Explore the vast range of titles available.

The Arabidopsis thaliana F-box gene HAWAIIAN SKIRT (HWS) affects organ growth and the timing of floral organ abscission. The loss-of-function hws-1 mutant exhibits fused sepals and increased organ size. To understand the molecular mechanisms of HWS during plant development, we mutagenized hws-1 seeds with ethylmethylsulphonate (EMS) and screened for mutations suppressing hws-1 associated phenotypes. We isolated the shs1/ hws-1 (suppressor of hws-1) mutant in which hws-1 sepal fusion phenotype was suppressed. The shs1/hws-1 mutant carries a G -> A nucleotide substitution in the MIR164 binding site of CUP-SHAPED COTYLEDON 1 (CUC1) mRNA. CUC1 and CUP-SHAPED COTYLEDON 2 (CUC2) transcript levels were altered in shs1, renamed cuc1-1D, and in hws-1 mutant. Genetic interaction analyses using single, double and triple mutants of cuc1-1D, cuc2-1D (a CUC2 mutant similar to cuc1-1D), and hws-1, demonstrate that HWS, CUC1 and CUC2 act together to control floral organ number. Loss of function of HWS is associated with larger petal size due to alterations in cell proliferation and mitotic growth, a role shared with the CUC1 gene.

At an early stage of shoot regeneration from calli of Arabidopsis, pre-meristematic cell mounds develop in association with localized strong expression of CUP-SHAPED COTYLEDON (CUC) genes. Previous characterization of root initiation-defective 3 (rid3), an Arabidopsis mutant originally isolated as being temperature-sensitive for adventitious root formation, with respect to shoot regeneration implicated RID3 in the negative regulation of CUC1 expression and the restriction of cell division in pre-meristematic cell mounds. Positional cloning has identified RID3 as a WD40 repeat protein gene whose molecular function was not investigated before. Here we performed in silico analysis of RID3 and found that RID3 is orthologous to IPI3, which mediates pre-rRNA processing in Saccharomyces cerevisiae. In the rid3 mutant, rRNA precursors accumulated to a very high level in a temperature-dependent manner. This result indicates that RID3 is required for pre-rRNA processing as is IPI3. We compared rid3 with rid2, a temperature-sensitive mutant that is mutated in a putative RNA methyltransferase gene and is impaired in pre-rRNA processing, for seedling morphology, shoot regeneration, and CUC1 expression. The rid2 and rid3 seedlings shared various developmental alterations, such as a pointed-leaf phenotype, which is often observed in ribosome-related mutants. In tissue culture for the induction of shoot regeneration, both rid2 and rid3 mutations perturbed cell-mound formation and elevated CUC1 expression. Together, our findings suggest that rRNA biosynthesis may be involved in the regulation of CUC1 gene expression and pre-meristematic cell-mound formation during shoot regeneration.

Key Points Plant development is fundamentally different from developmental patterning in most animals in that most of the body plan is established after embryogenesis. This is possible because plants retain two populations of stem cell — the shoot meristem and the root meristem. Shoot meristems regulate organ formation be carefully balancing the maintenance of indifferentiated stem cells and the commitment of appropriately positioned cells towards differentiation. Genetics screens are now revealing the signalling pathways that regulate this balance and the mechanisms that subsequently mediate organ development. Screens in Arabidopsis thaliana for mutants that lack stem cells or have ectopic stem cells have uncovered a signal-transduction pathway that controls stem cell behaviour. The CLAVATA ( CLV ) signalling pathway components so far identified consist of a putative receptor kinase ( CLV1 ), its dimerization partner ( CLV2 ) and its proposed ligand ( CLV3 ). Other interacting proteins include the kinase associated protein phosphatase KAPP, which is proposed to negatively regulate CLV1, and ROP, a member of the small GTPase Ras superfamily, which might regulate downstream signalling. WUSCHEL, a putative homeodomain-containing transcription factor that signals the stem-cell fate to overlying cells at the shoot meristem, is regulated, at least indirectly, by the CLV proteins. A feedback loop has been shown between WUS and CLV3, which allows a equilibrium to be achieved, resulting in a stable population of stem cells. After the onset of differentiation, organs are initiated in distinct spatial patterns. Further analysis of PERIANTHIA , which is required for the right number of organs in the flower meristem, might shed light on how this occurs. Other mutants have been identified that mediate organ separation during the early stages of organ intiation. Loss of STM , CUC1 or CUC2 results in fusion of the earliest organs initiated. Organ polarity is initiated by asymmetric growth during the earliest stages of organ development. The molecules that direct this are being uncovered in snapdragon, maize and Arabidopsis , revealing conserved mechanisms. The regulation of cell differentiation at meristems is crucial to developmental patterning in plants. Rapid progress has been made in identifying the genes that regulate differentiation and the receptor-mediated signalling events that have a key role in this process. In particular, we are now learning how the CLAVATA receptor kinase signalling pathway promotes stem cell differentiation in balance with the initiation of stem cells by the transcription factor WUSCHEL.

