Jasmonic acid and ethylene crosstalk : regulation of growth, defense and metabolisms in native tobacco Nicotiana attenuata in response to herbivory
Herbivore attack is a common biotic stress for plants. Therefore, the development of effective defenses against herbivory is an important fitness-enhancing strategy. Defense mechanisms are considered to be costly because plants need to distribute their limited energetic and nutritional resources between growth and development, and metabolically demanding defense process. Recently, phytohormone crosstalk has been proposed as a mechanism to interconnect growth and defense, implying that this crosstalk could play an important role in economically allocating resources between growth and defense under stress conditions. The phytohormones jasmonic acid (JA) and ethylene (ET) activate a broad range of plant defenses against herbivores in N. attenuata. Although JA-ET crosstalk is known to have both synergistic and antagonistic effects on plant defense, it was largely unknown how JA-ET crosstalk affects plant growth during herbivory. The influence of JA-ET crosstalk on growth of N. attenuata plants after simulated herbivory was observed in JA-deficient (asLOX3) and in ET-insensitive (mETR1) plants, and in their genetic cross (mETR1asLOX3). A novel morphological change, the development of callus-like cells around puncture wounds, was observed only in W+OS-treated mETR1asLOX3 plants. Moreover the accumulation of IAA in the elicited mETR1asLOX3 leaf was strongly increased compared to WT plants, and this increase coincided with the callus-like development. These results suggest that JA-ET crosstalk negatively regulates IAA biosynthesis, which can prevent unwanted plant cell growth during herbivory. JA-ET crosstalk not only affects to plant growth but also affects to specific plant defense metabolite, dicaffeoyl spermidine (DCS) biosynthesis. We identified a novel acyl transferase gene (DH29) in N. attenuata as a candidate for DCS synthase. Silencing DH29 gene expression resulted in strongly reduced accumulation of DCS after W+OS treatment in N. attenuata plants. Biochemical analysis of the recombinant DH29 protein resulted in the formation of monocaffeoyl spermidine (MCS). In conclusion, DH29 represents a novel spermidine monocaffeoyl transferase (SmCT) in N. attenuata, encoding the first biosynthetic enzyme for MCS, and suggesting that DCS biosynthesis in N. attenuata may occur in two reaction steps.