Regulation of adaptive growth decisions via phosphorylation of the TRAPPII complex in Arabidopsis.

Plants often adapt to adverse or stress conditions via differential growth. The trans-Golgi Network(TGN) has been implicated in stress responses, but it is not clear in what capacity it mediates adaptive growth decisions. In this study, we assess the role of the TGN in stress responses by exploring the interactome of the Transport Protein Particle II (TRAPPII) complex, required for TGN structure and function. Together with yeast-two-hybrid screens, this identified shaggy-like kinases (GSK3/AtSKs) as TRAPPII interactors. Kinase assays and pharmacological inhibition provided in vitro and in vivo evidence that AtSKs target the TRAPPII-specific subunit AtTRS120. We identified three GSK3/AtSK phosphorylation sites in AtTRS120. These sites were mutated, and the resulting AtTRS120 phosphovariants subjected to a variety of single and multiple stress conditions. The non-phosphorylatable TRS120 mutant exhibited enhanced adaptation to multiple stress conditions and to osmotic stress whereas the phosphomimetic version was less resilient. This suggests that the TRAPPII phosphostatus mediates adaptive responses to abiotic stress factors. AtSKs are multitaskers that integrate a broad range of signals. Similarly, the TRAPPII interactome is vast and considerably enriched in signaling components. An AtSK-TRAPPII interaction would integrate all levels of cellular organization and instruct the TGN, a central and highly discriminate cellular hub, as to how to mobilize and allocate resources to optimize growth and survival under limiting or adverse conditions.