Termination of stem cell maintenance in Arabidopsis floral meristems by interactions between WUSCHEL and AGAMOUS.

Floral meristems and shoot apical meristems (SAMs) are homologous, self-maintaining stem cell systems. Unlike SAMs, floral meristems are determinate, and stem cell maintenance is abolished once all floral organs are initiated. To investigate the underlying regulatory mechanisms, we analyzed the interactions between WUSCHEL (WUS), which specifies stem cell identity, and AGAMOUS (AG), which is required for floral determinacy. Our results show that repression of WUS by AG is essential for terminating the floral meristem and that WUS can induce AG expression in developing flowers. Together, this suggests that floral determinacy depends on a negative autoregulatory mechanism involving WUS and AG, which terminates stem cell maintenance.     

Genetic regulation of vascular tissue patterning in Arabidopsis

Plants transport water and nutrients through a complex vascular network comprised of interconnected, specialized cell types organized in discrete bundles. To identify genetic determinants of vascular tissue patterning, we conducted a screen for mutants with altered vascular bundle organization in Arabidopsis cotyledons. Mutations in two genes, CVP1 and CVP2 (for cotyledon vascular pattern), specifically disrupt the normal pattern of vascular bundles in cotyledons, mature leaves, and inflorescence stems. The spatial distribution of the procambium, the precursor to mature vascular tissue, is altered in cvp1 and cvp2 embryos, suggesting that CVP1 and CVP2 act at a very early step in vascular patterning. Similarly, in developing stems of cvp1 and leaves of cvp2, the pattern of vascular differentiation is defective, but the maturation of individual vascular cells appears to be normal. There are no discernible alterations in cell morphology in cvp2 mutants. In contrast, cvp1 mutants are defective in directional orientation of the provascular strand, resulting in a failure to establish uniformly aligned vascular cells, and they also show a reduction in vascular cell elongation. Neither cvp1 nor cvp2 mutants displayed altered auxin perception, biosynthesis, or transport, suggesting that auxin metabolism is not generally affected in these mutants.     

A link between sterol biosynthesis, the cell wall, and cellulose in Arabidopsis

A crucial role for sterols in plant growth and development is underscored by the identification of three Arabidopsis sterol biosynthesis mutants that exhibit embryonic defects: fackel (fk), hydra1 (hyd1), and sterol methyltransferase 1/cephalopod (smt1/cph). We have taken a dual approach of sterol profiling and ultrastructural analysis to investigate the primary defects underlying the mutant phenotypes. Comprehensive gas chromatography GC-MS analysis of hyd1 in comparison to fk reveals an abnormal accumulation of unique sterol intermediates in each case. Sterol profiling of the fk hyd1 double mutant provides genetic evidence that FK C-14 reductase acts upstream of HYD1 C-8,7 isomerase. Despite distinct differences in sterol profiles, fk and hyd1 as well as smt1/cph share ultrastructural features such as incomplete cell walls and aberrant cell wall thickenings in embryonic and post-embryonic tissues. The common defects are coupled with ectopic callose and lignin deposits. We show that all three mutants exhibit a deficiency in cellulose, but are not reduced in pectin and sugars of the cell wall and cytosol. The sterol biosynthesis inhibitors 15-azasterol and fenpropimorph also cause cell wall gaps in dividing root cells and a reduction in bulk cellulose, corroborating that the cell wall abnormalities are due to altered sterol composition. Our results demonstrate that sterols are crucial for cellulose synthesis in the building of the plant cell wall.     

Conservation of floral homeotic gene function between Arabidopsis and antirrhinum.

Several homeotic genes controlling floral development have been isolated in both Antirrhinum and Arabidopsis. Based on the similarities in sequence and in the phenotypes elicited by mutations in some of these genes, it has been proposed that the regulatory hierarchy controlling floral development is comparable in these two species. We have performed a direct experimental test of this hypothesis by introducing a chimeric Antirrhinum Deficiens (DefA)/Arabidopsis APETALA3 (AP3) gene, under the control of the Arabidopsis AP3 promoter, into Arabidopsis. We demonstrated that this transgene is sufficient to partially complement severe mutations at the AP3 locus. In combination with a weak ap3 mutation, this transgene is capable of completely rescuing the mutant phenotype to a fully functional wild-type flower. These observations indicate that despite differences in DNA sequence and expression, DefA coding sequences can compensate for the loss of AP3 gene function. We discuss the implications of these results for the evolution of homeotic gene function in flowering plants.     

