Figure 8.

Proposed models for the roles of PAD4, SA, and NPR1 in defense gene expression. (Model 1) SA is necessary but not sufficient for activation of expression of defense genes including PAD4. Another component is required—either NPR1 or some unknown factor X from the pathogen. NPR1 also inhibits SA accumulation. (Model 2) Different SA levels modulate PAD4 activity differently. Low SA levels activate and very high SA levels inactivate PAD4. Activated PAD4, in turn, stimulates expression of defense genes and inhibits the repressing activity of NPR1 on the SA amplification loop. Very high SA levels turn PAD4 off. In this situation, NPR1 activity is required for defense gene expression.