Boron influences pollen germination and pollen tube growth in Picea meyeri

To study the role of boron in pollen germination and pollen tube growth of Picea meyeri Rehd. et Wils., pollen grains were cultured in standard medium or boron-deficient medium. Effects of boron on the localization of pectins and callose in the walls of pollen tubes were observed by laser scanning confocal microscopy after staining with aniline blue or immunolabeling with antibodies JIM5 and JIM7. Changes in the structures of pectins and phenolics were investigated by fourier transform infrared (FTIR) microspectroscopy. Pollen germination in boron-deficient medium ranged from 18 to 24%, whereas pollen germination in standard medium reached 61%. Callose accumulated in the tip-regions of pollen tubes cultured in boron-deficient medium, but not in standard medium. Immunolabeling with antibody JIM5 revealed that acidic pectin preferentially accumulated in the tip regions of pollen tubes cultured in boron-deficient medium, whereas acidic pectin was weakly distributed along the entire lengths of pollen tubes cultured in standard medium. Esterified pectin, detected by immunolabeling with antibody JIM7, showed a similar distribution pattern in pollen tubes in both the boron-deficient and standard treatments. The FTIR spectra indicated slight increases in contents of phenolics and carboxylic acids and a substantial decrease in the content of saturated esters in boron-deficient pollen tubes compared with normal pollen tubes. The FTIR spectra confirmed that boron deficiency enhanced acidic pectin accumulation in pollen tubes, which may be associated with the increased content of carboxylic acid. We conclude that boron has a regulatory role in pollen germination and pollen tube growth.