Pollen tube in hazel grows intermittently: Role of Ca2+ and expression of auto-inhibited Ca2+ pump

Abstract

Owing to the absence of the ovary when the female inflorescence blooms, hazel pollen tubes show an obvious intermittent growth pattern, which has an important impact on the number of fruits per cluster and yield, but little is known about the mechanisms governing this process. Spray treatments of Ca2+ and Ca2+-ATPase activator 5-aminolevulinic acid (ALA) in field experiment showed that Ca2+ treatments inhibited pistillate inflorescence drop and produced more fruits, while the effects of ALA treatments was just opposite to that of calcium treatments. Highly accumulated Ca2+ was observed at the rear part of the stigma and in the ovary primordial cell layers. Under in vitro culture conditions, pollen germination ratio and tube length were promoted and inhibited by Ca2+ and ALA, respectively. ALA treatment reduced the Ca2+ concentration in pollen tube, which was not conducive to the formation of a tip-focused Ca2+ gradient in pollen tube. The hazel genome encoded 17 Ca2+-ATPases, including 14 auto-inhibited Ca2+-ATPases and three ER-type Ca2+-ATPases. Hazel Ca2+-ATPase family proteins, including 11 common transmembrane domains, were highly conserved. Of 17 Ca2+-ATPases, qRT-PCR analysis showed that ChACA1 and ChECA2 were highly expressed in pollen tubes. In the range of 0–1.3 mM Ca2+, Ca2+ fluorescence intensity in pollen tubes, expression of ChACA1 continued to rise simultaneously, and they reached their maximum at 1.3 mM Ca2+, then declined at 1.6 mM Ca2+, which was consistent and inconsistent with changing trends of pollen tube length and ChECA2 respectively. Analysis of enzyme activity in pollen tubes showed that Ca2+-ATPase activity was promoted by both Ca2+ and ALA treatments significantly. Our findings suggested that ChACA1 may act as a key regulator of pollen germination and pollen tube development in hazel, and provide new insight into the mechanisms of unique pollen tube intermittent growth in hazel.