β
Figure 2. SO Signaling. Blue arrows represent EX1 signaling
events. Prior to SO stimulation, EX1 is present in the grana margins in
complex with several other proteins, but SO accumulation causes EX1 to
disassociate as a result of oxidative modification. EX1 can then 1)
translocate to the nucleus (dashed arrow) to promote SORG expression in
concert with WRKY transcription factors; or 2) undergo proteolysis by
FtsH2. Proteolysis also promotes SORG expression through an
as-yet-unknown mechanism. Orange arrows represent carotenoid signaling.
SO generated in the grana core oxidizes β-carotene, yielding
β-cyclocitral (β-CC). This signaling molecule promotes increased
expression of MBS1 and SCL14, which in turn upregulate genes for stress
adaptation. β-CC or some other unknown signal promotes expression of the
OXI1 kinase, which mediates cell death in response to SO from the grana
core. SAFEGAURD1 (SAFE1) also acts independently of EX1 to suppress
responses to SO caused by damage to the grana margins. This figure was
created with Biorender, and was inspired in part by figures from
(Woodson, 2019; Wang et al., 2020; Li et al., 2023).