CRISPR-based genome editing: Catching impossibles for citrus improvements

Abstract

Citrus is globally one of the major fruit crops, occupying a place of prominence in international trade and tariff through horticultural commodities. Despite such distinction, citrus crop is confronted with a variety of biotic and abiotic stresses, thereby, sustaining production is always a daunting task. The genome size of citrus is rather small, ranging from 265 to 400 MB, probably an advantage for controlled trait specific editing. The evolution of next generation sequencing has facilitated the whole genome sequencing of as many 10 citrus species with 16 draft genome sequences, offering near future possibility to develop genome tailored citrus species or inducing the desired genetic transformation to address the issues chronically ailing commercial citrus cultivation in India, which is by no mean, a simple task to accomplish. Despite genetically intrinsic challenges involved in generating transgenics in perennial crop like citrus, several transgenics have been developed in namely, sweet orange, lemon, and grapefruit loaded with some useful traits. But, the public perception and the time taken to develop trangenics in citrus and less success ratio led the researchers adapt alternate ways. Of late, the thumping success of genome editing tools, especially Clustered. Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas9 system has provided a new molecular tailoring machine for citrus improvement. In several citrus species like sweet orange, pummelo, and grapefruit CRISPR-Cas9 system has resulted in value added multiple traits-based transgenics. However, the major drawback of the CRISPR/Cas9 system is the generation of significant off-target cleavage sites as a result of complexing of gRNA with mismatched complementary target DNA within the genome. The use of CRISPR as genome editing technology is anticipated to induce many desired traits in citrus in years to come with more commercial applications in field for changed canopy structure, root traits, regular bearing, extended fruit maturity, besides multiple disease resistance.