PARENT SESSION

Symposium S2B Light, redox and metabolic regulation: Light reactions
Monday August 30th, 2004 2:40 PM-4:40 PM Room 210A
Chair: Frances-Andre Wollman
Co-Chair: Kris Niyogi

LHCII protein phosphorylation in leaves of Arabidopsis thaliana mutants deficient in non-photochemical quenching. Hanna-Leena Breitholtz, Renu Srivastava¹, Esa Tyystjärvi¹, Eevi Rintamäki^*,1, ¹ Department of Biology, Turku, Finland

ABSTRACT- Light-induced phosphorylation of the LHCII proteins is regulated via activation of the LHCII kinase by binding of plastoquinol to cytochrome b6f complex. Under high light conditions, however, LHCII protein phosphorylation is down-regulated in vivo, and we have recently demonstrated that this down-regulation is induced by a thylakoid membrane-bound thiol-redox mediator that becomes activated at high light and specifically inhibits the LHCII protein phosphorylation in chloroplasts. Due to this complex regulation engaging both an electron transfer component in thylakoid membranes and a thiol-redox mediator, the LHCII kinase possibly serves as a delicate sensor of imbalance between the light reactions and carbon metabolism. We studied the regulation of LHCII protein phosphorylation in the leaves of npq1-2 and npq4-1 mutants of Arabidopis thaliana. In comparison with wild type, both mutants showed reduced non-photochemical quenching and increased PSII excitation pressure under physiological light intensities. Peculiar regulation of LHCII protein phosphorylation, however, was observed in illuminated mutant leaves. Light intensity-dependent changes in the level of LHCII protein phosphorylation were smaller in the npq1-2 mutant than in wild type. Furthermore, regardless of the increased PSII excitation pressure, the npq4-1 mutant was able to maintain a high phosphorylation level of LHCII proteins at light intensities that induced inhibition of phosphorylation in wild type leaves. No significant differences in leaf thickness, dry weight, chlorophyll content, or in the amount of LHCII proteins were observed between two mutant and wild type lines. Moreover, the light-induced oxidation of the primary donor of PSI occurred similarly in both mutants and in wild type. The results suggest that the capacity for thermal dissipation of excitation energy has an effect on the regulation of LHCII protein phosphorylation.

KEY WORDS: npq1-2 and npq4-1 mutants, thylakoid protein kinase, light-harvesting complex, redox regulation