title: A novel protective function for cytokinin in the light stress response is mediated by the Arabidopsis histidine kinase2 and Arabidopsis histidine kinase3 receptors.
abstract: Cytokinins are plant hormones that regulate diverse processes in plant development and responses to biotic and abiotic stresses. In this study, we show that Arabidopsis (Arabidopsis thaliana) plants with a reduced cytokinin status (i.e. cytokinin receptor mutants and transgenic cytokinin-deficient plants) are more susceptible to light stress compared with wild-type plants. This was reflected by a stronger photoinhibition after 24 h of high light (approximately 1,000 µmol m(-2) s(-1)), as shown by the decline in maximum quantum efficiency of photosystem II photochemistry. Photosystem II, especially the D1 protein, is highly sensitive to the detrimental impact of light. Therefore, photoinhibition is always observed when the rate of photodamage exceeds the rate of D1 repair. We demonstrate that in plants with a reduced cytokinin status, the D1 protein level was strongly decreased upon light stress. Inhibition of the D1 repair cycle by lincomycin treatment indicated that these plants experience stronger photodamage. The efficiency of photoprotective mechanisms, such as nonenzymatic and enzymatic scavenging systems, was decreased in plants with a reduced cytokinin status, which could be a cause for the increased photodamage and subsequent D1 degradation. Additionally, slow and incomplete recovery in these plants after light stress indicated insufficient D1 repair. Mutant analysis revealed that the protective function of cytokinin during light stress depends on the Arabidopsis histidine KINASE2 (AHK2) and AHK3 receptors and the type B Arabidopsis response regulator1 (ARR1) and ARR12. We conclude that proper cytokinin signaling and regulation of specific target genes are necessary to protect leaves efficiently from light stress.
authors.affiliation: Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences, Freie Universität Berlin, D-14195 Berlin, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Cortleven
authors.forename: Silvia
authors.initials: S
authors.lastname: Nitschke
authors.forename: Marion
authors.initials: M
authors.lastname: Klaumünzer
authors.forename: Hamada
authors.initials: H
authors.lastname: Abdelgawad
authors.forename: Han
authors.initials: H
authors.lastname: Asard
authors.forename: Bernhard
authors.initials: B
authors.lastname: Grimm
authors.forename: Michael
authors.initials: M
authors.lastname: Riefler
authors.forename: Thomas
authors.initials: T
authors.lastname: Schmülling
mesh_terms: D000975:Antioxidants / Q000378:metabolism; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics / Q000502:physiology* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects / Q000528:radiation effects / Q000648:ultrastructure; D003583:Cytokinins / Q000494:pharmacology*; D004268:DNA-Binding Proteins / Q000378:metabolism; D018506:Gene Expression Regulation, Plant / Q000187:drug effects / Q000528:radiation effects; D017343:Genes, Plant; D000071677:Histidine Kinase; D008027:Light; D008954:Models, Biological; D018384:Oxidative Stress / Q000187:drug effects / Q000528:radiation effects; D010788:Photosynthesis / Q000187:drug effects / Q000528:radiation effects; D045332:Photosystem II Protein Complex / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism*; D017382:Reactive Oxygen Species / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics / Q000528:radiation effects; D013312:Stress, Physiological / Q000187:drug effects / Q000528:radiation effects*; D014157:Transcription Factors / Q000378:metabolism
chemical_list: C117138:ARR1 protein, Arabidopsis; C117139:ARR2 protein, Arabidopsis; D000975:Antioxidants; D029681:Arabidopsis Proteins; D003583:Cytokinins; D004268:DNA-Binding Proteins; D045332:Photosystem II Protein Complex; D017382:Reactive Oxygen Species; D014157:Transcription Factors; D011494:Protein Kinases; D000071677:Histidine Kinase; C000605492:AHK2 protein, Arabidopsis; C000605493:AHK3 protein, Arabidopsis