Changes in hydrogen peroxide homeostasis and cytokinin levels contribute to the regulation of shade-induced senescence in wheat leaves Humberto F. Causin a, *, Irma N. Roberts b , Marı ´a V. Criado b , Susana M. Gallego c , Liliana B. Pena c , Marı ´a del Carmen Rı ´os d , Atilio J. Barneix b a D.B.B.E., Facultad de Ciencias Exactas y Naturales, University of Buenos Aires, Ciudad Universitaria, 1428 C.A.B.A., Argentina b IBYF, CONICET, Facultad de Agronomı´a, Av. San Martı´n 4453, 1417 C.A.B.A., Argentina c CONICET, Ca ´tedra de Quı´mica Biolo ´gica Vegetal, Dto. de Quı´mica Biolo ´gica, Facultad de Farmacia y Bioquı´mica, University of Buenos Aires, Junı´n 956, 1113 C.A.B.A., Argentina d CONICET, Departamento de Quı´mica Biolo ´gica, Facultad de Ciencias Exactas y Naturales, University of Buenos Aires, Ciudad Universitaria, 1428 C.A.B.A., Argentina 1. Introduction Senescence is an age-dependent slow form of cell death, that primarily allows plants to remobilize nutrients to either actively growing and/or reproductive structures [1]. Even though it is a genetically controlled process, many exogenous factors can prompt it. Leaves beneath a dense canopy experience a marked reduction of the photon flux as well as of the red (660 nm) and blue (400– 450 nm) wavelengths due to the spectral properties of chlorophyll and other pigments, and it has been shown that changes in either light intensity or spectral quality can induce leaf senescence in an independent way (revised in [2]). While several reports on dicotyledonous species show that senescence rate is accelerated when leaves are exposed to a far-red (i.e. 730 nm) enriched environment [3–6], we have recently demonstrated that in excised leaves of wheat, chlorophyll and soluble protein degradation rates under shade conditions are only slightly affected by changes in the red to far-red ratio, while they are significantly increased when blue light is suppressed [7]. Moreover, a similar effect was also reported on intact leaves [2], indicating that leaf excision accelerates but does not alter the senescence pattern caused by changes in light quality. The development of senescence symptoms in excised wheat leaves is correlated to an increment in lipid peroxidation as well as to the activity of catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POX) and superoxide dismutase (SOD). Nevertheless, lipid peroxidation rate was found to be lower and the induction of CAT activity higher in the presence of blue light, suggesting that changes in blue light transmission might affect leaf senescence rate through the regulation of some key components of the antioxidant metabolism. In support of this hypothesis we showed that infiltration of leaves with a solution of CAT from bovine liver after 24 h or 48 h exposure to the shading treatments retarded chlorophyll degradation in the absence of blue Plant Science 177 (2009) 698–704 ARTICLE INFO Article history: Received 23 July 2009 Accepted 26 August 2009 Available online 29 August 2009 Keywords: Catalase Hydrogen peroxide Leaf senescence Light spectral quality Oxidative metabolism Triticum aestivum ABSTRACT In a previous work we demonstrated that the suppression of blue light in shaded leaves of wheat increases their senescence rate and the development of oxidative stress symptoms. In order to better understand the interaction between the oxidative metabolism and light spectral quality in the regulation of leaf senescence, we studied the evolution of H 2 O 2 concentration, protein oxidation, proteolytic activity and cytokinin content in excised leaves, either illuminated (control, ‘‘C’’) or shaded under blue (‘‘B’’, high blue light transmission) or green (‘‘G’’, very low blue light transmission) light filters. H 2 O 2 concentration significantly increased during the first 9 h after treatment initiation, an effect that was consistently higher in treatments B and C. Leaves from these treatments showed lower chlorophyll and protein degradation rates, lower concentration of oxidized proteins, and maintained higher levels of the cytokinin isopentenyl-adenosine than those from treatment G. When moderate H 2 O 2 concentrations were supplied during 6–9 h after the onset of the shade treatments, senescence rate in treatment G was delayed, while the opposite effect was observed in the presence of the H 2 O 2 scavengers catalase and, to a lesser extent, dimethylthiourea. These effects were accompanied by an increment or a decrement, respectively, of catalase activity, suggesting that the early changes in H 2 O 2 homeostasis in leaves from treatments B and C may contribute to the prevention rather than to the induction of further oxidative damage. Altogether our results show that the suppression of blue light transmission in shaded leaves act as a stress signal that increases their sensitivity to oxidative stress and accelerates cell death. ß 2009 Elsevier Ireland Ltd. All rights reserved. * Corresponding author. Tel.: +54 11 45763300. E-mail address: causin@bg.fcen.uba.ar (H.F. Causin). Contents lists available at ScienceDirect Plant Science journal homepage: www.elsevier.com/locate/plantsci 0168-9452/$ – see front matter ß 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2009.08.014