A tomato enzyme synthesizes (+)-7-iso-jasmonoyl-l-isoleucine in wounded leaves

Jasmonoyl-l-isoleucine (JA-Ile) is a key jasmonate signal that probably functions in all plant species. The JASMONATE RESISTANT 1 (JAR1) enzyme synthesizes JA-Ile in Arabidopsis [Arabidopsis thaliana (L.) Heynh.], but a similar enzyme from tomato [Solanum lycopersicum (L.)] was not previously described. Tomato SlJAR1 has 66% sequence identity with Arabidopsis JAR1 and the SlJAR1-GST fusion protein purified from Escherichia coli catalyzed the formation of JA-amino acid conjugates in vitro. Kinetic analysis showed the enzyme has a strong preference for Ile over Leu and Val and it was about 10-fold more active with (+)-7-iso-JA than with its epimer (-)-JA. Leaf wounding rapidly increased JA-Ile 50-fold to about 450 pmol g(-1) FW at 30 min after wounding, while conjugates with Leu, Phe, Val and Met were only marginally increased or not detected. Nearly all of the endogenous JA-Ile was the bioactive epimer (+)-7-iso-JA-Ile and there was no evidence for its conversion to (-)-JA-Ile up to 6 h after wounding. A transgenic RNAi approach was used to suppress SlJAR1 transcript that reduced JA-Ile accumulation by 50-75%, suggesting that other JA conjugating enzymes may be present. These results show that SlJAR1 synthesizes the bioactive conjugate (+)-7-iso-JA-Ile and this is the predominant isomer accumulated in wounded tomato leaves.