| Genomic and reverse genetic approaches were used to identify and study new genes important for plant development. Initially, a subtractive Arabidopsis floral cDNA library was characterized by sequencing and analyzing a subset of 1579 EST sequences that represented 1209 unigenes. These ESTs provided new evidence for the expression of putative or hypothetical genes and helped refine the transcript annotation for some genes. 86 regulatory genes including transcription factors and protein kinases were identified from this EST dataset. Expression of these floral genes was further characterized using home-designed and -printed cDNA microarray chips. It showed that many floral genes were expressed at low levels. Furthermore, 97 genes were identified as flower-preferential ones in comparison to the leaf.;The above work led to the identification of a novel plant-specific family of zinc finger homeodomain proteins (ZHD). Proteins of this family were also reported from the C4 plant Flaveria. ZHD proteins have an N-terminal putative zinc finger, and a C-terminal atypical homeodomain distantly related to all known homeodomains. Sequence and phylogenetic analysis indicated that the ZHD family originated as early as from the non-vascular plant Physcomitrella, and could be divided into 6 clades, with Clade I, II and III most conserved, Clade IV mid-level divergent, and Clade V and VI most divergent. A new subfamily named as MINI ZINC FINGER (MIF) was also identified that has only the N-terminal zinc finger domain of the ZHD proteins. Current evidence suggested that the MIF proteins were derived from the ZHD proteins by loss of the homeodomain and then evolved separately. In addition, examination of evolutionary relationship, gene duplication, gene expression and mutant screening of Arabidopsis ZHD genes implied a high level of gene functional redundancy among this family.;The function of three Arabidopsis MIF genes was characterized by gain-of-function approaches. Constitutive overexpression of MIF1 caused dramatic developmental defects, including dwarfism, reduced apical dominance, extreme longevity, dark-green leaves, altered flower morphology, poor fertility, reduced hypocotyl length, spoon-like cotyledons, reduced root growth, and ectopic root hairs on hypocotyls and cotyledons. In addition, 35S::MIF1 seedlings underwent constitutive photomorphogenesis in the dark, with root growth similar to that in the light. 35S::MIF1 seedlings were further demonstrated to be non-responsive to gibberellin (GA) for cell elongation, hypersensitive to the GA synthesis inhibitor PAC and abscisic acid (ABA), and hyposensitive to auxin, brassinosteroid and cytokinin. Consistent with these observations, genome-scale expression profiling revealed that 35S::MIF1 seedlings exhibited decreased expression of genes involved in GA, auxin and brassinosteroid signaling as well as cell elongation/expansion, and increased expression of ABA-responsive genes. 35S::MIF2 and 35S::MIF3 transgenic plants exhibited similar phenotypes of 35S::MIF1 plants. On the basis of these results, it was proposed that the MIF proteins or the protein(s) with which they interact are involved in mediating the regulation of plant development by multiple hormones.;Overexpression of MIF1 or MIF3 also disrupted the determinate growth of leaves by inducing ectopic shoot meristems along leaf margins. We demonstrated that the ectopic meristems occurred specifically in the marginal region corresponding to the serration sinus of late rosette leaves in an ERECTA-dependent manner. Expression of STM was activated in these ectopic meristems. Altered GA and auxin response and/or level were implicated in the occurrence of ectopic meristems. Thus, this study provided insights for the sinus of leaf margin as a suppressed domain that maintains the potential for regenerating meristems. Furthermore, 35S::MIF1 and 35S::MIF3 transgenic plants may provide a new genetic system for dissecting the molecular mechanisms that suppress indeterminate growth in leaves and for understanding the interactions between hormones and meristem activity. |
