Molecular insights into heat stress tolerance in citrus: Roles of antioxidant systems, carotenoids, cytochrome P450, and genome editing

Abstract

Climate changes have intensified the impact of heat stress and posing a significant threat to citrus production by affecting plant growth, fruit quality and yield. Citrus is a perennial fruit crop with a long juvenile phase and is especially sensitive to prolonged high temperature, which emphasis the need of development of heat resilience citrus cultivars with an urgency. Heat stress induces a network of physiological, biochemical and molecular processes in plants that operate to maintain cellular homeostasis and promote thermotolerance. Here, we reviewed the major possible physiological changes caused by heat stress in citrus, which leads to reduced photosynthesis, severe oxidative damage, membrane instability and water relations disturbance. Further, we discussed the molecular pathways involved in heat tolerance, such as abscisic acid (ABA) signal transduction, the generation of reactive oxygen species (ROS) and their detoxification, heat shock proteins (HSPs), heat shock factors (HSFs), and other stress response pathways. Particular attention is given to the roles of the metabolism of carotenoids and the gene family of cytochrome P450 (CYP450) in the antioxidant system, signaling stress, and maintaining cellular redox homeostasis. In the recent years, information resources on thermotolerance have been advanced by the identification of various candidate genes through genomics and functional studies, which are useful targets for crop improvement. CRISPR/Cas9 genome editing is also presented as a promising approach to modifying stress-induced genes with precision and efficiency to meet the urgent need to develop climate resilience in citrus cultivars, which also address the unturned stones of conventional breeding. Future integration of multi-omics, functional genomics and precision genome editing approaches will provide a powerful framework for developing the next-generation of citrus varieties that will be able to ensure sustainable citrus production under a changing climate.