Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding.
Stimuli like wounding can cause tension and compression of plant tissues, which pinches the cross section of the shoot.
Hydraulic signaling begins with a local response like water expulsion, creating a suction in the vascular system.
[4] In general terms, long distance signaling is defined as the ability to have a widespread response when just one distinct area is stimulated.
[1] Hydraulic signals can be propagated downward or upward, relaying water potential gradients throughout the entire plant.
[2] These synthetic plant systems are made from polydimethylsiloxane (PDMS) and 3D molded like branches and filled with a silicone oil (with viscosity 1 Pa*s).
Hydraulic pulses are then induced by automated linear motor deplacement, creating a bend in the synthetic branches which results in a rise in overpressure (Pref), reaching a value of |Pref-P0| or deltaP.
Therefore, the mechanism of generating hydraulic pressure is due to the coupling of bending and the transverse deformation of the elastic beam.
[5] Hydraulic signals are primarily detected as decreases in water potential,[1] usually caused by increases in solute concentration or drought.
The hydraulic sensor, which is yet to be known, resides on the inner membrane of the parenchyma cells and detects the decreases in pressure and solute potential through an unknown mechanism.
After detection, the unidentified sensor initiates a signal cascade, leading to a calcium transient and subsequent reactive oxygen species (ROS) formation.
In an example, ABA response to a hydraulic signal from the roots- a decrease in water potential- is thought to reach the guard cells to stimulate stomatal closure.
Despite an unidentified hydraulic sensor(s) and the mechanism of which this sensor detects decreases in pressure and solute potential in the parenchyma, this primary site of ABA biosynthesis is thought to additionally participate as the main location of hydraulic signal perception, vital to mediation of water potential in the plant.
ABA triggers root growth at low concentrations and closes stomata to prevent water loss from transpiration.
One candidate for a hydraulic signal sensor has been MCA1,[6] a plasma membrane protein correlated with mechanosensing via calcium-mediated influx in Arabidopsis thaliana.
Research has found that MCA1 increased cytoplasmic calcium concentrations in response to a mechanosensory input: plasma membrane distortion in Arabidopsis.
PERK4 specifically plays a crucial role in abscisic acid (ABA) signalling and response and has shown to be an ABA- and calcium-activated protein kinase.
[8] Proline-rich extensin-like receptor kinases 4 (PERK4) is a gene expressed in the roots and flowers in Arabidopsis thaliana that localizes in the plasma membrane and plays a role in ABA signaling.
The function of PERK4 has been proposed in early stage ABA signalling to inhibit root elongation by disturbing cytoplasmic calcium gradients.