Temporal profiling of physiological, histological, and transcriptomic dissection during auxin-induced adventitious root formation in tetraploid Robinia pseudoacacia micro-cuttings.

ABSTRACT

MAIN CONCLUSION: Optimal levels of indole-3-butyric acid (IBA) applied at the stem base promote adventitious root (AR) initiation and primordia formation, thus promoting the rooting of leafy micro-cuttings of tetraploid Robinia pseudoacacia. Tetraploid Robinia pseudoacacia L. is a widely cultivated tree in most regions of China that has a hard-rooting capability, propagated by stem cuttings. This study utilizes histological, physiological, and transcriptomic approaches to explore how root primordia are induced after indole butyric acid (IBA) treatment of micro-cuttings. IBA application promoted cell divisions in some cells within the vasculature, showing subcellular features associated with adventitious root (AR) founder cells. The anatomical structure explicitly showed that AR initiated from the cambium layer and instigate the inducible development of AR primordia. Meanwhile, the hormone data showed that similar to that of indole-3-acetic acid, the contents of trans-zeatin and abscisic acid peaked at early stages of AR formation and increased gradually in primordia formation across the subsequent stages, suggesting their indispensable roles in AR induction. On the contrary, 24-epibrassinolide roughly maintained at extremely high levels during primordium initiation thoroughly, indicating its presence was involved in cell-specific reorganization during AR development. Furthermore, antioxidant activities transiently increased in the basal region of micro-cuttings and may serve as biochemical indicators for distinct rooting phases, potentially aiding in AR formation. Transcriptomic analysis during the early stages of root formation shows significant downregulation of the abscisic acid and jasmonate signaling pathways, while ethylene and cytokinin signaling seems upregulated. Network analysis of genes involved in carbon metabolism and photosynthesis indicates that the basal region of the micro-cuttings undergoes rapid reprogramming, which results in the breakdown of sugars into pyruvate. This pyruvate is then utilized to fuel the tricarboxylic acid cycle, thereby sustaining growth through aerobic respiration. Collectively, our findings provide a time-course morphophysiological dissection and also suggest the regulatory role of a conserved auxin module in AR development in these species.