Regulation of endogenous phytohormones and essential metabolites in frankincense-producing Boswellia sacra under wounding stress

Abstract

Boswellia sacra is an economically and ecologically important frankincense-producing tree, which is wounded to extract the aromatic resin. However, the underlying physiological mechanisms following this wounding stress are unknown. Our current goal was to elucidate the regulation of key physio-molecular determinants of wounded and preserved B. sacra populations. Wounding caused a twofold increase in calcium/magnesium content and a reduction in essential macronutrient (nitrogen) levels. Total amino acids were also reduced 1.74-fold; however, the levels of γ-amino butyric acid, hydroxyl-proline, and β-alanine were significantly higher (1- to 2.2-fold). In contrast, the fatty acids (linolenic, palmitic, stearic, and linoleic acids) were significantly higher in the preserved trees. Endogenous jasmonic acid (JA) was also significantly higher (67%) in the wounded trees, as was the expression of the JA-related genes allene oxide synthase and allene oxide cyclase. A similar twofold increase in stress-responsive abscisic acid was observed in the wounded trees. However, salicylic acid was down-regulated, and the pathogenesis-related genes PR1 and PR3 exhibited varying responses in the wounded plants. The presence of physiologically active (GA1, GA4, and GA3) and inactive (GA12, GA19, and GA20) gibberellins in both the wounded and control trees revealed similarity with the known GA biosynthesis in B. sacra. Both GA4 and GA3 were each significantly synthesized, which were buoyed by the increased expressions of ent-copalyl diphosphate synthase, cytochrome P450 monooxygenases, and gibberellin 20 oxidases under wounding stress. In conclusion, B. sacra responds to extraction of resin by regulating essential endogenous hormones and related transcripts, which in return retard tree growth and development.