Revisiting carotenoids and their role in plant stress responses: From biosynthesis to plant signaling mechanisms during stress

Stress in plants can be defined as any external factor that negatively influences plant growth, productivity, reproductive capacity or survival. As with any living organism, plants have an optimal temperature range at which growth and crop yield are best. Plants also require a certain amount of water for optimal survival; too much water (flooding stress) can cause plant cells to swell and burst, whereas drought stress (too little water) can cause the plant to dry up-a condition called desiccation. If the temperature is too cold for the plant, it can lead to cold stress, also called chilling stress. Cold temperatures can affect the amount and rate of uptake of water and nutrients, leading to cell desiccation and starvation. Hot weather can affect plants adversely, too. Intense heat can cause plant cell proteins to break down, a process called denaturation. Cell walls and membranes can also ‘melt’ under extremely high temperatures, and the permeability of the membranes is affected. Other abiotic stresses are less obvious, but can be equally as lethal. In farming systems, the use of agrochemicals such as fertilizers and pesticides, either in excess or in deficit, can also cause abiotic stress to the plant. The plant is affected through an imbalance of nutrition or via toxicity. High amounts of salt taken up by a plant can lead to cell desiccation, since elevated levels of salt outside a plant cell will cause water to leave the cell, a process called osmosis. Plant uptake of heavy metals can lead to complications with basic physiological and biochemical activities such as photosynthesis. Soil salinization also affects plants’ osmotic potential and inhibits many of a plant’s cellular functions including photosynthesis and stomatal opening. Such different types of stresses can ultimately cause closure of stomata, disrupt the membrane-bound electron transport system, damage of photosynthetic machinery and the production of toxic active oxygen species. Over generations, many plants have mutated and evolved with different mechanisms to counter stress effects. These include a range of different mechanisms such as facultative inducible metabolic adaptations (i.e., excretion of organic acids; osmotic adjustment; accumulation of sugars, amino acids and polyols; induction of glycolytic enzymes; c-aminobutyrate (GABA) accumulation; induction of fatty acid desaturases and heat shock proteins; activation of phytochelatin synthase and metallothioneins; activation of alternative respiratory pathways; induction of polyamine synthesis; production of antioxidant enzymes such as superoxide dismutase, ascorbate peroxidase, catalase, monodehydroascorbate and glutathione reductases) and ecophysiological (carbon assimilation) adaptations such as increased isoprene synthesis, which includes the large and crucial group of carotenoids. Carotenoids are essential in different plant processes and are potential antioxidants during plant stress. They act as light harvesters, quenchers and scavengers of triplate state chlorophylls and singlet oxygen species, dissipators of excess harmful energy during stress condition and membrane stabilizers.