Localization of a calcium channel-like protein in the sieve element plasma membrane

There is evidence that calcium (Ca2+) activity in sieve elements is much higher than in adjacent cells, which raises questions about Ca2+ exchange across the sieve element plasma membrane. We looked for the presence of Ca2+ channels in tobacco (Nicotiana tabacum L.) and Pistia stratiotes (L.) using a monoclonal antibody to a dihydropyridine (DHP)-type Ca channel (MAB427). Immunolabeling at the light microscope level gave strong signals along the sieve element of both species, and with the transmission electron microscope this label was found associated with the plasma membrane. Western blot analysis using the MAB427 antibody detects a protein between 175 and 220 kDa (approximately the size expected of the alpha-1 subunit of the Ca2+ channel protein in mammals) in blots containing either Pistia or tobacco leaf microsomal membrane protein extracts. As a further test for Ca2+ channels, hand-sectioned young Pistia leaves were treated with the fluorescent-tagged Ca2+ channel blocker 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-3-(indacene) propionic acid (DM-BODIPY)- DHP. The phloem, as well as calcium oxalate idioblast cells, exhibited strong fluorescence compared to general parenchyma cells. The binding of DM-BODIPY-DHP was inhibited by pre-treatment of sections with nifedipine, a competitive inhibitor of DHP-type Ca2+ channels in animal systems. The results indicate that sieve elements may be enriched with Ca2+ channels which might be partly responsible for the high Ca2+ activities observed in sieve sap, and also for rapid phloem responses, such as Ca2+ activated callose deposition.