ALTERED EXPRESSION OF FLOWERING CLASS-B AND CLASS-C GENES IN THE APPENDIX TOBACCO MUTANT

In the tobacco (Nicotiana tabacum) Appendix mutant, anthers are tipped by a miniature style and stigma. The outgrowth appears on the anther when it is already differentiating and follows the developmental timin g of the central carpel. The Appendix mutation thus represents a late homeotic transformation suggesting that the APPENDIX (APX) gene either could be a misregulated organ identity gene or could be involved in r egulating the expression of such genes. RFLP analysis with two class B (TM6 and NTGLO) and a class C (NAG) probes revealed that the Appendix phenotype is not caused by a mutation in one of these genes. However, in situ hybridization showed important changes in the expression of N TGLO and NAG in the mutant when compared with wild-type tobacco. Surpr isingly, although no phenotypic alteration other than the style and st igma outgrowth is observed in the Appendix mutant, changes in class B and class C gene expession were not restricted to the anther tip cells from which the outgrowth originates. As expected, NAG was expressed i n the Appendix outgrowth but it was also overexpressed in the normal t hird and fourth whorl organs at the time the outgrowth, as well as the central styles and stigmas, differentiated. Overexpression of a class C gene is probably responsible for the Appendix phenotype. In normal and mutant flowers, NTGLO was expressed in the second, third and fourt h whorls up to the time of carpel fusion. Expression of this class B g ene then ceased in the fourth whorl organs but was reactivated at late r stages only in the styles and stigmas as well as in the outgrowths o f the mutant. It thus seems that the function of the APX gene is eithe r to regulate the late expression of organ identity genes or to contro l cell proliferation in such a way that, in the mutant, some cells are in a state where they respond in an unusual way to developmental sign als.