Donor- and Acceptor-Side Protection Against Photosystem I Photoinhibition in Arabidopsis thaliana

Photosystem I (PSI) photoinhibition (PI(I)) is gaining traction as a potentially more significant threat to plant performance than photoinhibition of photosystem II (PSII). The increased focus is facilitated by the implementation of specific protocols that induce PI(I), such as artificial fluctuating light (FL) and repetitive short saturating pulses (rSPs). rSPs were long considered a specific sub-case of FL. However, recent evidence suggests that PI(I) proceeds via at least two distinct, treatment-dependent mechanisms, leading to damage at the donor or acceptor side of PSI. This discovery suggests that rSPs and FL represent distinct photoinhibitory stresses and that different mechanisms protect PSI against FL and rSPs. This study comparatively analyzed the effects of FL and rSPs on PSI activity in Arabidopsis thaliana wild-type plants and a selection of mutants (pgr5, pgrl1, stn7, tap38/pph1, and pgr1), previously noted or hypothesized to have altered PI(I) sensitivity relative to the wild type. The results of this work, particularly the contrasting sensitivity of tap38/pph1 compared to the wild type under FL and rSP conditions, strongly suggest that pulsed illumination and fluctuating light are distinct photoinhibitory treatments, and different mechanisms protect PSI against them.