with circadian rhythm (the body's biological clock). The protein's role in sensing depletion of blue light had been known, but this study is the first to show how cryptochromes promote growth in a shaded environment.-"The team placed normal and mutant Arabidopsis plants in a light-controlled room where blue light was limited. The mutant plants lacked either cryptochromes or a PIF transcription factor, a type of protein that binds to DNA to control when genes are switched on or off. PIFs typically make direct contact with red light sensors, called phytochromes, to initiate shade avoidance growth. The researchers compared the responses of the mutant and normal plants in the varying blue light conditions by monitoring the growth rate of the stems and looking at contacts between cryptochromes, PIFs and chromosomes.-"'We found that cryptochromes contact these transcription factors on DNA, activating genes completely different than what other photoreceptors activate," says Ullas Pedmale, first author of the work and a Salk research associate. "This is also a very short pathway so plants can rapidly respond to their light environment.'"-Comment: This is a fully automatic response through chemical reactions that trigger different DNA responses. This is the same way I envision automatic responses in animal organisms. Again, it is difficult to see how a non-directed Darwinian evolution evolved this mechanism.