Phenotypic characterization of an Arabidopsis stn7 stn8 pck2 triple mutant reveals only minor effects on chloroplast functions

Abstract

Reversible protein phosphorylation is a key regulator of chloroplast function. Although chloroplast kinases STN7, STN8, and plastid casein kinase 2 (pCK2) regulate photosynthetic acclimation and other chloroplast processes, their functional interplay remains poorly understood. We generated and characterized the first Arabidopsis thaliana stn7 stn8 pck2 triple knockout mutant. Biochemical analyses confirmed the loss of all three kinase activities on established substrates. Growth resembled the pck2 single mutant, whereas photosynthetic fluorescence parameters reflected the stn7 stn8 double mutant. Thus, simultaneous disruption of all three kinases primarily combined the parental phenotypes, with little evidence of additive or synergistic effects under standard growth conditions. This triple mutant enables studies of chloroplast phosphorylation networks and demonstrates the remarkable resilience of chloroplast physiology to combined kinase loss.