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. 2018 Oct 8;9:1387. doi: 10.3389/fpls.2018.01387

FIGURE 4.

FIGURE 4

MAPKs in ET biosynthesis and signaling. (A) An external stimulus leads to activation of ET biosynthesis predominantly through the MKK9-MPK3/MPK6 cascade. Alternatively, MKK7 may also be involved in MPK3/MPK6 activation due to similarities with the MKK9 sequence and its activation mechanisms. MPK3 and MPK6 can also be activated by MKK4 and MKK5, which act in a redundant fashion upstream of MPK3/MPK6, especially after wounding-induced ET biosynthesis. SIMK (MsMPK6) and NSIPK(NtMPK6-1) are homologs of AtMPK6 from alfalfa and tobacco, respectively. Active MPK6 phosphorylates ACS2/ACS6, which initiates ET biosynthesis. (B) ET is perceived by five different receptors (ETR1, ETR2, ERS1, ERS2, EIN4) localized in the endoplasmic reticulum (ER) membrane and this leads to inhibition of CTR1 kinase activity, which is the primary negative regulator of ET signaling. As a consequence, MKK9 is released from CTR1 inhibition and translocates to the nucleus, where it activates MPK3 and MPK6. Moreover, inactive CTR1 is no longer able to phosphorylate the C-terminal domain (CEND) of EIN2. Dephosphorylated CEND moves to the nucleus and takes part in EIN3 stabilization. (C) In the nucleus, active MPK3/MK6 promotes the stability of the main plant-specific ET-dependent transcription factors (EIN3 and EIL1). Phosphorylation of EIN3 at the T174 position blocks its proteasomal degradation and enables it to activate ET-responsive genes.