Scientists gain insight on how plants bend toward light

Angus Murphy, a professor of horticulture, said the process, called phototropism, is well-documented in grasses, but has been difficult to resolve in dicots, a large group of flowering plants that includes many agricultural crops. Research in the model plant Arabidopsis thaliana now shows that the hormones that cause phototropism are distributed from the tip of the plant rather than where the plant's stem bends as had been thought.

Charles Darwin and his son, Francis, described phototropic bending in the late 19th century based on experiments in which they were able to block light from reaching the tips of plant shoots and keep the plants from bending toward the light. Their work led to the discovery of auxin, a plant hormone that controls growth functions.

Murphy said the mechanisms that caused phototropism in dicots have been difficult to understand, however. For the past decade, it had been thought that auxin was transported down the stem of dicot plants and then moved laterally to epidermal areas to cause bending. Murphy's team, which included collaborators John Christie of the University of Glasgow and Wendy Peer, an assistant professor of horticulture at Purdue, found that the auxin instead is redistributed laterally at the plant tip and then moves down to the bending point, as is the case in grasses.

"We were looking in the wrong place at the right time," said Murphy, whose findings were reported Tuesday (June 7) in the journal PLoS Biology.

Observing the phenomenon in dicots has been difficult, Murphy said, because testing has to be done on plants grown in the dark. Once those plants receive blue light, which regulates growth, other photomorphogenic processes, such as the uncurling of the top of the shoot and leaf production, begin and make the chemical reactions causing phototropism difficult to observe.

Murphy's team found a simple solution. The plants were grown in the dark, exposed to light to allow photomorphogenic processes occur and then placed back in the dark.

"It basically resets the phototropic response," Murphy said.

Once exposed to light again, the phototropic response could be observed and measured with sensors without the other reactions occurring simultaneously.

"Phototropism is something that everyone who grows a plant in their window knows about," Murphy said. "However, it is also critical to the survival of crop plants, especially at the seedling stage."

The paper in PLoS Biology also showed that when the plant is exposed to blue light, a receprot protein called photropin1 blocks another protein, ABCB19, which is essential for auxin to transport from tip down the plant's stem. This creates a pool of auxin at the tip, which can then be distributed laterally before moving down the stem to the bending point.

"We now know what occurs in the first and last stages of phototropism, but still need to understand how lateral auxin distribution actually occurs," Murphy said. "As part of this work, we tested mutations in every known auxin transporter gene and all of them showed phototropic bending."

More information: Phot1 Inhibition of ABC19 Primes Lateral Auxin Fluxes in the Shoot Apex Required for Phototropism, PLoS Biology (2011)

Abstract
It is well accepted that lateral redistribution of the phytohormone auxin underlies the bending of plant organs toward light. In monocots, photoreception occurs at the shoot tip above the region of differential growth. Despite more than a century of research, it is still unresolved how light regulates auxin distribution and where this occurs in dicots. Here, we establish a system in Arabidopsis thaliana to study hypocotyl phototropism in the absence of developmental events associated with seedling photomorphogenesis. We show that auxin redistribution to the epidermal sites of action occurs at and above the hypocotyl apex, not at the elongation zone. Within this region, we identify the auxin efflux transporter ATP-BINDING CASSETTE B19 (ABCB19) as a substrate target for the photoreceptor kinase PHOTOTROPIN 1 (phot1). Heterologous expression and physiological analyses indicate that phosphorylation of ABCB19 by phot1 inhibits its efflux activity, thereby increasing auxin levels in and above the hypocotyl apex to halt vertical growth and prime lateral fluxes that are subsequently channeled to the elongation zone by PIN-FORMED 3 (PIN3). Together, these results provide new insights into the roles of ABCB19 and PIN3 in establishing phototropic curvatures and demonstrate that the proximity of light perception and differential phototropic growth is conserved in angiosperms.

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