Managing Biotic and Abiotic Stress at Chromosomal Level in Citrus: A Comprehensive Review

Citrus is a globally important fruit crop, widely valued for its nutritional and economic significance. However, citrus production is increasingly constrained by a multitude of abiotic stresses such as salinity, drought, heat, and boron toxicity as well as biotic pressures including Huanglongbing (HLB), citrus tristeza virus (CTV), root rot, and nematode infestations. These stressors severely affect plant growth, yield, and fruit quality. This review provides a comprehensive examination of genetic, physiological, and molecular strategies aimed at enhancing citrus resilience to these challenges. The manuscript highlights the pivotal role of genetic transformation in citrus, where genes sourced from other plants, microorganisms, and even insects have been utilized to create transgenic varieties with improved stress responses. Although biotic stress resistance has seen more genetic intervention than abiotic, the advent of genome sequencing and gene cloning technologies is bridging this gap. Polyploidy, particularly in rootstock development, is explored as a powerful tool for improving tolerance to salinity, drought, and pathogen attack. Tetraploid and allotetraploid rootstocks have demonstrated superior water-use efficiency, ion exclusion capacity, and reduced oxidative damage under stress. The review further discusses methods of gene isolation—including map-based cloning, homology-based cloning, and transcriptomics and their roles in identifying candidate genes responsible for stress tolerance. Several key genes, such as p5cs, HAL2, CS-ACS1, and others, have been linked to tolerance mechanisms against drought, salt, and temperature extremes. Despite promising advancements, challenges remain in field validation, gene functional characterization, and integration of stress-tolerant traits into commercial cultivars. The public availability of citrus genome data is expected to accelerate research in gene discovery and trait development. This review underscores the necessity of combining traditional breeding, biotechnology, and genomic tools to secure sustainable citrus cultivation amid evolving climate and pest threats.