Biosynthesis for phenylphenalenone-type compounds in Haemodoraceae plants and their phytopathological role in the interaction Musa : Mycosphaerella fijiensis
Phenylphenalenones are polycyclic phenolic natural products occurring in the four monocotyledoneous plant families Musaceae, Haemodoraceae, Pontederiaceae and Strelitziaceae. Previous biosynthetic studies have hypothesized that the substitution pattern at the lateral phenyl ring attached to the phenalenone nucleus depends on the intact incorporation of the precursor used. Feeding experiments performed in Anigozanthos preissii with 13C-labeled ferulic acid and the unusual phenylpropanoid 4-methoxycinnamic acid were incorporated into phenylphenalenones bearing the same substitution at the phenyl ring as the precursor. 4'-O-methylanigorufone is reported here for the first time as a natural product.Little is known about the role of the phenylphenalenones in plants of the Haemodoraceae family. However, plants of the Musaceae (banana) family provided a good experimental system for exploring the ecological role of these metabolites in plant chemical defense. The level of their biosynthesis has been correlated to some extent with the resistance against pathogens and pests. Therefore, the metabolic changes in the chemical responses of the susceptible 'Williams' and the resistant 'Khai Thong Ruang' ('KTR') Musa varieties were studied. Mycosphaerella fijiensis, the causing agent of the most devastating disease in banana crops named Black Leaf Sigatoka Disease (BLSD), was used as the fungal pathogen. Phenylphenalenones were identified as the major induced metabolites in the infected tissue. The number and turnover of phenylphenalenones biosynthesized in the local infected tissue together with the slow progression of the disease was correlated with the resistance level observed for the 'KTR' Musa variety. However, a virulent strain of M. fijiensis was able to overcome plant resistance by converting fungitoxic phenylphenalenones to sulfate conjugates. Phenylphenalenone sulfates are reported here as metabolic detoxification products to evade chemical defense in Musa.