Widely targeted metabolic, physical and anatomical analyses reveal diverse defensive strategies for pseudobulbs and succulent roots of orchids with industrial value

Abstract

Storage in pseudobulbs and succulent roots is vulnerable to attack from herbivores while play an important role in industrial utilization of orchids. However, we know little about the defensive strategies of pseudobulbs and succulent roots. Here, we characterized the possible physical and chemical defenses of pseudobulbs and succulent roots of five orchids with industrial value (Bletilla striata, Dendrobium chrysotoxum, Cymbidium tracyanum, C. sinense, and Acampe rigida) using X-ray micro-computerized tomography scanning and optical microscope observation as well as widely targeted metabolic profiling. We found that pseudobulbs and succulent roots of different orchid species combine different physical and chemical defensive strategies, and the relationship between defense and storage functions depend on species and age. Of the pseudobulbs of five species, C. tracyanum recruited the strongest defensive strategies, consisting of a thick epidermis, larger proportion of needle-like calcium oxalate crystals, and higher content of alkaloids and quinones. The pseudobulbs of C. sinense had a higher proportion of vascular bundle, and those B. striata had the strongest tissue biomechanical resistance. However, pseudobulbs of D. chrysotoxum were the most vulnerable to herbivores with relatively higher nutrient content and weaker defensive protection. For the succulent roots of five species, A. rigida had the strongest storage ability, and correspondingly the most chemical defensive components, while a thicker velamen and higher proportion of needle-like calcium oxalate crystals were found in the roots of C. tracyanum and C. sinense respectively. Our results suggest that diverse combinations of defensive strategies play a role protecting pseudobulbs and succulent roots of orchids from biotic stress. These findings contribute to the cultivation management of orchids with industrial value.