Pollen as the link between floral phenotype and fitness

Pollen plays a key role in plant reproductive biology. Despite the long history of research on pollen and pollination, recent advances in pollen-tracking methods and statistical approaches to linking plant phenotype, pollination performance, and reproductive fitness yield a steady flow of exciting new insights. In this introduction to the Special Issue “Pollen as the Link Between Phenotype and Fitness,” we start by describing a general conceptual model linking functional classes of floral phenotypic traits to pollination-related performance metrics and reproductive fitness. We use this model as a framework for synthesizing the relevant literature, highlighting the studies included in the Special Issue, and identifying gaps in our understanding and opportunities for further development of the field. The papers that follow in this Special Issue provide new insights into the relationships between pollen production, presentation, flower morphology, and pollination performance (e.g., pollen deposition onto stigmas), the role of pollinators in pollen transfer, and the consequences of heterospecific pollen deposition. Several of the studies demonstrate exciting experimental and analytical approaches that should pave the way for continued work addressing the intriguing role of pollen in linking plant phenotypes to reproductive fitness.     

Correlation between number and position of floral organs in Arabidopsis

Background and Aims

The study of variation in number, position and type of floral organs may serve as a key to understanding the mechanisms underlying their variation, and will make it possible to improve the analysis of gene function in model plant species by means of a more accurate characterization of mutant phenotypes. The present analysis was carried out in order to understand the correlation between number and position of floral organs in Arabidopsis thaliana.

Methods

An analysis of number and position of organs in flowers of wild type as well as in a series of mutations with floral organ position alterations was carried out, using light and electron microscopy. Variation common to different genotypes was analysed by means of individual diagrams, upon which generalized diagrams depicting variation in number and position of organs, were built by superimposition.

Key Results and Conclusions

It is shown that in the Arabidopsis flower a correlation exists between positions of petals and sepals, as well as between positions of stamens and carpels, whereas the position of carpels does not seem to depend on number and position of petals and stamens. This suggests that the position of organs in the basal (sepals) and apical (carpels) parts of the flower are determined before that in the intermediate zone. This assumption is consistent with the results of mathematical modelling and is supposed to be the consequence of stem-cell activity in the flower.     

Isolation of COV1, a gene involved in the regulation of vascular patterning in the stem of Arabidopsis

The molecular mechanisms that control the ordered patterning of vascular tissue development in plants are not well understood. Several models propose a two-component system for vascular differentiation. These components include an inducer of vascular tissue development and an inhibitor that prevents the formation of vascular bundles near pre-existing bundles. We have identified two recessive allelic mutants in Arabidopsis, designated continuous vascular ring (cov1), that display a dramatic increase in vascular tissue development in the stem in place of the interfascicular region that normally separates the vascular bundles. The mutant plants exhibited relatively normal vascular patterning in leaves and cotyledons. Analysis of the interaction of cov1 with a known auxin signalling mutant and direct analysis of auxin concentrations suggests that cov1 affects vascular pattering by some mechanism that is independent of auxin. The COV1 protein is predicted to be an integral membrane protein of unknown function, highly conserved between plants and bacteria. In plants, COV1 is likely to be involved in a mechanism that negatively regulates the differentiation of vascular tissue in the stem.     

A molecular link between autophagy and circadian rhythm in plants

Extremely high or low autophagy levels disrupt plant survival under nutrient starvation. Recently, autophagy has been reported to display rhythms in animals. However, the mechanism of circadian regulation of autophagy is still unclear. Here, we observed that autophagy has a robust rhythm and that various autophagy-related genes (ATGs) are rhythmically expressed in Arabidopsis. Chromatin immunoprecipitation (ChIP) and dual-luciferase (LUC) analyses showed that the core oscillator gene TIMING OF CAB EXPRESSION 1 (TOC1) directly binds to the promoters of ATG (ATG1a, ATG2, and ATG8d) and negatively regulates autophagy activities under nutritional stress. Furthermore, autophagy defects might affect endogenous rhythms by reducing the rhythm amplitude of TOC1 and shortening the rhythm period of CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1). Autophagy is essential for the circadian clock pattern in seedling development and plant sensitivity to nutritional deficiencies. Taken together, our studies reveal a plant strategy in which the TOC1-ATG axis involved in autophagy-rhythm crosstalk to fine-tune the intensity of autophagy.     

