Glucosinolates are a group of sulphur-rich thioglucosides known to play an important role in plant defense. The accumulation of indolic glucosinolates (IGS) is strong induced by jasmonate (JA). Many studies have demonstrated that coronatine-insensitive 1 (COI1) is important in JA signaling, and is involved in almost all the JA-responsive pathways. However, the underlying molecular mechanisms for JA-induced IGS accumulation remain partially understood. In this study, the coi1-22 and coi1-23 mutants and wild-type (WT) Arabidopsis thaliana plants were used to investigate the role of COI1 in MeJA-induced IGS metabolism.
The effect of MeJA treatment on IGS contents in coi1-22 and coi1-23 plants was analyzed. The results showed that the contents of IGS were significantly increased in MeJA-treated WT plants compared with the control, while the IGS levels were not changed notably in MeJA-treated coi1-22 and coi1-23 plants, indicating that the accumulation of IGS is insensitive to MeJA treatment in the mutants. Thus, coi1-22 and coi1-23 are good materials for studying the mechanisms of JA-induced IGS biosynthesis. Transcription factors are important regulators on the expression of genes. The R2R3-MYB transcription factors ATR1/MYB34 and HIG1/MYB51 have been reported to act as the direct biosynthetic transcriptional activators for IGS, and they regulate many IGS biosynthetic genes and the tryptophan (IGS precursor) biosynthetic genes. The expression levels of MYB34 and MYB51 in MeJA-treated coi1-22, coi1-23 and WT plants were analyzed. The results showed that in WT plants, the expression levels of MYB34 and MYB51 were significantly higher in MeJA-treated plants than the control after 3 h MeJA-treatment, while it is not the case in coi1-22 and coi1-23 plants, indicating that the loss of function of COI1 blocks the MeJA-induced MYB34 and MYB51 expression. Furthermore, it is interesting to find that the expression levels of MYB34 in coi1-22 and coi1-23 mutants were significantly higher than in the control plants, suggesting that another signal pathway other than COI1 might be involved in JA-regulated MYB34 expression. Moreover, the expression levels of tryptophan biosynthetic genes ASA1, TSA1 and TSB1 and IGS biosynthetic genes CYP79B2, CYP79B3 and CYP83B1 were further analyzed. The results showed that the induced expression of ASA1, TSA1, TSB1, CYP79B2, CYP79B3 and CYP83B1 was substantially attenuated in two coi1 mutants after 3 h of MeJA treatment, indicating that the loss of function of COI1 blocks the MeJA-induced tryptophan biosynthetic genes and IGS biosynthetic genes expression, and the expression of ASA1, TSA1, TSB1, CYP79B2, CYP79B3 and CYP83B1 might be regulated by COI1 via MYB34 and MYB51. Lastly, the expression levels of ASA1, TSA1 and TSB1 in coi1-22 and coi1-23 mutants after 6 h of MeJA treatment and those of CYP79B2 and CYP79B3 in coi1-22 and/or coi1-23 after 24 h of MeJA treatment were significantly higher than in the control plants, suggesting that another signal pathway other than COI1 might also be involved in the JA-regulated expression of them.
In conclusion, COI1 is involved in JA-regulated biosynthesis of indolic glucosinolates in A. thaliana. JA induces the expression of tryptophan biosynthetic genes ASA1, TSA1 and TSB1 and IGS biosynthetic genes CYP79B2, CYP79B3 and CYP83B1 through MYB34 and MYB51 via COI1. In addition, another pathway might exist in JA-induced IGS biosynthesis when COI1 is not functional.