Histological dissection of cutting-inducible adventitious rooting in Platycladus orientalis reveals developmental endogenous hormonal homeostasis

Abstract

De novo root regeneration in stem cutting is essential for massive propagation of Platycladus orientalis (L.) Franco, which is one of the major ecological and economic tree species in China. However, the details of adventitious root (AR) formation and how it is controlled by inherent hormones are still ambiguous. In this study, histological analysis, immunohistochemical localization combined with quantitative hormonal analysis were conducted to investigate how cutting induces the AR formation of P. orientalis. It was explicitly seen that AR emerged from either calli- or non-calli- covered barks. The anatomical structure showed that cutting initiated the inducible formation of AR primordia in multiple sites, including xylem, phloem, medullary ray, and annulus randomly. The tissue-specific accumulations of indole-3-acetic acid (IAA) surged in cambia-derived calli, basal boundaries of AR primordia, and AR, supporting the idea that the mode action of auxin was key to diverse types of root organogenesis. Meanwhile, the data showed that similar to that of IAA, the contents of abscisic acid (ABA) and zeatin (ZT) peaked at early stages of AR formation and deceased gradually in both phloem and xylem across the later stages, suggesting their indispensable roles in AR initiation. On contrary, jasmonic acid (JA) roughly maintained at extremely high levels in phloem comparing with the xylem tissues thoroughly, indicating its presence was involved in phloem-specific reorganization during AR formation. Collectively, the results provide a time-course histological dissection of AR formation, revealing developmental homeostasis of four main endogenous hormones, and providing theoretically a potential way to improve performance of stem-cutting in P. orientalis through regulating hormone actions in the future.