CELL ABLATION REVEALS THAT EXPRESSION FROM THE PHASEOLIN PROMOTER IS CONFINED TO EMBRYOGENESIS AND MICROSPOROGENESIS

Most previous studies of the beta-phaseolin (phas) gene, which encodes the major storage protein in bean (Phaseolus vulgaris L.), have shown its expression to be rigorously confined to the developing seed, both in bean and transgenic tobacco (Nicotiana tabacum L. cv Xanthi) plant s. To confirm unequivocally the lack of phas expression in vegetative tissues, we placed the diphtheria toxin A-chain (DT-A) coding region u nder the control of beta-phaseolin promoter sequences. Tobacco plants transgenic for phas/DT-A were phenotypically normal until flowering, w hen they produced anthers that were externally normal but contained no viable pollen. Microscopic examination of immature anthers revealed a normal tapetum, but the pollen mother cells did not undergo meiosis a nd subsequently degenerated, resulting in male-sterile plants. This de monstration of phas expression during microsporogenesis was corroborat ed by the expression of beta-glucuronidase in pollen of plants transfo rmed with comparable phas/uidA constructs. Although these findings sug gested that similarities in phas expression may exist between seed and pollen maturation, no phas activity could be detected in bean pollen. After fertilization of the DT-A-transformed plants with pollen from w ild-type tobacco, 50% of the resulting embryos aborted at the heart st age, defining this as the earliest time for phas expression during emb ryogenesis.