Figure 7.

Working model for MEL regulation of plant broad-spectrum resistance against pathogens. Microtubule-localized C4HC3 RING-type E3 ligase (MEL) transcript is activated by the invasion of pathogens. MEL forms homodimers through intermolecular disulfide bonds depending on the SWIM domain and then hijacks its substrate SHMT1 through the YφNL motif, triggering SHMT1 ubiquitination and 26S proteasome-dependent degradation. SHMT1 localize on mitochondria. The degradation of SHMT1 initiates a plant immune signaling cascade, including enhancement of mtROS generation, activation of MAPK cascades, and activation of defense-related genes, ultimately conferring broad-spectrum resistance to multiple pathogen species in both the dicot N. benthamiana and the monocot O. sativa.