A Branched SNF1-Related Protein Kinase 2 Signalling Cascade Controls ABA-Induced Ethylene Production and Regulates Both Fruit Ripening and Reproductive Growth.

Control of fruit ripening, quality and yield is of major scientific, nutritional and commercial importance. The burst of ethylene (ET) production at the initiation of ripening is the most critical event controlling climacteric (CL) fruit ripening, yet little is known about how it is initiated. ABA is known to be capable of inducing ET production in many biological processes. However, the mechanism for this ABA induced ET production (AEP) and its potential importance in the burst of ET production that initiates ripening are unclear. Here, we report a branched signalling network involving ABA-activation of multiple SlSnRK2 (SNF1-related protein kinase 2) kinases, which, when overexpressed in tomato, stimulated ABA-induced ET production. Two key components, SlSnRK2.1 and SlSnRK2.4, phosphorylate an HD-Zip homeobox transcription factor, SlHB1, which transcriptionally activates ACC oxidase (SlACO1), required for ethylene synthesis. Concurrently, SlSnRK2.1 and SlSnRK2.4 phosphorylate two mitogen-activated protein kinases, SlMPK1/2, resulting in the post-translational stabilisation of ACC synthase (SlACS2), which generates the precursor 1-aminocyclopropane-1-carboxylic acid (ACC) that is converted to ET by ACO1. Removal of SlSnRK2.1 by CRISPR/Cas9 mutation was sufficient to alter the progress of fruit ripening. These results indicate that ABA is a primary hormonal signal modulating CL fruit ripening that stimulates ethylene production by targeting different steps in the ethylene biosynthesis pathway by both transcriptional and post-translational mechanisms. Further analysis revealed that removal of SlSnRK2.1 signalling also affected other aspects of the life cycle by prolonging the flowering period and suppressing seed development, indicating the potential for modifying fruit cropping and seedlessness.