Abscisic Acid Inhibits Root Growth In Arabidopsis Through Inducing Ethylene Biosynthesis

Plants are faced with a variety of abiotic and biotic stresses in their entire life cycle. The sessile nature determines that they have to cope with the environmental changes only through adjusting their life activities. The plant resistance is formed in the process of gradually adaption to the environment. As an important organ, root not only bears the functions of absorption, transport and storage of nutrients, but also feels the endogenous signals and exogenous environmental stimuli, so that plants could grow and develop much better. Root morphogenesis directly affects the growth of the whole plant, therefore is closely related to yield. Especially in poor soil or stressful conditions, such as drought, the function of root is more important.During the whole life of plants, phytohormones play crucial roles. They interact with each other and co-regulate the physiological processes of plants. Abscisic acid (ABA) as an important phytohormone, regulates the growth and development in plant. Over a long period, it is known that high concentrations of ABA inhibit root growth. However, the molecular mechanism by which ABA regulates root growth remains unclear.In Arabidopsis, the previous studies showed that ABA inhibits root growth through ethylene signaling, but not ethylene biosynthesis. In this study, we found that ABA inhibits root growth by regulating ethylene biosynthesis in Arabidopsis. The ethylene biosynthesis inhibitor aminoethoxyvinyl glycine (AVG) reduced the inhibition of root growth by ABA, and multiple ACS (1-aminocyclopropane-1-carboxylate synthase) mutants were more insensitive to ABA in terms of root growth than was the wild type. In addition, it was shown that ABA induced the ethylene biosynthesis by the gas chromatography testing.ACC synthases, the key enzymes of ethylene biosynthesis pathway, are less abundant and unstable. Type1and type2ACS isozymes carry the predictive CDPK phosphorylation sites, and previous study showed that phosphorylation at specific sites by CDPKs could stabilize the protein production. Here, we reported that two ABA-activated CDPK protein kinases, CPK4and CPK11, phosphorylated the C-terminus of ACS6and stabilized ACS6protein. The transgenic plants expressing ACS6mutant form that mimics the CDPK phosphorylation produce more ethylene than dose the wild type, moreover, were more insensitive to ABA in terms of root growth and seeds germination greening than was the wild type.This study helps us to further understand the molecular mechanism by which ABA regulates root growth. It will also shed more light on the mechanisms of crosstalk between ABA and ethylene signaling.