A molecular link between malaria and Epstein-Barr virus reactivation

Although malaria and Epstein-Barr (EBV) infection are recognized cofactors in the genesis of endemic Burkitt lymphoma (BL), their relative contribution is not understood. BL, the most common paediatric cancer in equatorial Africa, is a high-grade B cell lymphoma characterized by c-myc translocation. EBV is a ubiquitous B lymphotropic virus that persists in a latent state after primary infection, and in Africa, most children have sero-converted by 3 y of age. Malaria infection profoundly affects the B cell compartment, inducing polyclonal activation and hyper-gammaglobulinemia. We recently identified the cystein-rich inter-domain region 1alpha (CIDR1alpha) of the Plasmodium falciparum membrane protein 1 as a polyclonal B cell activator that preferentially activates the memory compartment, where EBV is known to persist. Here, we have addressed the mechanisms of interaction between CIDR1alpha and EBV in the context of B cells. We show that CIDR1alpha binds to the EBV-positive B cell line Akata and increases the number of cells switching to the viral lytic cycle as measured by green fluorescent protein (GFP) expression driven by a lytic promoter. The virus production in CIDR1alpha-exposed cultures was directly proportional to the number of GFP-positive Akata cells (lytic EBV) and to the increased expression of the EBV lytic promoter BZLF1. Furthermore, CIDR1alpha stimulated the production of EBV in peripheral blood mononuclear cells derived from healthy donors and children with BL. Our results suggest that P. falciparum antigens such as CIDR1alpha can directly induce EBV reactivation during malaria infection that may increase the risk of BL development for children living in malaria-endemic areas. To our knowledge, this is the first report to show that a microbial protein can drive a latently infected B cell into EBV replication.     

Uncovering genetic and molecular interactions among floral meristem identity genes in Arabidopsis thaliana

The inflorescence meristem produces floral primordia that remain undifferentiated during the first stages of flower development. Genes controlling floral meristem identity include LEAFY (LFY), APETALA1 (AP1), CAULIFLOWER (CAL), LATE MERISTEM IDENTITY 1 (LMI1), SHORT VEGETATIVE PHASE (SVP) and AGAMOUS-LIKE24 (AGL24). The lfy mutant shows partial reversions of flowers into inflorescence shoot-like structures and this phenotype is enhanced in the lfy ap1 double mutant. Here we show that combining the lfy mutant with agl24 and svp single mutants or with the agl24 svp double mutant enhances the lfy phenotype and that the lfy agl24 svp triple mutant phenocopies the lfy ap1 double mutant. Analysis of the molecular interactions between LFY, AGL24 and SVP showed that LFY is a repressor of AGL24 and SVP, whereas LMI1 is a positive regulator of these genes. Moreover, AGL24 and SVP positively regulate AP1 and LFY by direct binding to their regulatory regions. Since all these genes are important for establishing floral meristem identity, regulatory loops are probably important to maintain the correct relative expression levels of these genes.     

LEAFY controls floral meristem identity in Arabidopsis.

The first step in flower development is the generation of a floral meristem by the inflorescence meristem. We have analyzed how this process is affected by mutant alleles of the Arabidopsis gene LEAFY. We show that LEAFY interacts with another floral control gene, APETALA1, to promote the transition from inflorescence to floral meristem. We have cloned the LEAFY gene, and, consistent with the mutant phenotype, we find that LEAFY RNA is expressed strongly in young flower primordia. LEAFY expression procedes expression of the homeotic genes AGAMOUS and APETALA3, which specify organ identify within the flower. Furthermore, we demonstrate that LEAFY is the Arabidopsis homolog of the FLORICAULA gene, which controls floral meristem identity in the distantly related species Antirrhinum majus.     

The role of floral meristems in patterning

The flower is one of the most complex and varied structures found in plants. Over the past decade, we have begun to understand how floral patterning is established in a handful of model species. Recent studies have identified the presence of several potential pathways for organ patterning. Many genes that are involved in these pathways have been cloned, providing opportunities for further fruitful investigations into the genetic components of flower development.