Lipases and unsaturated free fatty acids mediate the defense response in corn (Zea mays)
The feeding of an insect herbivore on a corn (Zea mays) leaf triggers the plant’s defense response, resulting in 1) the release of green leaf volatiles (GLV), 2) the accumulation of jasmonic acid (JA), 3) the emission of herbivore-induced plant volatiles (HIPV) and 4) the activation of other defense response measures like the production of proteinase inhibitors and toxic secondary metabolites. GLV are immediately released from all plant leaves upon tissue damage and disperse into the atmosphere. GLV can be sensed by systemic leaves and intact plants nearby and prepare them against the impeding danger. Plants that perceived such a GLV signal previously, often show increased JA accumulation and HIPV emission when actually attacked by an insect herbivore. JA is the major regulator of plant defense response against insect herbivores. JA is biosynthesized from an unsaturated fatty acid, α-linolenic acid (LnA) in chloroplasts, where LnA is bound to membrane lipid as an acyl chain. To finish the biosynthesis of JA, LnA and its derivatives need to be released from the membrane lipids and enter peroxisome for final processing. In corn, the lipase(s) that cleave(s) LnA and its derivatives remain(s) unknown. Our previous works discovered three lipase-like proteins (lipase I, II and III) whose expressions were up-regulated shortly after GLV exposure. Given their responses to GLV exposure, we investigated these three lipase-like proteins for their potential involvement in the biosynthesis of JA and their roles in the defense response. All three proteins showed lipase-like activities toward para-nitrophenyl myristate and, lipase I showed sn-2 positional specificity toward monogalactosyl diacylglyceride (MGDG). In addition, three lipases responded to biotic stresses, demonstrated as transcript accumulations after insect elicitor (IE) treatment, mechanical wounding, GLV exposures and treatments with JA, salicylic acid (SA), LnA and palmitoleic acid (PeicA). Also, we discovered that in response to GLV exposure, unsaturated free fatty acid level increased in both monocotyledon (monocot) and dicotyledon (dicot) plants, implying a common signaling pathway mediated by the unsaturated free fatty acids. We established a positive correlation between LnA and JA, where elevated level of LnA increased JA accumulation after IE treatment, while reduced LnA levels (via physical movement of the plants) decreased JA accumulation after IE treatment. Furthermore, we demonstrated a priming effect of the non-jasmonate producing unsaturated free fatty acid like PeicA acid and γ-linolenic acid (gLnA), by showing that the incubation of plant with these unsaturated fatty acid led to enhanced HIPV emission when they were challenged with IE. Our results shed new light to GLV priming effect by showing 1) GLV exposure increased unsaturated free fatty acids in both monocot and dicot plants, 2) the increased level of unsaturated free fatty acids have priming effect, similar to GLV and 3) the presence of non-jasmonate producing unsaturated fatty acids primed the defense response, rather than incorporated into JA biosynthesis pathway.