Alternative pathways of tobacco placental development: time of commitment and analysis of a mutant

Previous work from our lab identified mutants, Mgr3 and Mgr9, of tobacco (Nicotiana tabacum) that produced unusual elongated green outgrowths from placentae in vivo. Similarly appearing stigmatoid growths were described developing from some in vitro cultures of excised placentae of tobacco (Hicks and McHughen, 1974, 1977). Here we report a developmental analysis and comparison of the unusual stigmatoid outgrowths seen in in vitro cultures of wild-type and mutant placentae, as well as the green outgrowths seen in vivo in the mutants. The growths produced by wild-type and mutant placental cultures in vitro, and the growths produced by the mutants in vivo, are identified as abnormal stigmas and styles. Wild-type in vitro placental cultures also produce outgrowths identified as homologs of whole carpels. Carpel fusion is not required for differentiation of stigma, style, transmitting tract, vascular traces, ovary, and ovules in these structures. The type and extent of stigmatoid growth production depends upon the age of the explant at excision and culture initiation. Before ovule primordium initiation, few growths are seen in culture; for a short window of time afterward, the primordia are competent to give rise to stigmatoid and carpelloid growths when cultured. After commitment to ovule development occurs, the primordia produce only ovules when cultured. The behavior of the mutant placental cultures is dimorphic. Explants from early stages behave similarly to wild-type when cultured, but differences between wild-type and the mutant behaviors in culture arise at the time when the stigmatoid growths begin to appear in vivo in the mutants. These results imply that ovule primordia pass through stages of distinct sequential restrictions of fate, first to growth as gynoecia, and then second to growth as ovules. The mutant strains described here perturb the commitment to differentiation as ovules.