Effect of Exogenous Abscisic acid (ABA) on the Morphology, Phytohormones, and Related Gene Expression of Developing Lateral Roots in ‘Qingzhen 1’

Lateral roots (LRs) are critical for plant stress tolerance and productivity. Understanding how hormones and genes interact in a fluctuating environment to coordinate LR development is a major challenge. Abscisic acid (ABA) is the primary stress-responsive hormone and mediates LR development in various plant species. However, the effect of exogenous ABA on LR development has not been elucidated in apple. In this study, ‘Qingzhen 1’ was treated with exogenous 5 µM ABA for 20 days to investigate the regulation mechanism of ABA on LR development. Morphological observations advocated that ABA inhibited both LR and shoot development in ‘Qingzhen 1’ apple plants, where the root number was 16.94%, the root length was 30.32%, the plant height was 10.88%, and the stem thickness was 8.08% lower than those in the control plants. Meanwhile, the endogenous ABA concentration was significantly increased, but the indole-3-acetic acid (IAA), zeatin riboside (ZR), and jasmonic acid (JA) concentrations were significantly decreased with ABA treatment. Furthermore, the expression levels of ABA-related genes ( MdCYP707A2 , MdABI1 , MdAREB2 , and MdABF3 ) were significantly upregulated, while the expression levels of auxin-related genes ( MdYUCCA3 , MdYUCCA8 , MdPIN1 MdPIN2, MdPIN3 , and MdARF19 ), root development-related genes ( MdWOX5 and MdWOX11 ), and cell cycle-related genes ( MdCYCD1;1 and MdCYCD3;1 ) were significantly downregulated at the early stage of ABA treatment, which act together on the inhibition of LR development. Taken together, the changes in hormone levels and gene expression resulted in inhibited LR development of apple plants in response to ABA.