Involvement of Ethylene in Adventitious Root Formation of Red-Stalked Rhubarb In Vitro

Irregular rooting in vitro is a major problem in the micropropagation of culinary rhubarb (<i>Rheum rhaponticum</i>), a vegetable crop rich in bioactive compounds. To date, little is known about the factors and mechanisms underlying adventitious root (AR) formation in rhubarb under in vitro conditions. Here, we studied the effects of indole-3-butyric acid (IBA) and its interaction with ethylene (ET) on AR development in rhubarb 'Raspberry' selection. To evaluate the ET-effect, we applied a precursor of ET biosynthesis-1 aminocyclopropane-1-carboxylic acid (ACC); an inhibitor of ET synthesis-aminoethoxyvinylglycine (AVG); and an inhibitor of ET action-silver nitrate (AgNO₃). The best results (96.9% rooting frequency, 12.7 roots/shoot) were obtained after adding ACC to the IBA-containing medium. The positive effect of ET was linked to decreased levels of cytokinin and auxins in the rhubarb shoot bases at the initiation and expression stages of rooting. Moreover, the enhanced expression levels of genes involved in auxin signalling and homeostasis (<i>IAA17</i>, <i>GH3.1</i>) and ABA catabolism (<i>CYP707A1</i>) were observed. The blocking of ethylene synthesis significantly increased JA production, and the rooting frequency decreased to 29.8%. The presence of AgNO₃ in the auxin medium resulted in a significant reduction in root number, which was consistent with the enhanced levels of ABA and the expression of genes related to ABA biosynthesis and signalling (<i>PP2C49</i> and <i>CBF4</i>), as well as ET synthesis (<i>ACO5</i>).