By comparing expression levels of MADS box transcription factor genes between near-isogenic winter and spring lines of bread wheat, Triticum aestivum, we have identified WAP1 as the probable candidate for the Vrn-1 gene, the major locus controlling the vernalization flowering response in wheat. WAP1 is strongly expressed in spring wheats and moderately expressed in semispring wheats, but is not expressed in winter wheat plants that have not been exposed to vernalization treatment. Vernalization promotes flowering in winter wheats and strongly induces expression of WAP1. WAP1 is located on chromosome 5 in wheat and, by synteny with other cereal genomes, is likely to be collocated with Vrn-1. These results in hexaploid bread wheat cultivars extend the conclusion made by Yan et al. Many plants from temperate regions are induced to flower by an extended exposure to low temperature: vernalization. In winter cereal crops, such as wheat and barley, plant breeders have selected for variation in vernalization responsiveness to produce cultivars suited to plantings in different climatic zones. Winter cultivars are sown in autumn, vernalized by the low temperatures of winter, and subsequently flower and develop grain in spring. Spring cultivars do not require vernalization and usually are planted in the late winter period (reviewed in refs.

Late-flowering ecotypes and mutants of Arabidopsis thalana and the related crucifer Thlaspi arvense flower early after cold treatment (vernalization). Treatment with the DNA demethylating agent 5-azacytidine induced nonvernalized plants to flower significantly earlier than untreated controls. Cytidine at similar concentrations had no effect on time to flower. In contrast, late-flowering mutants that are insensitive to vernalization did not respond to 5-azacytidine treatment. Normal flowering time was reset in the progeny of plants induced to flower early with 5-azacytidine, paralleling the lack of inheritance of the vernalized condition. Arabidopsis plants, either cold-treated or 5-azacytidine-treated, had reduced levels of 5-methylcytosine in their DNA compared to nonvernalized plants. A Nicotiana plumbaginifolia cell line also showed a marked decrease in the level of 5-methylcytosine after treatment with either 5-azacytidine or low temperature. We suggest that DNA methylation provides a developmental control preventing early flowering in Arabidopsis and Thlaspi ecotypes. Vernalization, through its general demethylating effect, releases the block to flowering initiation. We propose that demethylation of a gene critical for flowering permits its transcription. We further suggest, on the basis of Thlaspi data, that the control affects transcription ofkaurenoic acid hydroxylase, a key enzyme in the gibberellic acid biosynthetic pathway.In many plants the transition of the shoot apical meristem from the vegetative to the flowering state occurs in response to environmental cues. Day length is one such cue, some plants requiring short, others long, days to initiate flowering. The site of perception of day length is the leaf, where a signal is generated and subsequently translocated to the shoot apex, where a new pattern of cell division forms the flowering meristem that subsequently produces the inflorescence (1).Exposure to low temperature can induce flowering in many temperate monocots and dicots, the cold treatment being termed vernalization (1). Experiments involving cooling treatments localized to parts of plants have shown that the shoot apical meristem is itself a site of perception of the low-temperature treatment (2, 3). Wellensiek (4) provided evidence that cell division during vernalization is necessary for thermoinduction in Lunaria annua and that flowering structures are ultimately derived from the mitotically active cells that were subjected to vernalizing temperatures. He concluded that thermoinduction is a cell-autonomous process that is mitotically propagated.The observation that tissues other than the shoot apical meristem also have the capacity to support thermoinductive responses to vernalization is consistent with the concept of mitotic transmission. Shoots regenerated from leaf cuttings of both L. annua (4) and Thlaspi arvense (3) exhibited a developmental state identical to that of plants from which they were obtained: flowering shoots developed from leaves of cold-treated plants, where...

