A Distinct Endosomal Ca21/Mn21 Pump Affects Root Growth through the Secretory Process1(C)(W)(OA)

its endogenous Ca21 pumps conferred the ability to grow on Ca21-depleted medium and tolerance to toxic levels of Mn21. A green fluorescent protein-tagged AtECA3 was functionally competent and localized to intracellular membranes of yeast, suggesting that Ca21 and Mn21 loading into internal compartment(s) enhanced yeast proliferation. In mesophyll protoplasts, AtECA3-green fluo- rescent protein associated with a subpopulation of endosome/prevacuolar compartments based on partial colocalization with the Ara7 marker. Interestingly, three independent eca3 T-DNA disruption mutants showed severe reduction in root growth normally stimulated by 3 mM Ca21, indicating that AtECA3 function cannot be replaced by an ER-associated AtECA1. Furthermore, root growth of mutants is sensitive to 50 mM Mn21, indicating that AtECA3 is also important for the detoxification of excess Mn21. Curiously, Ateca3 mutant roots produced 65% more apoplastic protein than wild-type roots, as monitored by peroxidase activity, suggesting that the secretory process was altered. Together, these results demonstrate that the role of AtECA3 is distinct from that of the more abundant ER AtECA1. AtECA3 supports Ca21-stimulated root growth and the detoxification of high Mn21, possibly through activities mediated by post-Golgi compartments that coordinate membrane traffic and sorting of materials to the vacuole and the cell wall.