The Role of Calcium Ion and Autophagy in Regulating Hydrotropism in Roots: Insights from Arabidopsis thaliana

Abstract

Plants are sessile organisms with different mechanisms that intervene during water scarcity. One of them is hydrotropism, which consists of modifying the growth and movement of the root towards areas where there is more water. This movement is essential for the survival of the plant. Another mechanism is autophagy, described not only as a defense against drought but also as a catabolic process conserved in most eukaryotic cells, through which unwanted or dysfunctional cytoplasmic material is transported to the vacuole of plant cells for degradation and recycling. In the hydrotropic response, an accumulation of autophagosomes and H₂O₂ occurs in the zone of root curvature. For this reason, autophagy reduces oxidative stress caused by water stress during hydrotropism.

The calcium ion (Ca²⁺) is a second messenger that regulates stress reactions. When the cell perceives a stimulus such as water scarcity, there is a transient or sustained increase in the concentration of Ca²⁺ in the cytosol that triggers temporary cellular processes when detected by Ca²⁺-binding proteins such as calmodulin, located in the plasma membrane and in the membrane of cellular organelles. Root curvature during hydrotropism depends on Ca²⁺ accumulation in the cytosol through blockade of the Ca²⁺ ATPase ECA1 by the MIZ1 protein. Moreover, calmodulin-like protein 24 (CML24) potentially binds Ca²⁺ in response to abiotic stress, and by interacting with the autophagy protein ATG4, may affect its progress. In this work, a review of the basic concepts of hydrotropism, autophagy and Ca²⁺ flow was carried out with the intention of expanding knowledge about the interaction of these processes and the cellular defense of Arabidopsis thaliana against water scarcity.