In vivo evidence for a regulatory role of phosphorylation of Arabidopsis Rubisco activase at the Thr78 site

Abstract

ArabidopsisRubisco activase (Rca) is phosphorylated at threonine-78 (Thr78) in low light and in the dark, suggesting a potential regulatory role in photosynthesis, but this has not been directly tested. To do so, we transformed anrca-knockdown mutant largely lacking redox regulation with wild-type Rca-β or Rca-β with Thr78-to-Ala (T78A) or Thr78-to-Ser (T78S) site–directed mutations. Interestingly, the T78S mutant was hyperphosphorylated at the Ser78 site relative to Thr78 of the Rca-β wild-type control, as evidenced by immunoblotting with custom antibodies and quantitative mass spectrometry. Moreover, plants expressing the T78S mutation had reduced photosynthesis and quantum efficiency of photosystem II (ϕPSII) and reduced growth relative to control plants expressing wild-type Rca-β under all conditions tested. Gene expression was also altered in a manner consistent with reduced growth. In contrast, plants expressing Rca-β with the phospho-null T78A mutation had faster photosynthetic induction kinetics and increased ϕPSIIrelative to Rca-β controls. While expression of the wild-type Rca-β or the T78A mutant fully rescued the slow-growth phenotype of therca-knockdown mutant grown in a square-wave light regime, the T78A mutants grew faster than the Rca-β control plants at low light (30 µmol photons m−2s−1) and in a fluctuating low-light/high-light environment. Collectively, these results suggest that phosphorylation of Thr78 (or Ser78 in the T78S mutant) plays a negative regulatory role in vivo and provides an explanation for the absence of Ser at position 78 in terrestrial plant species.

Probing the rice Rubisco–Rubisco activase interaction via subunit heterooligomerization

“…3A). This finding is consistent with a recent report showing that Arabidopsis plants expressing a hyperphosphorylated Rca mutant displayed reduced photosynthetic performance (59).…”

A role for differential Rubisco activase isoform expression in C4 bioenergy grasses at high temperature

“…That is, a Rca-β isoform appears to be adaptive under heat stress in wheat in sustaining photosynthesis, whereas we found the Rca-α isoform appears adaptive in C 4 grasses may be yet another example of the highly diverse regulation-and expression-by-environment interactions for Rca across species. Another example of this diversity is the phosphorylation of Rca T78 that occurs in Arabidopsis in the dark or in low light and downregulates photosynthesis (Kim et al, 2016(Kim et al, , 2019 but does not occur in Rca-α of C 4 grasses that also have this phospho-residue ( Figure S1). This result may indicate that the proper kinases for Rca phosphorylation do not exist in the bundle sheath cell in sorghum or that the phosphatase activity is so high that it negates the kinase activity.…”

“…The Rca-α isoforms are sensitive to redox regulation due to the C-terminal extension (CTE) that contains two conserved redox-sensing cysteine residues (Zhang & Portis, 1999). In Arabidopsis (Arabidopsis thaliana), phosphorylation of Rca at the Thr78 site also occurs under low light and darkness, which plays an additional regulatory role in photosynthesis in vivo (Kim et al, 2016(Kim et al, , 2019.…”

Arabidopsis Plant Natriuretic Peptide Is a Novel Interactor of Rubisco Activase

“…Such a mechanism for protecting RCA from thermal denaturation during heat stress and allowing acclimation of photosynthesis to heat stress, has been proposed for its interaction with chaperonin-60β [ 51 ]. It is also conceivable that AtPNP-A interaction with RCA may change its susceptibility for being post-translationally modified [ 52 ] (e.g., phosphorylation at Thr78 serves a negative regulatory role [ 53 ]) or its ability to oligomerize and proneness to proteolytic degradation. Detailed biochemical and physico-chemical characterization of the AtPNP-A interaction with RCA would provide insight into these aspects of the interaction.…”