Ni2+ effects on Nicotiana tabacum L. pollen germination and pollen tube growth

To investigate the mechanisms of Ni2+ effects on initiation and maintenance of polar cell growth, we used a well-studied model system—germination of angiosperm pollen grains. In liquid medium tobacco pollen grain forms a long tube, where the growth is restricted to the very tip. Ni2+ did not prevent the formation of pollen tube initials, but inhibited their subsequent growth with IC50 = 550 μM. 1 mM Ni2+ completely blocked the polar growth, but all pollen grains remained viable, their respiration was slightly affected and ROS production did not increase. Addition of Ni2+ after the onset of germination had a bidirectional effect on the tubes development: there was a considerable amount of extra-long tubes, which appeared to be rapidly growing, but the growth of many tubes was impaired. Studying the localization of possible targets of Ni2+ influence, we found that they may occur both in the wall and in the cytoplasm, as confirmed by specific staining. Ni2+ disturbed the segregation of transport vesicles in the tips of these tubes and significantly reduced the relative content of calcium in the aperture area of pollen grains, as measured by X-ray microanalysis. These factors are considered being critical for normal polar cell growth. Ni2+ also causes the deposition of callose in the tips of the tube initials and the pollen tubes that had stopped their growth. We can assume that Ni2+-induced disruption of calcium homeostasis can lead to vesicle traffic impairment and abnormal callose deposition and, consequently, block the polar growth.