pH modulates interaction of 14-3-3s with pollen PM H+ ATPases independently from phosphorylation

Pollen grains transport the sperm cells through the style tissue via a fast growing pollen tube to the ovaries where fertilisation takes place. This tube growth process requires a precisely regulated network of cellular as well as molecular courses of events including the activity of the plasma membrane H + ATPase (PM H + ATPase), which is known to be regulated by reversible protein phosphorylation and subsequent binding of 14-3-3 isoforms. Immunodetection of the phosphorylated penultimate threonine residue of the pollen PM H + ATPase LilHA1 of Lilium longiflorum pollen revealed a sudden increase in phosphorylation with the start of pollen tube growth. In addition to phosphorylation, pH modulated the binding of 14-3-3 isoforms to the regulatory domain (R domain) of the H + ATPase, whereas metabolic components had only little effects on 14-3-3 binding as tested in in vitro assays using recombinant produced 14-3-3 isoforms and phosphomimicking substitutions of the threonine residue. In consequence of these results, local H + influxes and effluxes as well as pH gradients in the pollen tube tip are generated by localised regulation of the H + ATPase activity and not only by heterogeneous distribution in the plasma membrane.