Plants switch off growth signals by targeting 'used' receptor molecules for destruction

August 12, 2011
Figure 1: After six weeks of growth, bri1-5 mutant Arabidopsis plants (left) are developmentally stunted. By comparison, plants with the sbi1 and bri1-5 mutations together grow as well as wild-type plants (middle), and plants with sbi1 mutations alone exhibit even more uninhibited growth. Credit: 2011 AAAS

Plants coordinate growth using hormones called brassinosteroids (BR), and defects in the associated signaling pathway can result in profoundly stunted development. For example, researchers have identified numerous mutations within the gene encoding the BR receptor, BRI1, which yield plants with a dwarf phenotype.

The bri1-5 mutation does not directly disrupt , but nevertheless inhibits growth of thale cress, Arabidopsis thaliana, plants by somehow accelerating the rate of receptor degradation. The discovery of an additional mutation that fully counteracts this effect has now revealed valuable insights into how plants manage to keep a tight rein on growth signals1.

When Guang Wu and Joanne Chory of the Salk Institute for Biological Studies, USA, identified this enigmatic sbi1 mutant (Fig. 1), they partnered with Yuji Kamiya, a biologist at the RIKEN Plant Science Center in Yokohama. “We wanted to know the reason why sbi1 plants recover from the dwarf phenotype,” says Kamiya, “and so Wu visited my laboratory to study the mechanism of the BRI1 gene.”

An initial series of experiments provided strong evidence that the sbi1 mutant disrupts the function of a negative regulator of BRI1, which appears to act on the receptor after it has been activated by binding BR. The researchers were subsequently able to uncover the affected SBI1 gene, which encodes a member of the leucine carboxylmethyltransferase (LCMT) enzyme family.

LCMTs selectively attach methyl chemical groups onto the catalytic subunit of protein phosphatase 2A (PP2A), a multi-protein complex that deactivates a variety of receptors and other signaling proteins. Accordingly, Wu, Kamiya and colleagues found evidence that SBI1 methylates PP2A within Arabidopsis cells. This chemical modification activates the complex and alters its localization within the cell, bringing it into close proximity to BRI1 and thereby enabling it to switch off the receptor. SBI1 production is directly stimulated by BR signaling, further reinforcing this negative feedback loop.

These newly inactivated receptor molecules appear to be subsequently targeted for destruction. “We found that activated that have bound BR transfer their signal to the nucleus and then get degraded, while unbound BR receptor is recycled rather than being degraded,” explains Kamiya. “By this mechanism, receptor levels are controlled in plants.”

Although similar regulatory systems are known to operate in animal cells, PP2A function is poorly understood in , and further investigation will be needed to determine whether this represents a general mechanism for constraining receptor signaling in Arabidopsis and other species.

Explore further: Plants on Steroids: Key Missing Link Discovered

More information: Wu, G., et al. Methylation of a phosphatase specifies dephosphorylation and degradation of activated brassinosteroid receptors. Science Signaling 4, ra29 (2011).

Related Stories

Plants on Steroids: Key Missing Link Discovered

September 8, 2009

(PhysOrg.com) -- Researchers at the Carnegie Institution's Department of Plant Biology have discovered a key missing link in the so-called signaling pathway for plant steroid hormones (brassinosteroids). Many important signaling ...

In plants, small changes make big impact

October 7, 2010

(PhysOrg.com) -- You can’t see them or feel them, but right now countless biochemical interactions in your body affect your life in countless ways. These interactions are important because if they go poorly, really bad ...

Mastermind steroid found in plants

November 15, 2010

Scientists have known for some time how important plant steroids called brassinosteroids are for regulating plant growth and development. But until now, they did not know how extensive their reach is. Now researchers, including ...

Nailing down a crucial plant signaling system

January 23, 2011

Plant biologists have discovered the last major element of the series of chemical signals that one class of plant hormones, called brassinosteroids, send from a protein on the surface of a plant cell to the cell's nucleus. ...

Recommended for you

Genomes uncover life's early history

August 24, 2015

A University of Manchester scientist is part of a team which has carried out one of the biggest ever analyses of genomes on life of all forms.

Rare nautilus sighted for the first time in three decades

August 25, 2015

In early August, biologist Peter Ward returned from the South Pacific with news that he encountered an old friend, one he hadn't seen in over three decades. The University of Washington professor had seen what he considers ...

Why a mutant rice called Big Grain1 yields such big grains

August 24, 2015

(Phys.org)—Rice is one of the most important staple crops grown by humans—very possibly the most important in history. With 4.3 billion inhabitants, Asia is home to 60 percent of the world's population, so it's unsurprising ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.