Avoiding Salt

In a newly identified tropism, plant roots steer clear of salinity.

By | January 1, 2014

BYPASS THE SALT: The roots of Arabidopsis seedlings (controls on left) change their downward course when they encounter salinity (right). Colors have been enhanced.CARLOS GALVAN-AMPUDIA

EDITOR'S CHOICE IN PHYSIOLOGY

The paper
C.S. Galvan-Ampudia et al., “Halotropism is a response of plant roots to avoid a saline environment,” Curr Biol, 23:2044-50, 2013.

The context
Gravity guides the course of root development in plants through a process called gravitropism. Salt may also change the architecture of plants’ root systems, but until now, it was unclear whether this response was directional—a true tropism—or an effect of salt that somehow made the roots less responsive to gravity.

The finding
A team led by Christa Testerink of the University of Amsterdam grew Arabidopsis and tomato seedlings in agar prepared with a diagonal salt gradient. The roots of the seedlings changed direction when they encountered the salinity, a response the group termed halotropism. “We could see that they are not randomly following gravity, but really growing away from the salt,” said Testerink. She added that this is the first time that scientists have observed “that a toxic substance, like salt, could cause a tropism.”

The mechanism
The researchers showed that salinity induced endocytosis of the cell membrane protein PIN-FORMED 2 (PIN2), a regulator of auxin transport, on the side of the root encountering the salt. Consequently, concentrations of auxin, a hormone that helps determine the direction of root growth, were redistributed, and the direction of root growth changed course.

The implications
The paper “highlights the need to really look at many environmental stresses . . . at this high spatial resolution,” says plant biologist José Dinneny of the Carnegie Institution for Science in California. “It may be that a lot of stimuli—nutrients, heavy metals—induce similar tropisms. Simply, people haven’t looked carefully enough at those stimuli to really know if that’s the case.”

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