*3.4. Abiotic Stress Tolerance*

*Brassica* tolerance towards key abiotic stresses such as heat, drought, cold, and salinity are critical for *Brassica* breeding programs. The physiological changes of canola and the signaling pathways involved in response to the various abiotic responses are often interconnected across the various abiotic stress types [146]. For example, the stress response genes such as the homeodomain-leucine zipper subfamily transcription factors play a role in drought tolerance of *B. napus*[147] and other abiotic stresses, supported by the findings of 113 homeobox genes being identified in Chinese cabbage as differentially expressed under multiple stresses such as cold and osmotic stress [148]. Other examples of genes related to multiple abiotic stresses include the membrane-bound FAD genes in canola that also play a role in improving oil quality. The FAD genes were found to be differentially expressed under cadmium and salinity stresses using qRT-PCR [80]. The FAD candidate genes identified from Xu et al. [80] are highly promising for breeding cadmium and salt tolerant canola. In addition, phytohormones such as abscisic acid (ABA) are commonly involved in abiotic stress response besides regulating plant growth and development [149]. Targeting the coding and transcription factor genes involved in the ABA-mediated signaling pathway and using these genes for creating transgenics *B. napus* that are tolerant towards drought, cold, osmotic, and salinity stresses is very promising [150]. Regulatory pathways of abiotic stress responses in *B. napus*, typically drought, salinity, extreme temperature, and cadmium toxicity, along with the gene families identified in *B. napus* were reviewed in detail by Lohani et al. [146] which highlighted multigenic engineering approach in coping with multiple abiotic stresses and CRISPR/Cas9 as a versatile tool to achieve this target. Mutant *B. napus* lines of paralogous genes of *Bna.RGA* family, orthologues of Arabidopsis REPRESSOR OF GA1-3 (RGA) genes functioning as repressor in gibberellin signaling, were generated via CRISPR/Cas9 with high efficiency [151] in which *bnaa6.rga-D* (gain-of-function mutant) presented enhanced drought tolerance and its stomata were hypersensitive to abscisic acid (ABA) signal, while *bnarga* (loss-of-function mutant) possessed reduced drought tolerance and less sensitivity to ABA treatment [152]. Using the CRISPR/Cas9 tool will efficiently exploit and elucidate roles of identified major genes/factors regulating stress responses in *B. napus*, providing qualified germplasm for breeding of abiotic stress tolerance.
