Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance

Abstract

Abiotic stresses, such as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalty worldwide. It is getting imperative to equip crops with multistress tolerance to relieve the pressure of environmental changes and to meet the demand of population growth, as different abiotic stresses usually arise together in the field. The feasibility is raised as land plants actually have established more generalized defenses against abiotic stresses, including the cuticle outside plants, together with unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes inside cells. In stress response, they are orchestrated by a complex regulatory network involving upstream signaling molecules including stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors, particularly transcription factors. In this review, we aimed at presenting an overview of these defensive systems and the regulatory network, with an eye to their practical potential via genetic engineering and/or exogenous application.

Keywords: abiotic stresses; general defenses; land plants; multistress tolerance; regulatory network.

FIGURE 1

The general defense systems and the underlying regulatory network in botanic responses to abiotic stresses. Different abiotic stresses, such as cold, heat, drought, flood, and salt can provoke common cellular disorder and secondary stresses, including membrane injury, reactive species (RS) damage, protein denaturation, and osmotic stress, which are also interconnected with each other. Accordingly, land plants have resorted to unsaturated fatty acids, RS scavengers, molecular chaperones, and compatible solutes. Some compatible solutes may also be involved in counteracting other adverse effects, as indicated with dotted inhibitory lines. Besides, the cuticle serves as the universal outermost shield. Upon stress stimulation, signaling molecules mobilize the downstream effectors, primarily protein kinases and transcription factors, leading to altered gene expression and protein/enzyme activities, thereby launching the defense systems. Notably, phytochrome B (PHYB) is emerging as a negative regulator in stress tolerance. 18:3, linolenic acid; APX, ascorbate peroxidase; GST, glutathione S-transferase; HSP, heat shock protein; Pro, proline; GB, glycine betaine; ABA, abscisic acid; PAs, polyamines; MAPK, mitogen-activated protein kinase; DREB, dehydration responsive element binding factor.

FIGURE 2

Crosstalk between signaling molecules focused in the review in botanic responses to abiotic stresses. Once triggered, abscisic acid (ABA), H₂O₂, H₂S, NO, polyamines (PAs), phytochrome B (PHYB), and Ca²⁺, extensively interplay with others at various levels, synergistically or antagonistically. For simplification, the two effects are shown in combination. Dashed line is used between PHYB and H₂O₂ as PHYB is emerging to play a negative role in its scavenging. Of note, H₂O₂, H₂S, NO, and PAs can actually block each other via chemical reaction, though not indicated.