Cellular sensors enable plants to find sunlight, study says

Washington: Plants wage wars to absorb sunlight and outgrow each other by using their cellular sensors that can set off an internal alarm when threatened by the shade of other plants, scientists have found. If a plant is shaded by another, it becomes cut off from essential sunlight it needs to survive. 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 have found a way by which plants assess the quality of shade to outgrow menacing neighbours, a finding that could be used to improve the productivity of crops. "With this knowledge and discoveries like it, maybe you could eventually teach a plant to ignore the fact that it's in the shade and put out a lot of biomass anyway," said senior author Joanne Chory, director of Salk Institute's Plant Molecular and Cellular Biology. It was known that plants respond to diminished red light by activating a growth hormone called auxin to outpace its neighbours.

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 finding could help researchers learn how to modify plant genes to optimise growth to, for example, coerce soy or tomato crops grow more aggressively and give a greater yield even in a crowded, shady field. Cryptochromes are blue light-sensitive sensors responsible for telling a plant when to grow and when to flower.

The 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," said first author Ullas Pedmale, a Salk research associate. "Ultimately, we could help farmers grow crops very close together by changing how plants put out leaves, how fast the leaves grow and at what angles the leaves grow relative to each other and the stem. This will help increase yield in the next few generations of crop plants," said Chory. The study was published in the journal Cell.