Effects of the Relative Extrachloroplastic Concentrations of Inorganic Phosphate, 3-Phosphoglycerate, and Dihydroxyacetone Phosphate on the Rate of Starch Synthesis in Isolated Spinach Chloroplasts

Resumo

Foi examinado o efeito do fosfato inorgânico externo (Pi) na síntese de amido em cloroplastos isolados de espinafre (Spinacia oleracea American Hybrid No. 424) na presença de concentrações milimolares de 3-fosfoglicerato (PGA) e/ou fosfato de dihidroxiacetona (DHAP). Enquanto a fixação de CO2 foi relativamente constante, pois a proporção do fosfato externo para o PGA + DHAP variou de 1:3 a 3:1, a síntese de amido variou de 17% a 2% da taxa de fixação de CO2. Somente com DHAP, a síntese máxima de amido foi de cerca de 10% da taxa de fixação de CO2. Os dados demonstram que a relação Pi/(PGA + DHAP) no citoplasma das células vegetais pode servir para regular o fluxo de carbono recém-fixado para o amido sem alterações na taxa de fixação de CO2.

Abstract

The effect of external inorganic phosphate (Pi) on starch synthesis in isolated spinach (Spinacia oleracea American Hybrid No. 424) chloroplasts in the presence of millimolar concentrations of 3-phosphoglycerate (PGA) and/or dihydroxyacetone phosphate (DAP) was examined. Whereas CO2 fixation was relatively constant as the ratio of the external phosphate to the PGA + DAP varied from 1:3 to 3:1, starch synthesis varied from 17% to 2% of the CO2 fixation rate. With DAP alone, maximal starch synthesis was about 10% of the CO2 fixation rate. The data demonstrate that the Pi/(PGA + DAP) ratio in the cytoplasm of plant cells could serve to regulate the flow of newly fixed carbon into starch without alterations in the rate of CO2 fixation.