Developmental Cell: Reveal the molecular mechanism of salt avoidance in plant roots

The salinization of soil in China leads to insufficient arable land, which affects the effective supply of
food.
Exploring the sensing mechanism of plants to salt stress and elucidating the strategies for plants to adapt to salt stress will provide new ideas and molecular targets for the genetic improvement of crop stress resistance, which has important theoretical significance and practical application value
.

The salinization of soil in China leads to insufficient arable land, which affects the effective supply of
food.
Exploring the sensing mechanism of plants to salt stress and elucidating the strategies for plants to adapt to salt stress will provide new ideas and molecular targets for the genetic improvement of crop stress resistance, which has important theoretical significance and practical application value
。 On October 14, the Zhao Yang Research Group of the Center for Excellence in Molecular Plant Science, Chinese Academy of Sciences, published a research paper
entitled Root twisting drives halotropism via stress-induced microtubule reorientation online in Developmental Cell 。 This study elucidated the mechanism of ABA-activated phosphorylation of SnRK2 protein kinase to modify microtubule-binding protein SP2L to mediate apical salt avoidance, which provided a new strategy and molecular target for the cultivation of stress-resistant and stable yield crops
.

Living beings have the ability to
seek advantages and avoid harms.
Single-celled paramecium has the property of tending to favor stimuli while avoiding noxious stimuli; Animals move to escape hazards and other adverse environments
.
Although plants cannot move like animals, they can grow in response to environmental stimuli in a directional manner, thus avoiding adverse environments, a phenomenon called tropic movement
.
The growth direction of plant organs facing the stimulating side is positive growth, and the negative growth
is negative for the back stimulating side.
The negative growth of the root tip to avoid the high concentration of salt ions in the soil is called halotropism
.
The salt distribution in the soil is uneven, and the salt damage in deep soil is more serious
than in shallow soil.
Therefore, salt avoidance is one of
the important strategies for plants to cope with salt stress.
It plays an important role
in improving plant salt tolerance by enhancing the salt avoidance of plant roots and reducing the damage to plants in the high salt environment of the soil from the source.
However, the cytological processes and molecular mechanisms of salt avoidance at plant root tips are unclear
.

The researchers constructed a separator research system that simulated the gradient distribution of soil salt concentration, and observed the salt-avoidance growth
of the root tip of the model plant, Arabidopsis.
thaliana.
It was found that the ABA concentration at the root tip increased rapidly during the salt avoidance response.
ABA synthesis mutant nced3/5, ABA receptor duoplex mutant pyls and ABA signaling core protein kinase triple mutant SNRK2.
2/3/6 had defects of apical salt avoidance and cell extension direction change in root tip transition region, indicating that plant root salt avoidance depended on ABA-mediated change
of cell extension direction in the root tip transition region.

Through the selection of mutant salt-avoidance phenotype, the microtubule-binding protein mutant SP2L-4 was found to have a loss of
salt-avoidance.
Salt stress rapidly induced the rearrangement of the microtubule skeleton, and the treatment of microtubule depolymerizer and the microtubule cleavage protein mutant LEU1 led to salt avoidance defects, indicating that the rearrangement of microtubules controlled apical salt avoidance
.
However, salt stress under the background of sp2l-4 mutant could not induce microtubule rearrangement, indicating that the microtubule-binding protein SP2L controlled salt-induced microtubule rearrangement
.
ABA signaling core protein kinase SnRK2.
6 interacts with SP2L protein and phosphorylates serine at position 406 of SP2L; salt and ABA induce phosphorylation modification of serine at position 406 of SP2L in vivo; simulating the inactivation of this phosphorylation site (S406A) cannot complement the sp2l-4 salt avoidance loss phenotype
.
The above results confirmed that salt-induced phosphorylation modification of serine at position 406 of SP2L in vivo mediates plant root salt avoidance
at biochemical and genetic levels.

The microtubule skeleton is connected to the cellulose synthase CesA complex through CSI protein, which affects the arrangement of cellulose microfibrils in the cell wall and regulates the direction
of cell growth.
The cesa1, cesa3, cesa6 and CSI mutants all exhibited salt avoidance defects and salt stress-mediated cell anisotropy extension defects, indicating that SP2L-mediated microtubule rearrangement affected the arrangement of cellulose microfibrils, controlled the direction of cell anisotropic extension, and drove plants to avoid salt growth
.

Salt stress activates ABA-dependent protein kinase SnRK2.
6 and phosphorylates the microtubule-binding protein SP2L, thereby regulating microtubule arrangement and redirection, guiding the arrangement of cellulose microfibrils, controlling the anisotropic extension direction of cells in the root tip transition region, and driving root cells to curl to produce root salt avoidance
.
The study revealed the mechanism
of action of plant root salt avoidance at the biochemical, cellular and genetic levels.
The research work was supported
by the National Natural Science Foundation of China, the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences, the Shanghai Municipal Science and Technology Commission, and the Shanghai Research Center for Plant Stress Biology of the Chinese Academy of Sciences.
Researchers from the University of Copenhagen in Denmark, Purdue University in the United States and Southern University of Science and Technology also made important contributions
to this research.

ABA and SP2L mediate root salt avoidance