Chemist Finds Key To Light's Effect On Plant Growth

Pill-Soon Song, Dow Chemical Co. professor of chemistry at the University of Nebraska-Lincoln, and molecular geneticist Gitsu Choi of South Korea's Kumho Life Science Laboratory, announced that their team of researchers has found the missing connection between a plant's initial detection of light and its physiological response at the molecular and cellular levels.

Song explained that plants possess tiny but indispensable quantities of a pigment protein called phytochrome, which acts as a sort of biological clock. Phytochrome, he said, specifically detects light in red wavelength.

"The phytochrome pigment exists in the so-called 'Pr' form -- 'P' for phytochrome and subscript 'r' for red light," he said. "This red-light detecting phytochrome detects red light and that initiates chemical reactions to the phytochrome itself and it becomes another form called 'Pfr,' for phytochrome far-red absorbing."

Song said that while human eyes are not very sensitive to far-red light, which has much longer wavelengths than red light, plants can detect red and far-red light equally well, and that's where the biological clock comes in. Early in the morning, most of the sunlight that reaches plants is in the far-red spectrum and plants don't respond. But by 9 or 10 a.m., plants receive more red light than far-red light and chemical processes start to happen.

"You have to have red light to transform phytochrome from its Pr form to its Pfr form, and the Pfr then triggers a series of molecular and cellular processes that transform plants into photosynthetically active green plants," Song said. "In order to have photosynthesis, you have to have chlorophyll, then you have to have cellular-molecular machinery to do the photosynthetic reactions within the plant cells. Most of those things are developed as a result of light and that process is mediated by phytochrome in its Pfr form."

At dusk, when plants begin to receive more far-red light than red light, Pr phytochrome is no longer transformed into the active Pfr form, and they go to sleep for the night.

The existence of this biological clock has been known for some time, Song said, but the connection between the physiological response and the light signal hasn't been well-understood -- until now. The molecular biological research he performed with funding from the National Institutes of Health and Choi's genetic research have found the connection.

"We identified one specific component protein which recognizes the Pfr form and then triggers this pathway that connects the visual signal and the final response," Song said. "This protein turned out to be nucleoside diphosphate kinase, or 'NDPK.'

"What is surprising about that is this is a very well-known enzyme, especially in animal and human systems, and it is an extremely unusual enzyme that is known to have multiple functions in animals and humans."

Known as NM23 in humans, NDPK is a cancer suppressant in humans and mice. In fruit flies, it's a gene necessary for wing development. NDPK is also unusual in that it acts as a DNA-binding protein while most enzymes (other than those involved in DNA and RNA enzyme reactions) do not interact with DNA.

"Our guess is that NDPK could be active as a gene-activating factor," Song said. "In our research, we found that the Pr form of phytochrome is found outside the nucleus of a cell. But when it is converted to Pfr form, it is relocated to the nucleus, where genes are located in the form of DNA, and those genes which are sensitive to light must be turned on.

"That's one possibility. Another is that NDPK in its enzymatic function activates G proteins, which mediate signal response in cells. In other words, NDPK mediates external environmental signals eventually into internal cellular response. But these possibilities are speculative right now. That's what we're going to study in the next phase of our research."

The discovery by Song and colleagues has some intriguing practical applications that may be possible and they have to do with counteracting an evolutionary trait called "shade-avoidance." To survive, a green plant has to break the soil before the nutrient in its seed runs out, then it has to outgrow neighboring plants to stay out of the shade -- where it would receive mostly far-red light, the Pr-Pfr-NDPK chain reaction wouldn't be activated and it would die.

If NDPK is activated without red light (and this is possible, Song said), or if the structure of phytochrome can be changed through genetic engineering to absorb far-red light, shade avoidance can be avoided. The fewer resources plants expend in raising their stems to compete with their neighbors for red light, the more resources they save for growing seeds and expanding their root systems.

Plants such as rice would develop more and larger grains. They would also be healthier, Song said, because the stress of shade-avoidance weakens them in the face of fungal and other plant diseases. Plants would also require less water, fertilizer and pesticide.

Those potential benefits apply to lawn grass, too, which is good news for homeowners on at least two counts. First, a lawn that isn't a money pit for water, fertilizer and pesticide would ease the pressure on a lot of families' budgets. Second, grass blades that aren't madly competing to get to the top of the canopy would grow more slowly than regular grass -- and therefore wouldn't have to be mowed as often.

"This idea came from that," Song said. "I hate having to mow my lawn every week."

To that end, he and his colleagues are working on generating such genetically engineered lawn grass seeds.