Role of brassinosteroids in alleviating toxin-induced stress of Verticillium dahliae on cotton callus growth.

Brassinosteroids are well known to mitigate biotic stresses; however, their role to induce tolerance against Verticillium dahliae is unknown. The current study employed V. dahliae (Vd) toxin as pathogen-free model system to induce stress on cotton callus growth, and its amelioration was investigated using 24-epibrassinolide (EBR). Results revealed that EBR has ameliorative effects against Vd toxin with greater seen effect when callus was treated with EBR prior to its exposure to Vd toxin (pre-EBR treatment) than EBR applied along with Vd toxin simultaneously (co-EBR treatment). Pre-EBR-treated calli remained green, while 65 and 90% callus browning was observed in co-EBR- and Vd toxin-alone-treated callus, respectively. Likewise, the fresh weight of the pre-EBR-treated callus was 52% higher than Vd toxin-alone treatment, whereas this increase was only 23% in co-EBR-treated callus. Meanwhile, EBR treatment of the cotton callus has also increased the contents of chlorophylls a and b, carotenoids, total phenols, flavonoids, soluble sugars, and proteins and increased the activity of enzymes involved in secondary metabolism like polyphenol oxidase (PPO), phenylalanine ammonialyase (PAL), cinnamyl alchol dehydrogenase (CAD), and shikimate dehydrogenase (SKDH) over Vd toxin-alone treatment with higher increments being observed in pre-EBR-treated callus. Furthermore, EBR treatment mimicked the DNA damage and improved the structure of mitochondria, granum, stroma thylakoids, and the attachment of ribosomes with the endoplasmic reticulum. This EBR-mediated mitigation was primarily associated with substantially increased contents of photosynthetic pigments and regulation of secondary metabolism.