Ca2+-binding sites and phosphatase activities in sieve element reticulum and P-protein of chick-pea phloem. A cytochemical and X-ray microanalysis survey

Summary In stem ofCicer arietinum, the loss of ribosomes attached to the rough ER cisternae during sieve element ontogeny results in the formation of sieve element reticulum (SER). By enhancing contrast of the SER, the OsFeCN postfixation/staining of material prefixed in glutaraldehyde in presence of calcium enables a good visualization of this membrane system. The pattern of staining in the SER is slightly lower when Mg²⁺ is substituted for Ca²⁺. These results support the view that the OsFeCN staining requires divalent cations and that the SER can accumulate Ca²⁺. The detection of Ca²⁺ by means of the pyroantimonate method in conjunction with X-ray microanalysis and the cytochemical localization of Ca²⁺ -ATPase in the SER cisternae provides evidence for Ca²⁺ sequestration by the SER. On the other hand, Ca²⁺-binding sites and ATPase activity are localized in P-protein. The ability to bind Ca²⁺ seems to enable the SER to function as an effective Ca²⁺ sink which may participate—with the sieve tube plasma membrane and mitochondria—in the maintenance of low Ca²⁺ concentration in phloem sap. In addition, the close association between P-protein and SER membranes exhibiting Ca²⁺-binding capabilities suggests that a Ca²⁺-mediated functional relationship may exist between the two structures. It is postulated that the SER may play a role in putative Ca²⁺ control of P-protein organization.Abbreviations SER sieve element reticulum - ER endoplasmic reticulum - P protein, phloem protein - OsFeCN method, osmium tetroxide-ferricyanide method - EDTA ethylenediamine tetraacetic acid - EGTA ethyleneglycol-bis-(-aminoethyl ether) N,N-tetraacetic acid - ATP adenosine 5-triphosphoric acid - ATPase adenosine triphosphatase - PCMB p chloromercuribenzoate - IDP inosine diphosphate