Hydrotropism

Hydrotropism (hydro- "water"; tropism "involuntary orientation by an organism, that involves turning or curving as a positive or negative response to a stimulus")[1] is a plant's growth response in which the direction of growth is determined by a stimulus or gradient in water concentration.

Recent identification of a mutant plant that lacks a hydrotropic response may help to elucidate its role in nature.

[4] This behavior is thought to have been developed millions of years ago when plants began their journey onto dry land.

[7] However until very recently, only within the last decade, have scientists found a likely receptor in root caps for signals of water potential gradients.

Receptor-like kinases (RLKs) appear to be responsible for this sensing of water potential gradients because of their apt location in the cell membranes of root caps as well as their interactions and effect on a type of aquaporin water channel known as plasma membrane intrinsic protein (PIP).

[9][10] Dietrich hypothesizes that a signal of lower water potential likely affects the interaction between the PIPs and RLKs resulting in differential cell elongation and growth due to fluxes in abscisic acid (ABA) and its following pathways.

The mutants were found to have decreased hydrotropic responses such that their root growth towards higher water potentials was not significant.

After application of ABA, however, heightened responses of root growth towards higher water potentials were observed.

This leads us to believe that hydrotropic response can counteract the gravitropic desire to move toward the center of the Earth and allows root systems to spread toward higher water potentials.

These findings also raised the question of how plants distinguish the vibrations produced by water in comparison to other environmental factors, such as insects or wind.

[17] ABA treatment, in addition to blue light irradiation, and stressful environment conditions, increase MIZ1 expression in plants.

In gravitropism, the gradient between cytosolic and apoplastic calcium levels plays a large role in initiating a physiological response in other tropisms, and it is hypothesized that a similar process occurs in hydrotropism.