Lobo Ana Karla Moreira, de Oliveira Martins Marcio, Lima Neto Milton Costa, Machado Eduardo Caruso, Ribeiro Rafael Vasconcelos, Silveira Joaquim Albenisio Gomes
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.
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.
J Plant Physiol. 2015 May 1;179:113-21. doi: 10.1016/j.jplph.2015.03.007. Epub 2015 Mar 26.
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.
糖对光合作用的调节作用已为人所知多年，但对这一过程的生化和分子理解仍很欠缺。我们研究了向叶片供应外源蔗糖如何影响四个月龄甘蔗植株叶片、叶鞘和茎中的糖代谢并抑制光合作用。向附着叶片喷施50mM外源蔗糖会严重损害净CO2同化作用（PN）并降低瞬时羧化效率（PN/Ci），这表明光合作用的损害是由生化限制引起的。光系统II活性也受到过量蔗糖的影响，表现为表观电子传递速率、有效量子产率降低以及非光化学猝灭增加。在叶片切段中，蔗糖积累与可溶性酸性和中性转化酶、蔗糖合酶以及蔗糖磷酸合酶活性增加有关，而果糖含量增加，葡萄糖含量略有下降。完整植株的叶片、叶鞘和茎中蔗糖水解和合成酶活性以及糖谱的变化不足以确定哪些糖或酶直接参与光合作用调节。然而，外源蔗糖能够引发核酮糖-1,5-二磷酸羧化酶（Rubisco）丰度、活化状态和酶活性的下调。尽管PN/Ci因蔗糖而显著降低，但磷酸烯醇式丙酮酸羧化酶（PEPCase）的体外活性和丰度并未改变，这表明该酶在体内受到调节。数据表明，叶片中的蔗糖和/或其他衍生糖通过下调Rubisco的合成和活性来抑制甘蔗光合作用。我们的数据还表明，糖调节并非由未成熟甘蔗茎中糖积累诱导的反馈机制所发挥作用。
