Triggered in distress: a miRNA-controlled switch for drought-induced ABA biosynthesis in rice

Abstract

Abiotic stress conditions such as drought, salinity, and flood challenge agricultural production worldwide by adversely affecting crop growth and yield. Plants inherently possess sophisticated response mechanisms for adjusting their physiology to improve survival under stress conditions. Understanding these mechanisms and enhancing them through breeding and biotechnological means are important steps toward developing crop varieties that can perform well under unpredictable climatic extremities of the future.

Abscisic acid (ABA) is a key phytohormone that mediates stress responses in all land plants. Stress conditions induce ABA biosynthesis through a series of steps that occur in plastids and cytosol, and the rate-limiting step in this process is catalyzed by enzymes known as 9-cis-epoxycarotenoid dioxygenases (NCEDs) (Nambara and Marion-Poll, 2005). ABA activates a signaling cascade featuring a set of receptors that function redundantly, type 2C protein phosphatases (PP2Cs) that act as co-receptors, sucrose non-fermenting 1-related protein kinase 2 subfamily proteins (SnRK2s) that channel the signal from cytosol to nucleus, and an array of transcription factors that mediate stress-responsive gene expression (Chen et al., 2020). Since elevated levels of ABA can often impart negative effects on growth, a tight modulation of ABA biosynthesis and signaling is necessary to balance growth and stress responses (Chen et al., 2020). In addition to transcriptional regulation, the levels and activities of ABA signaling components are optimized at the post-transcriptional level by miRNAs and at the protein level through post-translational modifications (Nadarajah and Kumar, 2019; Zhang et al., 2019; Chen et al., 2020). However, the precise mechanisms by which ABA biosynthesis is rapidly induced under stress conditions and turned off immediately when the normal growth conditions are restored remain poorly understood.