How plants compete in battle for sunlight

Light sensors cause plants to react | Too much shade triggers protein-producing hormones that stimulate stem growth

Plants may appear companionably benign, but a vicious turf war is being waged beneath that shady canopy.

A plant’s goal is to get ahead of its neighbour, upwardly speaking, and grow toward the light while literally overshadowing the competition.

Scientists at the Salk Institute for Biological Studies in La Jolla, California, together with colleagues from the University of California, San Diego, and the Swedish University of Agricultural Sciences, have now discovered just how leaves respond to ratios of red light to stimulate stems to grow faster and get out of a shady place.

Their report was recently published in Genes and Development.

A protein called a phytochrome interacting factor 7 (PIF7) is the trigger between a plant’s cellular light sensors and the production of hormones called auxins that stimulate stem growth.

“Plants use a photoreceptor, called Phytochrome B (PHYB), to sense a difference in the ratio of red to far-red light,” said professor Joanne Chory, director of the Salk’s Plant Biology Laboratory and a Howard Hughes Medical Institute investigator.

“When the ratio goes below one, a plant thinks it’s in the shade of another plant,” she said.

“This then causes a modification in PIF7 that allows it to bind to regulatory sequences in the genes that encode auxin.

“These genes become expressed at higher levels, allowing the plant to make more auxin in the leaves. This auxin is then transported to stems, which elongate (within 45 to 60 minutes) to try to get above the plant that is putting it in the shade.”

She said the main auxin found in plants is a small indolic (aromatic) compound called IAA for indole-3-acetic acid. IAA is a phytohormone that regulates many of the growth and developmental processes of plants, including embryogenesis (seed development from a fertilized ovule), tropic growth (growth in response to an environmental stimulus), leaf formation, stem elongation, root elongation and fruit development.  

Plants sense their exposure to light, including whether they are being surrounded by other light-hogging plants, through the photosensitive molecules in their leaves.

These sensors, responding to the wavelength of red light striking the leaves, react to the degree to which the plant is either basking in full sunlight or struggling in the shade of other plants.

The pigment PHYB reacts to the wavelengths of light that both drive photosynthesis and respond to shady spots.

This is the first time scientists have found a direct link between the response to light and chain reaction that drives the hormone-driven auxin growth response to shade.

Chory conducted her studies on thale cress, which thrives in direct sunlight. Through its response to light and shade, she was able to document the sensor reaction that triggers the stem to grow toward sunlight.

“The lower the ratio of red to far-red light, the bigger the response,” said Chory. “Elongation is positively correlated with shade.”

The danger arises when a plant is forced to remain in the shade for a period of time. The lack of direct sunlight may force it to flower early, producing fewer seeds, which are then broadcast by wind or insects to germinate in direct sunlight.

In agriculture, this response is known as shade avoidance syndrome and can lead to loss in crop yields. When seeds are row planted too close together, young plants will struggle to grow upward toward the sun, blocking each other’s light as a consequence.

As a result, high density planting can be counter-productive because it can lead to production loss.

Chory and her colleagues used biochemical and gene analyses to identify PIF7 as the key molecular link between a plant’s light sensors and the production of auxins.

“PIF7 was identified originally as one of about a dozen proteins that both bind DNA and phytochrome,” she said.

“But what its target genes were was unknown. We identified PIF7 in a screen designed to identify proteins that bind to a specific shade-induced light regulatory element in the promoters of shade-regulated genes.”

Her colleagues then identified proteins that could bind to the light regulatory elements. When they knocked out the function of PIF7 in a background where all other genes were expressed normally, the PIF7 mutant made less auxin in the shade and the plant did not elongate its stem.

From this research, they were able to show that when a thale cress plant was denied access to sunlight, it triggered a molecular domino effect in the cells of the leaves.

The PHYB photoreceptor causes chemical changes in PIF7, which then activated genes that direct the cell to produce auxin. The auxin hormone stimulates stem growth.

From an agricultural viewpoint, this knowledge could offer opportunities for developing crops that will have a stem structure less prone to shade avoidance syndrome when seeds are planted tightly in field rows.

However, plants must currently respond quickly when shaded and beat the competition for access to sunlight.

“A plant that senses competition from other plants needs to make its move fast,” said Chory.

“Since the plant can’t (physically) move, it makes its move by growing and it grows via increasing auxin levels within shoots. We can detect physical growth by one hour.”

She said researchers don’t know for sure if the molecular/chemical process is the same in all plant species. The phytochrome B, the main photoreceptor, plays the role of shade detector in species of flowering plants, but the entire signalling pathway is not known in other plants.

“I would speculate that this is a highly conserved process and it is something we are eager to obtain answers to.”

She said the researchers plan to trick a plant in the early stages of shade avoidance to assume that it has escaped the shade.

“If we can do this successfully, then the plant won’t initiate the syndrome of shade avoidance, which leads to early flowering and loss of biomass and yield.”

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