Degraded Metabolites of Phlorizin Promote Germination of Valsa mali var. mali in its Host Malus spp.

Abstract

Plant pathogenic fungi are able to utilize the principal metabolites of their hosts, which is one reason pathogens can so seriously harm the plants, although the mechanisms behind this utilization are not always clear. Valsa mali var. mali is a pathogenic fungus specific to the plant genus Malus. The fungus can seriously endanger apple crops and has caused serious economic losses. Phlorizin (1), the principal component in the stems, roots, and leaves of Malus pumila and M. sieversii, was able to promote spore germination of Valsa mali var. mali (Vmm-30) significantly over 120-168 h in a non-nutritional suspension. Compared with the control, the concentrations of nine phenolic compounds (3-11) in the stems of M. pumila increased after inoculation with Vmm-30. Moreover, compounds 3, 4, and 9-11 were able to promote the germination of Vmm-30 spores over 24-36 h, which was a significantly shorter time than that of phlorizin. High-performance liquid chromatography with diode-array detection (HPLC-DAD) and ultraperformance liquid chromatography with tandem mass spectrometry (UPLC-MS/MS) analyses further suggested that compounds 2-11 were the degradation products of phlorizin (1) and are produced through carbon oxidation cracking, decarboxylation, and oxidation reactions. This suggests that the degradation of phlorizin is able to effectively promote the growth of Vmm-30. The Vmm-30 strain is therefore able to utilize the principal metabolite phlorizin to generate a series of degradation products, which further promote its germination and the infection of its host plants in the genus Malus.