Pollen Tube Growth and Early Embryogenesis in Wheat x Maize Crosses Influenced by 2,4-D

Abstract

Pollen tube growth and embryogenesis were investigated in the intergeneric Triticum aestivum × Zea mays cross. Emasculated wheat florets were pollinated with maize pollen, and at one day after pollination, the wheat plants were injected with 2,4-dichlorophenoxyacetic acid (2,4-D). The influence of 2,4-D on pollen tube behaviour was determined applying callose staining in whole mount preparations of pistils. Changes in the embryo sac and early embryogenesis were analysed on sections after various histochemical stainings. Maize pollen tubes germinated within 30 min and grew much slower through the pollen tube pathway compared with selfings of both maize and wheat. Deviations in pollen tube growth occurred such as coiling, widening and forking, irrespective of treatment with 2,4-D. Pollen tubes reached the micropyle between 5 and 24 h after pollination. 2,4-D treatment increased the number of the pollen tubes that reached the micropyle, and additionally, multiplication of sperm cells was found in the maize pollen tubes. Embryo formation was analysed at 2 d after pollination. Only when pollen tubes were in the micropyle, zygotes and embryos were observed. This points to their hybrid origin. Moreover, the embryos are likely of zygotic origin since multicellular embryos exhibited micronuclei, a sign of chromosome elimination not occurring in parthenogenic embryos and leading to haploidy of the embryos. Treatment with 2,4-D increased successful intergeneric fertilization from 18·7 to 69·3 %. Immature embryos were rescued by in vitro culture from 14 % of pollinated florets when excised at 14 d after pollination.