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Throughout development the Arabidopsis shoot apical meristem successively undergoes several major phase transitions such as the juvenile-to-adult and floral transitions until, finally, it will produce flowers instead of leaves and shoots. Members of the Arabidopsis SBP-box gene family of transcription factors have been implicated in promoting the floral transition in dependence of miR156 and, accordingly, transgenics constitutively over-expressing this microRNA are delayed in flowering. To elaborate their roles in Arabidopsis shoot development, we analysed two of the 11 miR156 regulated Arabidopsis SBP-box genes, i.e. the likely paralogous genes SPL9 and SPL15. Single and double mutant phenotype analysis showed these genes to act redundantly in controlling the juvenile-to-adult phase transition. In addition, their loss-of-function results in a shortened plastochron during vegetative growth, altered inflorescence architecture and enhanced branching. In these aspects, the double mutant partly phenocopies constitutive MIR156b over-expressing transgenic plants and thus a major contribution to the phenotype of these transgenics as a result of the repression of SPL9 and SPL15 is strongly suggested.

Gibberellins (GAs) are important plant growth regulators, regulating many plant developmental processes, including seed germination, root and stem elongation, rosette expansion, floral induction and anther development. The diverse effects of GAs on plant development make it critical to maintain an appropriate endogenous GA level and a fine-tuned GA signalling. Some global regulators in GA signalling have been identified but little is known about genes specifically involved in local implementation of GA signalling. Here we report that the Arabidopsis thaliana SBP-box gene SQUAMOSA-PROMOTER-BINDING-PROTEIN-LIKE8 (SPL8) acts as a local regulator in a subset of GA-dependent developmental processes. Previous knowledge holds that SPL8 is involved in reproductive development as deduced from its loss-of-function phenotype (Unte et al. (2003) Plant Cell 15:1009-1019). We now determined that constitutive overexpression of SPL8 affects fertility due to non-dehiscent anthers, likely resulting from a constitutive GA response, suggesting a positive role of SPL8 in GA-mediated anther development. On the other hand, SPL8 gain- and loss-of-function mutants showed opposite responses to GA and its biosynthetic inhibitor paclobutrazol (PAC) with respect to seed germination and root elongation during the seedling stage. Genes involved in GA biosynthesis and signalling are transcriptionally affected by altered SPL8 expression. Our study uncovered a tissue-dependent regulatory role for SPL8 in the response to GA signalling in plant development.

Theissen and Saedler argue that Goethe was right when he proposed that flowers are modified leaves. Recent research found that four genes involved in plant development must be expressed together to turn leaves into floral organs.

The MADS box motif is common to genes that regulate the pattern of flower development. To determine whether MADS box genes also play a role in differentiation of the sexes in dioecious plants, we isolated cDNAs (SLM1 to SLM5, for Silene latifolia MADS) with MADS box homology from transcripts of male flower buds of the model dioecious species white campion and compared their expression in developing female and male flowers. SLM1 had extensive sequence similarity to the snapdragon MADS box gene PLENA, SLM2 to GLOBOSA, SLM3 to DEFICIENS, and both SLM4 and SLM5 were similar to SQUAMOSA. Each of the white campion MADS box genes was expressed in the same floral whorls as their respective most homologous snapdragon genes. The sex of the plant affected the pattern of SLM2 and SLM3 expression in the petal and stamen whorls, resulting in a smaller fourth whorl in male flowers than in female flowers. This was correlated with repressed gynoecium development in male flowers. The expression of SLM4 and SLM5 in both sexes differed from that of SQUAMOSA in one important aspect. Unlike SOUAMOSA, they were expressed in inflorescence meristems. This may reflect differences in growth pattern between white campion and snapdragon

Class B floral homeotic genes specify the identity of petals and stamens during the development of angiosperm flowers. Recently, putative orthologs of these genes have been identified in different gymnosperms. Together, these genes constitute a clade, termed B genes. Here we report that diverse seed plants also contain members of a hitherto unknown sister clade of the B genes, termed B(sister) (B(s)) genes. We have isolated members of the B(s) clade from the gymnosperm Gnetum gnemon, the monocotyledonous angiosperm Zea mays and the eudicots Arabidopsis thaliana and Antirrhinum majus. In addition, MADS-box genes from the basal angiosperm Asarum europaeum and the eudicot Petunia hybrida were identified as B(s) genes. Comprehensive expression studies revealed that B(s) genes are mainly transcribed in female reproductive organs (ovules and carpel walls). This is in clear contrast to the B genes, which are predominantly expressed in male reproductive organs (and in angiosperm petals). Our data suggest that the B(s) genes played an important role during the evolution of the reproductive structures in seed plants. The establishment of distinct B and B(s) gene lineages after duplication of an ancestral gene may have accompanied the evolution of male microsporophylls and female megasporophylls 400-300 million years ago. During flower evolution, expression of B(s) genes diversified, but the focus of expression remained in female reproductive organs. Our findings imply that a clade of highly conserved close relatives of class B floral homeotic genes has been completely overlooked until recently and awaits further evaluation of its developmental and evolutionary importance. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00438-001-0615-8.

