Boron deficiency alters cytosolic Ca2+ concentration and affects the cell wall components of pollen tubes in Malus domestica

Abstract

Boron (B) is essential for normal plant growth, including pollen tube growth. B deficiency influences various physiological and metabolic processes in plants. However, the underlying mechanism of B deficiency in pollen tube growth is not sufficiently understood. In the present research, the influence of B deficiency on apple (Malus domestica) pollen tube growth was studied and the possible regulatory mechanism evaluated. Apple pollen grains were cultured under different concentrations of B. Scanning ion-selective electrode technique, fluorescence labelling and Fourier-transform infrared (FTIR) analysis were used to detect calcium ion flux, cytosolic Ca²⁺ concentration ([Ca²⁺ ]cyt), actin filaments and cell wall components of pollen tubes. B deficiency inhibited apple pollen germination and induced retardation of tube growth. B deficiency increased extracellular Ca²⁺ influx and thus led to increased [Ca²⁺ ]cyt in the pollen tube tip. In addition, B deficiency modified actin filament arrangement at the pollen tube apex. B deficiency also altered the deposition of pollen tube wall components. Clear differences were not observed in the distribution patterns of cellulose and callose between control and B deficiency treated pollen tubes. However, B deficiency affected distribution patterns of pectin and arabinogalactan proteins (AGP). Clear ring-like signals of pectins and AGP on control pollen tubes varied according to B deficiency. B deficiency further decreased acid pectins, esterified pectins and AGP content at the tip of the pollen tube, which were supported by changes in chemical composition of the tube walls. B appears to have an active role in pollen tube growth by affecting [Ca²⁺ ]cyt, actin filament assembly and pectin and AGP deposition in the pollen tube. These findings provide valuable information that enhances our current understanding of the mechanism regulating pollen tube growth.

Keywords: Malus domestica; Actin; B deficiency; calcium; cell wall components.