Abscisic acid and fruit ripening: Multifaceted analysis of the effect of abscisic acid on fleshy fruit ripening

The plant hormone abscisic acid (ABA) acts as a developmental signal and as an integrator of environmental cues such as drought and cold. Key players in ABA signal transduction include the type 2C protein phosphatases (PP2Cs) ABI1 and ABI2, which act by negatively regulating ABA responses. In this study, we identify interactors of ABI1 and ABI2 which we have named regulatory components of ABA receptor (RCARs). In Arabidopsis, RCARs belong to a family with 14 members that share structural similarity with class 10 pathogen-related proteins. RCAR1 was shown to bind ABA, to mediate ABA-dependent inactivation of ABI1 or ABI2 in vitro, and to antagonize PP2C action in planta. Other RCARs also mediated ABA-dependent regulation of ABI1 and ABI2, consistent with a combinatorial assembly of receptor complexes.

Abscisic acid (ABA) regulates numerous developmental processes and adaptive stress responses in plants. Many ABA signaling components have been identified, but their interconnections and a consensus on the structure of the ABA signaling network have eluded researchers. Recently, several advances have led to the identification of ABA receptors and their three-dimensional structures, and an understanding of how key regulatory phosphatase and kinase activities are controlled by ABA. A new model for ABA action has been proposed and validated, in which the soluble PYR/PYL/RCAR receptors function at the apex of a negative regulatory pathway to directly regulate PP2C phosphatases, which in turn directly regulate SnRK2 kinases. This model unifies many previously defined signaling components and highlights the importance of future work focused on defining the direct targets of SnRK2s and PP2Cs, dissecting the mechanisms of hormone interactions (i.e., cross talk) and defining connections between this new negative regulatory pathway and other factors implicated in ABA signaling.

Elucidating the mechanisms involved in ripening of climacteric fruit and the role that ethylene plays in the process are key to understanding fruit production and quality. In this review, which is based largely on research in tomato, particular attention is paid to the role of specific isoforms of ACC synthase and ACC oxidase in controlling ethylene synthesis during the initiation and subsequent autocatalytic phase of ethylene production during ripening. Recent information on the structure and role of six different putative ethylene receptors in tomato is discussed, including evidence supporting the receptor inhibition model for ripening, possible differences in histidine kinase activity between receptors, and the importance of receptor LeETR4 in ripening. A number of ethylene-regulated ripening-related genes are discussed, including those involved in ethylene synthesis, fruit texture, and aroma volatile production, as well as experiments designed to elucidate the ethylene signalling pathway from receptor through intermediate components similar to those found in Arabidopsis, leading to transcription factors predicted to control the expression of ethylene-regulated genes.