Land plants (embryophytes) are characterized by an alternation of two generations, the haploid gametophyte and the diploid sporophyte. The development of the small and simple male gametophyte of the flowering plant Arabidopsis (Arabidopsis thaliana) critically depends on the action of five MIKC* group MCM1-AGAMOUS-DEFICIENS-SRF-box (MADS-box) proteins. In this study, these MIKC* MADS-box genes were isolated from land plants with relatively large and complex gametophyte bodies, namely the bryophytes. We found that although the gene family expanded in the mosses Sphagnum subsecundum, Physcomitrella patens, and Funaria hygrometrica, only a single homologue, Marchantia polymorpha MADS-box gene 1 (MpMADS1), has been retained in the liverwort M. polymorpha. Liverworts are the earliest diverging land plants, and so a comparison of MpMADS1 with its angiosperm homologues addresses the molecular evolution of an embryophyte-specific transcription factor over the widest phylogenetic distance. MpMADS1 was found to form a homodimeric DNA-binding complex, which is in contrast to the Arabidopsis proteins that are functional only as heterodimeric complexes. The M. polymorpha homodimer, nevertheless, recognizes the same DNA sequences as its angiosperm counterparts and can functionally replace endogenous MIKC* complexes to a significant extent when heterologously expressed in Arabidopsis pollen. The 11 MIKC* homologues from the moss F. hygrometrica are highly and almost exclusively expressed in the gametophytic generation. Taken together, these findings suggest that MIKC* MADS-box proteins have largely preserved molecular roles in the gametophytic generation of land plants.

The genome of Arabidopsis (Arabidopsis thaliana) encodes over 100 MADS-domain transcription factors, categorized into five phylogenetic subgroups. Most research efforts have focused on just one of these subgroups (MIKCc), whereas the other four remain largely unexplored. Here, we report on five members of the so-called Mδ or Arabidopsis MIKC* (AtMIKC*) subgroup, which are predominantly expressed during the late stages of pollen development. Very few MADS-box genes function in mature pollen, and from this perspective, the AtMIKC* genes are therefore highly exceptional. We found that the AtMIKC* proteins are able to form multiple heterodimeric complexes in planta, and that these protein complexes exhibit a for the MADS-family unusual and high DNA binding specificity in vitro. Compared to their occurrence in promoters genome wide, AtMIKC* binding sites are strongly overrepresented in the proximal region of late pollen-specific promoters. By combining our experimental data with in silico genomics and pollen transcriptomics approaches, we identified a considerable number of putative direct target genes of the AtMIKC* transcription factor complexes in pollen, many of which have known or proposed functions in pollen tube growth. The expression of several of these predicted targets is altered in mutant pollen in which all AtMIKC* complexes are affected, and in vitro germination of this mutant pollen is severely impaired. Our data therefore suggest that the AtMIKC* protein complexes play an essential role in transcriptional regulation during late pollen development.

In vegetative leaf tissues, cuticles including cuticular waxes are important for protection against nonstomatal water loss and pathogen infection as well as for adaptations to environmental stress. However, their roles in the anther wall are rarely studied. The innermost layer of the anther wall (the tapetum) is essential for generating male gametes. Here, we report the characterization of a T-DNA insertional mutant in the Wax-deficient anther1 (Wda1) gene of rice (Oryza sativa), which shows significant defects in the biosynthesis of very-long-chain fatty acids in both layers. This gene is strongly expressed in the epidermal cells of anthers. Scanning electron microscopy analyses showed that epicuticular wax crystals were absent in the outer layer of the anther and that microspore development was severely retarded and finally disrupted as a result of defective pollen exine formation in the mutant anthers. These biochemical and developmental defects in tapetum found in wda1 mutants are earlier events than those in other male-sterile mutants, which showed defects of lipidic molecules in exine. Our findings provide new insights into the biochemical and developmental aspects of the role of waxes in microspore exine development in the tapetum as well as the role of epicuticular waxes in anther expansion.

Pod corn is a classic morphological mutant of maize in which the mature kernels of the cob are covered by glumes, in contrast to generally grown maize varieties in which kernels are naked. Pod corn, known since pre-Columbian times, is the result of a dominant gain-of-function mutation at the Tunicate (Tu) locus. Some classic articles of 20th century maize genetics reported that the mutant Tu locus is complex, but molecular details remained elusive. Here, we show that pod corn is caused by a cis-regulatory mutation and duplication of the ZMM19 MADS-box gene. Although the WT locus contains a single-copy gene that is expressed in vegetative organs only, mutation and duplication of ZMM19 in Tu lead to ectopic expression of the gene in the inflorescences, thus conferring vegetative traits to reproductive organs.