Vacuolar and ER-Ca2+-ATPases regulate calcium dynamics during pollen tube growth in <i>Arabidopsis thaliana</i>

Calcium (Ca2+) plays a pivotal role in plant reproduction. Pollen tubes exhibit polarized growth, regulated among other factors, by an apical oscillatory Ca2+ gradient. This gradient is controlled by the Ca2+ influx from the apoplast mediated by plasma membrane channels and the efflux from the cytosol to organelles and the apoplast, which is controlled by pumps and exchangers. Nevertheless, the precise mechanism by which Ca2+ efflux is regulated during pollen tube growth remains unclear. Here, we study the function of the pollen-expressed Arabidopsis thaliana autoinhibited Ca2+ ATPases (ACAs), particularly those localized in the vacuole (ACA4 and ACA11) and in the endoplasmic reticulum (ER) (ACA2 and ACA7). We found that double mutants (aca4-1 aca11-1, aca2-2 aca7-1 and aca2-2 aca11-1) and triple mutants (aca2-2 aca4-1 aca11-1 and aca2-2 aca7-1 aca11-1) show reduced pollen tube growth velocity compared to WT. Moreover, we found that aca4-1 aca11-1 displays a higher frequency of pollen tube growth and cytosolic Ca2+ oscillations, higher amplitude values of Ca2+ oscillations, and slower pollen tube growth compared to WT. Our results suggest that the absence of the two pollen vacuolar ACAs (ACA4 and ACA11) has a more pronounced impact on cytosolic Ca2+ dynamics and pollen tube growth than that of the ER ACAs (ACA2 and ACA7). Our findings align with a model in which Ca2+ is transported to internal stores, vacuoles, and the ER, thereby regulating the establishment and maintenance of the Ca2+ gradient during pollen tube growth.