Gene expression in leaves of Theobroma cacao in response to mechanical wounding, ethylene, and/or methyl jasmonate

Abstract

Yield of Theobroma cacao (cacao), the source of chocolate, is limited by disease and insect pests. Developmental stage influences the resistance of cacao leaves to disease and insect pests. Mechanical wounding, ethylene, and methyl jasmonate induce resistance to pests in many plant species. The effects of mechanical wounding, ethylene, and methyl jasmonate on gene expression was studied in cacao leaves at two developmental stages, young red (YR) and mature green (MG). Differential expression was observed for genes putatively encoding a DNA binding protein ( TcWRKY-1), a protein regulating cell division ( TcORFX-1), a Type III peroxidase ( TcPer-1), an endo-1,4-β-glucanase ( TcGlu-1), a class VII chitinase ( TcChiB), a caffeine synthase ( TcCaf-1), and a light-harvesting complex protein ( TcLhca-1). Wounding induced TcWRKY-1 and TcORFX-1 in YR and MG leaves. Elevated TcPer-1 mRNA levels were detected in YR and MG leaves after wounding. Wounding induced TcChiB in YR leaves and repressed TcLhca-1 in MG leaves. Ethylene (12 μL L −1) induced TcPer-1 in YR and MG leaves but induced TcGlu-1 in MG leaves only. Ethylene repressed TcLhca-1 and TcCaf-1 in YR leaves. Ethylene repressed TcLhca-1 and TcChiB in MG leaves. Methyl jasmonate (0.2 mM) induced TcCaf-1 and TcChiB in YR leaves and TcPer-1 and TcChiB in MG leaves. Ethylene/methyl jasmonate combined induced TcChiB in YR leaves and TcGlu-1 in MG leaves. 1-Methylcyclopropene, an inhibitor of ethylene action, blocked ethylene induced responses but did not block responses to wounding or methyl jasmonate. The cacao response to wounding, ethylene, and/or methyl jasmonate was influenced by developmental stage. Cross-talk between ethylene and methyl jasmonate action on cacao gene expression resulted in synergistic and antagonistic responses. It is critical to account for tissue developmental stage when studying the molecular responses of cacao to mechanical wounding, ethylene, and methyl jasmonate. The constituative and inducible defense strategies used by cacao are dependent on the developmental stage of the tissues involved.