title: Exogenous sucrose supply changes sugar metabolism and reduces photosynthesis of sugarcane through the down-regulation of Rubisco abundance and activity.
abstract: Photosynthetic modulation by sugars has been known for many years, but the biochemical and molecular comprehension of this process is lacking. We studied how the exogenous sucrose supplied to leaves could affect sugar metabolism in leaf, sheath and stalk and inhibit photosynthesis in four-month old sugarcane plants. Exogenous sucrose 50mM sprayed on attached leaves strongly impaired the net CO2 assimilation (PN) and decreased the instantaneous carboxylation efficiency (PN/Ci), suggesting that the impairment in photosynthesis was caused by biochemical restrictions. The photosystem II activity was also affected by excess sucrose as indicated by the reduction in the apparent electron transport rate, effective quantum yield and increase in non-photochemical quenching. In leaf segments, sucrose accumulation was related to increases in the activities of soluble acid and neutral invertases, sucrose synthase and sucrose phosphate synthase, whereas the contents of fructose increased and glucose slightly decreased. Changes in the activities of sucrose hydrolyzing and synthesizing enzymes in leaf, sheath and stalk and sugar profile in intact plants were not enough to identify which sugar(s) or enzyme(s) were directly involved in photosynthesis modulation. However, exogenous sucrose was able to trigger down-regulation in the Rubisco abundance, activation state and enzymatic activity. Despite the fact that PN/Ci had been notably decreased by sucrose, in vitro activity and abundance of PEPCase did not change, suggesting an in vivo modulation of this enzyme. The data reveal that sucrose and/or other derivative sugars in leaves inhibited sugarcane photosynthesis by down-regulation of Rubisco synthesis and activity. Our data also suggest that sugar modulation was not exerted by a feedback mechanism induced by the accumulation of sugars in immature sugarcane stalk.
authors: Laboratório de Metabolismo de Plantas, Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Ceará, Avenida Humberto Monte, S/N, CP 6004, CEP 60440-970 Fortaleza, Ceará, Brazil.
authors: Ana Karla Moreira
authors: AK
authors: Lobo
authors: Laboratório de Metabolismo de Plantas, Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Ceará, Avenida Humberto Monte, S/N, CP 6004, CEP 60440-970 Fortaleza, Ceará, Brazil.
authors: Marcio
authors: M
authors: de Oliveira Martins
authors: Laboratório de Metabolismo de Plantas, Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Ceará, Avenida Humberto Monte, S/N, CP 6004, CEP 60440-970 Fortaleza, Ceará, Brazil.
authors: Milton Costa
authors: MC
authors: Lima Neto
authors: Laboratório de Fisiologia Vegetal "Coaracy M. Franco", Centro de Pesquisa e Desenvolvimento de Ecofisiologia e Biofísica, Instituto Agronômico (IAC), Avenida Barão de Itapura, 1481, CP 28, CEP 13012-970 Campinas, São Paulo, Brazil.
authors: Eduardo Caruso
authors: EC
authors: Machado
authors: Department of Plant Biology, Institute of Biology, University of Campinas (UNICAMP), Rua Monteiro Lobato, 255, CEP 13083-862 Campinas, São Paulo, Brazil.
authors: Rafael Vasconcelos
authors: RV
authors: Ribeiro
authors: Laboratório de Metabolismo de Plantas, Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Ceará, Avenida Humberto Monte, S/N, CP 6004, CEP 60440-970 Fortaleza, Ceará, Brazil. Electronic address: silveira@ufc.br.
authors: Joaquim Albenisio Gomes
authors: JA
authors: Silveira
mesh_terms: D015153:Blotting, Western; D050260:Carbohydrate Metabolism / Q000187:drug effects*; D002245:Carbon Dioxide / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects*; D004579:Electron Transport / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D005947:Glucose / Q000378:metabolism; D010730:Phosphoenolpyruvate Carboxylase / Q000378:metabolism; D010788:Photosynthesis / Q000187:drug effects*; D045332:Photosystem II Protein Complex / Q000378:metabolism; D054046:Plant Stomata / Q000187:drug effects / Q000201:enzymology / Q000502:physiology; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D031786:Saccharum / Q000187:drug effects / Q000502:physiology*; D013395:Sucrose / Q000378:metabolism / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D045332:Photosystem II Protein Complex; D002245:Carbon Dioxide; D013395:Sucrose; D010730:Phosphoenolpyruvate Carboxylase; D012273:Ribulose-Bisphosphate Carboxylase; D005947:Glucose
keywords: CO(2) assimilation; PEPCase activity; Photosynthetic modulation; Saccharum spp.; Source-sink