Controlled nuclear import of the transcription factor NTL6 reveals a cytoplasmic role of SnRK2.8 in the drought-stress response

Controlled proteolytic activation of membrane-anchored transcription factors provides an adaptation strategy that guarantees rapid transcriptional responses to abrupt environmental stresses in both animals and plants. NTL6 is a plant-specific NAC [NAM/ATAF1/2/CUC2] transcription factor that is expressed as a dormant plasma membrane-associated form in Arabidopsis. Proteolytic processing of NTL6 is triggered by abiotic stresses and ABA (abscisic acid). In the present study, we show that NTL6 is linked directly with SnRK (Snf1-related protein kinase) 2.8-mediated signalling in inducing a drought-resistance response. SnRK2.8 phosphorylates NTL6 primarily at Thr142. NTL6 phosphorylation by SnRK2.8 is required for its nuclear import. Accordingly, a mutant NTL6 protein, in which Thr142 was mutated to an alanine, was poorly phosphorylated and failed to enter the nucleus. In accordance with the role of SnRK2.8 in drought-stress signalling, transgenic plants overproducing either NTL6 or its active form 6ΔC (35S:NTL6 and 35S:6ΔC) exhibited enhanced resistance to water-deficit conditions such as those overproducing SnRK2.8 (35S:SnRK2.8). In contrast, NTL6 RNAi (RNA interference) plants were susceptible to dehydration as observed in the SnRK2.8-deficient snrk2.8-1 mutant. Furthermore, the dehydration-resistant phenotype of 35S:NTL6 transgenic plants was compromised in 35S:NTL6 X snrk2.8-1 plants. These observations indicate that SnRK2.8-mediated protein phosphorylation, in addition to a proteolytic processing event, is important for NTL6 function in inducing a drought-resistance response.