Dual control of nuclear EIN3 by bifurcate MAPK cascades in C2H4 signalling

A principal question in MAP kinase ( MAPK/ MPK) cascade signalling is how similar components dictate different specificity in the information- processing machineries from yeast to humans and plants. In Arabidopsis, how MPK3/ 6modulates distinct outputs in diverse signal transduction pathways remains elusive. By combining systematic cellular and genetic screens, here we uncover a previously unexpected MKK9 - MPK3/ MPK6 cascade promoting ethylene- insensitive 3 ( EIN3)- mediated transcription in ethylene signalling. The mkk9 mutant exhibits a broad spectrum of moderate ethylene- insensitive phenotypes, and translocated MKK9 governs nuclear signalling downstream of receptors. Breaking a linear model and conventional MAPK signalling, ethylene inactivates the negative regulator constitutive triple response 1 ( CTR1, a Raf- like MAPK kinase kinase ( MAPKKK)) to activate the positive MKK9 - MPK3/ 6 cascade. The bifurcate and antagonistic CTR1 and MKK9 pathways are both critical in determining ethylene- signalling specificity through two MAPK phosphorylation sites with opposite effects on EIN3 stability. The results suggest a new paradigm for linking intertwined MAPK cascades to control quantitative responses and specificity in signalling networks.