Genetic analysis of incurvata mutants reveals three independent genetic operations at work in arabidopsis leaf morphogenesis

In an attempt to identify genes involved in the control of leaf morphogenes is, we have studied 13 Arabidopsis thaliana mutants with curled, involute l eaves, a phenotype herein referred to as Incurvata (Icu), which were isolat ed by G. Robbelen and belong to the Arabidopsis Information Service Form Mu tants collection. The Icu phenotype was inherited as a single recessive tra it in 10 mutants, with semidominance in 2 mutants and with complete dominan ce in the remaining 1. Complementation analyses indicated that the studied mutations correspond to five genes, representative alleles of which were ma pped relative to polymorphic microsatellites. Although most double-mutant c ombinations displayed additivity of the Icu phenotypes, those of inc1 icu2 and icu3 icu4 double mutants were interpreted as synergistic, which suggest s that the five genes studied represent three independent genetic operation s that are at work for the leaf to acquire its final form at full expansion . We have shown that icu1 mutations are alleles of the Polycomb group gene CURLY LEAF ( CLF) and that the leaf phenotype of the icu2 mutant is suppres sed in an agamous background, as is known for clf mutants. In addition, we have tested by means of multiplex RT-FCR the transcription of several flora l genes in Icu leaves. Ectopic expression of AGAMOUS and APETALA3 was obser ved in clf and icu2 but not in icu? icu4, and icu5 mutants. Taken together, these results suggest that CLF and ICU2 play related roles, the latter bei ng a candidate to belong to the Polycomb group of regulatory genes. We prop ose that, as flowers evolved, a new major class of genes, including CLF and ICU2, may have been recruited to prevent the expression of floral homeotic genes in the leaves.