ATPase and Proton Pumping Activities in Cotyledons and other Phloem-Containing Tissues2Ricinus communis

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Abstract
ATPase activity was investigated in phloem-containing tissues of Ricinus communis in relation to its proposed role in phloem loading. Cytochemical staining of cotyledons revealed an ATP-hydrolysing activity on the plasma membrane of the sieve tube/companion cell complex. Microsomal fractions prepared from cotyledons and main veins contained a Mg2+-dependent ATPase activity which showed low stimulation by KC1 particularly at pH 6.5. The pH optimum was at pH 8.5 to 90, although the effect of azide indicated the presence of mitochondrial ATPase. At pH 6.5, the cited optimum for plasma membrane ATPase, the activity showed strong inhibition by PCMBS, vanadate and DCCD. A high pyrophosphatase activity was observed at pH 8.5. Acidification of the medium by intact cotyledons was increased by fusicoccin and inhibited by PCMBS, NEM and vanadate. Proton pumping by microsomal vesicles as measured by quinacrine fluorescence was also inhibited by PCMBS, NEM and vanadate. Sucrose uptake by cotyledon discs showed strong inhibition by PCMBS, NEM and CCCP but was little affected by vanadate. Sucrose uptake varied with the developmental stage of the cotyledons and this correlated with microsomal ATPase activity measured at pH 6.5, although the precise cellular origin of this activity is not certain. The results are discussed in relation to the role of ATPase activity and proton pumping in phloem loading.