Figure 10.

Summary of the Roles of Pip, FMO1, and SA in Arabidopsis SAR and Associated Defense Priming, and Modes of the Synergistic Interplay between Pip and SA.

(A) to (C) Regulation of the SAR transcriptional response, SAR establishment, and SAR-associated defense priming by Pip, FMO1, and SA.

(A) The situation in wild-type plants. The full SAR and priming responses are established by elevated levels of ALD1-generated Pip, the action of FMO1 downstream of Pip, and ICS1-synthesized SA in 2° leaf tissue. The Pip/FMO1 module acts as an indispensable switch for SAR activation, and SA amplifies Pip-dependent responses to different degrees.

(B) The situation in sid2 mutant plants. In the absence of functional ICS1 and elevated SA, the Pip/FMO1 module is sufficient to induce a set of partially SA-independent responses to a certain level and trigger a moderate SAR response. Notably, an intact Pip/FMO1 module is also capable to prime plants for enhanced activation of partially SA-independent responses in the absence of SA elevations (bent red arrow).

(C) The situation in ald1 and fmo1 mutant plants. Functional ICS1 alone is not sufficient for SAR activation. Without Pip elevations or functional FMO1, SA biosynthesis is not activated. Failure of both Pip and SA elevations prevents the establishment of a primed state. Without the Pip/FMO1 module, inducing stimuli from 1° leaves are either not transduced into a meaningful response in 2° leaves or too weak to induce a noticeable response.

(D) and (E) Two modes of synergistic interplay between the immune signals Pip and SA.

(D) Elevated Pip levels amplify SA-induced PR1 expression. This amplification response is mediated by FMO1 but does not depend on PAD4.

(E) Elevated Pip levels induce PR1 expression via ICS1-triggered SA production. This signaling mode of Pip depends on both FMO1 and PAD4.