Role of cytokinins in arabidopsis flower development
This study was conducted to investigate the possible role of cytokinins (CKs) in the gene-controlled flowering program in Arabidopsis thaliana . Four different experimental approaches and two groups of floral mutants, i.e., floral meristem identity (FMI) and floral organ identity (FOI) mutants were used to analyze the roles of CKs during the development of flowers. In vivo application of both a synthetic and a natural CK induced abnormalities in floral phenotypes of wild type plants. In particular, application of 3 [mu]l of 10-3 M benzylaminopurine (BAP) induced floral phenocopies of FMI mutants in both the wild type and FOI mutants. BAP also the phenotype of the weak allele, apetala2-1 (ap2-6), to resemble the strong allele, ap2-6. BAP application to FMI mutants, ap1-1 and ap1-3 strong and weak alleles of APETALA1 respectively, resulted in the enhanced conversion of flowers to inflorescence-like structures. Similar conversions of flowers to inflorescence-like structures were also observed in BAP treated ap2-1 mutants. In vitro culture of inflorescence explants showed that wild type inflorescences produced flowers in the absence of CKs, and the addition of CKs to the culture medium induced abnormalities similar to those observed in in vivo experiments. These results demonstrate that in vitro growth of Arabidopsis flowers can take place in the absence of exogenous CKs, and suggest that exogenous CKs suppress FMI gene functions as observed in in vivo FMI experiments. This led to a study of mode of CK interaction with FMI genes. Two possible postulates were considered for the roles of CKs in FMI: (1) mutations in FMI genes alter the biosynthesis or metabolism of CKs, and (2) CKs suppress the expression of FMI genes. The first postulate was tested by analysing endogenous CKs in the wild type and FMI mutants, ap1-1 and clavata1-1 (clv1-1). The predominant CK in the wild type and in both FMI mutant tissues was dihydrozeatin (DZ). In the clv1-1 mutant, DZ levels were 10 to 13 fold higher in comparison to the wild type. The ap1-1 mutants did not show major changes in CK levels. This suggested that the CLV1 gene negatively regulates CK levels in the floral meristems of Arabidopsis. The second postulate was studied using altered meristem program1-1 ( amp1-1), a CK-overproducer mutant, to construct double mutant combinations with FMI mutants ap1-1, ap2-1and clv1-1. The double mutants, amp1-1 ap1-1, and amp1-1 ap2-1, showed enhanced conversions of flowers to inflorescence-like structures, supporting the view that high endogenous CKs suppress FMI function in the absence of a functional FMI gene. However, amp1-1 clv1-1 double mutant did not show any such conversions indicating that the role of CLV1 in the establishment of FMI is secondary, and that the role of CKs in FMI becomes obvious only in the absence of the AP1 or AP2 gene. Based on this study a model is proposed which suggests that in wild-type floral meristems, the 'CLV1' gene negatively regulates CK levels which allow the complete activation of FMI genes, and resulting in the formation of a flower.
Doctor of Philosophy (Ph.D.)