The Arabidopsis genome encodes 14 P-type calcium pumps (10 ACAs, autoin-hibited Ca2+-ATPase and 4 ECAs, ER-type Ca-ATPases) that are proposed to lower [Ca2+]c by extruding Ca2+ into intracellular compartments or outside the cell (Sze et al. 2000). The first null mutation reported for a plant ACA (aca9) was found to cause partial male sterility (Schiott et al. 2004). The primary defect was a failure of aca9 pollen to discharge sperm into the synergid. While mutant pollen tubes were able to reach ovules, they failed to complete fertilization in more than 50% of the interactions. Evidence indicates that the ACA9 pump is primarily located at the plasma membrane. A plasma membrane Ca2+ pump may either function in signal transduction by helping to control the magnitude or duration of a Ca2+ signal (signaling function), or in cell wall biogenesis by pumping Ca2+ directly into the wall (nutritional function), or both. The fact that ACAs are activated by Ca2+/calmodulin, indicates that they are directly connected to Ca2+ signaling.
The observation that the aca9 null phenotype does not result in complete male sterile phenotype leaves open the question of whether other isoforms provide some low level of functional redundancy. In mature pollen, expression profiling studies show that ACA9 is more highly expressed than any other ACA type calcium pump, including ACA2, ACA7 and ACA8 (Fig. 2). The expression of ACA8, a known plasma membrane Ca2+ pump (Bonza et al. 2000), under the control of the ACA9 promoter was shown to complement the aca9 null mutation. While this indicates that Arabidopsis pollen do express pumps that have the potential to be functionally redundant, expression levels are not sufficient to completely compensate for the loss of the most abundant isoform, ACA9.
Interestingly, pollen-expressed ACA9 is also preferentially expressed in roots based on the transcriptome data of several sporophytic tissues (Bock K, Sze H, unpublished). As root hairs are known to elongate by tip growth, it is tempting to speculate that ACA9 performs a similar function in tip growth of both pollen tube and root hairs.
Pumps that load Ca2+ into endomembrane compartments also influence pollen tube growth. Although several ECA genes are expressed in pollen, ho-mozygous mutants of ECA3 gene alone reduced pollen tube growth in vitro (Li X, Harper JF, Sze H, unpublished). Results suggest ECA3 affects lumenal [Ca2+] in compartments distinct from the ER where ECA1 and ACA2 are localized. It is possible that eca3 mutants altered one or more of the following: (i) [Ca2+]c dynamics needed for signaling; and (ii) lumenal [Ca2+] required to activate enzymes/proteins involved in protein sorting, modification or cell wall biosynthesis.
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