Most dioecious plant species are believed to derive from hermaphrodite ancestors. The regulatory pathways that have been modified during evolution of the hermaphrodite ancestors and led to the emergence of dioecious species still remain unknown. Silene latifolia is a dioecious plant species harboring XY sex chromosomes. To identify the molecular mechanisms involved in female organ suppression in male flowers of S. latifolia, we looked for genes potentially involved in the establishment of floral organ and whorl boundaries. We identified homologs of Arabidopsis thaliana SHOOTMERISTEMLESS (STM) and CUP SHAPED COTYLEDON (CUC) 1 and CUC2 genes in S. latifolia. Our phylogenetic analyses suggest that we identified true orthologs for both types of genes. Detailed expression analyses showed a conserved expression pattern for these genes between S. latifolia and A. thaliana, suggesting a conserved function of the corresponding proteins. Comparative in situ hybridization experiments between male, female, and hermaphrodite individuals reveal that these genes show a male-specific pattern of expression before any morphological difference become apparent. Our results make SlSTM and SlCUC strong candidates for being involved in sex determination in S. latifolia.

Axillary meristems (AMs) are secondary shoot meristems whose outgrowth determines plant architecture. In rice, AMs form tillers, and tillering mutants reveal an interplay between transcription factors and the phytohormones auxin and strigolactone as some factors that underpin this developmental process. Previous studies showed that knockdown of the transcription factor gene RFL reduced tillering and caused a very large decrease in panicle branching. Here, the relationship between RFL, AM initiation, and outgrowth was examined. We show that RFL promotes AM specification through its effects on LAX1 and CUC genes, as their expression was modulated on RFL knockdown, on induction of RFL:GR fusion protein, and by a repressive RFL–EAR fusion protein. Further, we report reduced expression of auxin transporter genes OsPIN1 and OsPIN3 in the culm of RFL knockdown transgenic plants. Additionally, subtle change in the spatial pattern of IR4 DR5:GFP auxin reporter was observed, which hints at compromised auxin transport on RFL knockdown. The relationship between RFL, strigolactone signalling, and bud outgrowth was studied by transcript analyses and by the tillering phenotype of transgenic plants knocked down for both RFL and D3. These data suggest indirect RFL–strigolactone links that may affect tillering. Further, we show expression modulation of the auxin transporter gene OsPIN3 upon RFL:GR protein induction and by the repressive RFL–EAR protein. These modified forms of RFL had only indirect effects on OsPIN1. Together, we have found that RFL regulates the LAX1 and CUC genes during AM specification, and positively influences the outgrowth of AMs though its effects on auxin transport.

An emerging aspect of plant development and adaptation studies is microRNA (miRNA)-mediated gene regulation and to understand post-transcriptional gene silencing (PTGS) dynamics. Studies performed within an evolutionary context, such as specific plant lineages and genome status (e.g. ploidy level) can highlight the selection pressure and constraints guiding the evolutionary trajectory of regulatory modules. We have investigated conservation and allelic diversity in one such module composed of CUC1 ( CUP-SHAPED COTYLEDON 1 ), a NAC domain family member, and miR164C , within Brassica species. Ancestral CUC1 sequences, including those belonging to Amborellales, Nymphaeales and Austrobaileyales, harbour miR164 binding sites, implying an ancient nature of this regulatory module. We found that sequence and length polymorphisms occurred in the miR164C precursor of diploid and amphidiploid Brassica species, although the 21-nt mature sequence was invariant. In contrast, the target sequence in CUC1 exhibited polymorphism at the “seed” and “cleavage” site, otherwise governed by stringent rules of pairing for successful PTGS, implying a “relaxed” selection pressure on CUC1 . Further studies are needed to improve our understanding of the functional implications of the polymorphism and single nucleotide polymorphisms that constitute allelic variation at the region responsible for PTGS within Brassica species, in context of ploidy level, and to unravel the evolutionary dynamics of the miR164C – CUC1 regulatory module within the plant kingdom.