We have identified three Arabidopsis genes with GAMYB-like activity, AtMYB33, AtMYB65, and AtMYB101, which can substitute for barley (Hordeum vulgare) GAMYB in transactivating the barley ␣-amylase promoter. We have investigated the relationships between gibberellins (GAs), these GAMYB-like genes, and petiole elongation and flowering of Arabidopsis. Within 1 to 2 d of transferring plants from short-to long-day photoperiods, growth rate and erectness of petioles increased, and there were morphological changes at the shoot apex associated with the transition to flowering. These responses were accompanied by accumulation of GAs in the petioles (GA 1 by 11-fold and GA 4 by 3-fold), and an increase in expression of AtMYB33 at the shoot apex. Inhibition of GA biosynthesis using paclobutrazol blocked the petiole elongation induced by long days. Causality was suggested by the finding that, with GA treatment, plants flowered in short days, AtMYB33 expression increased at the shoot apex, and the petioles elongated and grew erect. That AtMYB33 may mediate a GA signaling role in flowering was supported by its ability to bind to a specific 8-bp sequence in the promoter of the floral meristem-identity gene, LEAFY, this same sequence being important in the GA response of the LEAFY promoter. One or more of these AtMYB genes may also play a role in the root tip during germination and, later, in stem tissue. These findings extend our earlier studies of GA signaling in the Gramineae to include a dicot species, Arabidopsis, and indicate that GAMYB-like genes may mediate GA signaling in growth and flowering responses.Gibberellins (GAs) regulate many aspects of plant growth and development. In the seed and seedling these include the production of hydrolytic enzymes, germination, and growth. In the adult plant, GAs are important in leaf and stem elongation, flowering, anther development, and fruit set (Pharis and King, 1985).Two classes of mutants have contributed much to an understanding of GA action (Thornton et al., 1999). One class includes dwarf mutants that are defective in GA biosynthesis. The other class includes response mutants such as Arabidopsis spindly (spy), GA-insensitive (gai), repressor of GA1-3 (rga), and the rice (Oryza sativa) d1 mutant. Many of the genes defined by these mutants have been cloned, but their molecular role in GA signaling is not yet fully understood (Jacobsen et al., 1996; Peng et al., 1997; Silverstone et al., 1998; Ashikari et al., 1999).An alternative approach to understanding GA signal transduction has involved functional studies, particularly with aleurone cells of cereals. These studies have identified a number of early GA signaling steps that precede expression of hydrolytic enzymes such as ␣-amylase. These steps involve heterotrimeric G-proteins (Jones et al., 1998; Ueguchi-Tanaka et al., 2000) and cGMP (Penson et al., 1996), which may in turn control the barley (Hordeum vulgare) HvGAMYB gene, whose expression is induced by GAs (Gubler et al., 1995).HvGAMYB encodes a transcriptional act...

l h e long-day plant Arabidopsis tbaliana (L.) Heynh. flowers early in response to brief end-of-day (EOD) exposures to far-red light (FR) following a fluorescent short day of 8 h. FR promotion of flowering was nullified by subsequent brief red light (R) EOD exposure, indicating phytochrome involvement. The EOD response to R or FR is a robust measure of phytochrome action. Along with their wild-type (WT) parents, mutants deficient in either phytochrome A or B responded similarly to the EOD treatments. Thus, neither phytochrome A nor B exclusively regulated flowering, although phytochrome B controlled hypocotyl elongation. Perhaps a third phytochrome species is important for the EOD responses of the mutants and/or their flowering is regulated by the amount of the FR-absorbing form of phytochrome, irrespective of the phytochrome species. Overexpression of phytochrome A or phytochrome B resulted in differing photoperiod and EOD responses among the genotypes. The day-neutra1 overexpressor of phytochrome A had an EOD response similar to all of the mutants and WTs, whereas R EOD exposure promoted flowering i n the overexpressor of phytochrome B and FR EOD exposure inhibited this promotion. The comparisons between relative flowering times and leaf numbers at flowering of the overexpressors and their WTs were not consistent across photoperiods and light treatments, although both phytochromes A and B contributed to regulating flowering of the transgenic plants.

SummaryThe MADS-box protein encoded by FLOWERING LOCUS C (FLC) is a repressor of¯owering. Loci in the autonomous¯owering pathway control FLC levels. We show the epistatic groupings of autonomous pathway mutants fca/fy and fve/fpa, based on their effects on¯owering time, are consistent with their effects on FLC transcript and protein levels. We demonstrate that synergistic increases in FLC mRNA and protein expression occur in response to interactions between the autonomous pathway mutants fca and fpa and mutants in other pathways (fe, ft, fha) that do not regulate FLC when present as single mutants. These changes in FLC levels provide the molecular basis of the interactions previously shown in genetic analyses. The interactions between genes of multiple pathways emphasize the central position of FLC in the control of¯oral initiation. FLC protein levels match those of its mRNA for a range of genetic, developmental and environmental variables, indicating that control of FLC is at the level of transcription or transcript stability. The autonomous and photoperiod pathways also interact at the level of SOC1. FLC acts as a repressor of SOC1, and SOC1 levels are low when FLC levels are high. In C24 plants which have moderately high FLC levels,¯owering occurs without a decrease in FLC level, but the SOC1 level does increase. Thus SOC1 levels can be upregulated through the activities of other pathways, despite the repression by FLC.

Background and ObjectivesInteractions between plants and beneficial soil organisms (e.g. rhizobial bacteria, mycorrhizal fungi) are models for investigating the ecological impacts of such associations in plant communities, and the evolution and maintenance of variation in mutualisms (e.g. host specificity and the level of benefits provided). With relatively few exceptions, variation in symbiotic effectiveness across wild host species is largely unexplored.MethodsWe evaluated these associations using representatives of several legume genera which commonly co-occur in natural ecosystems in south-eastern Australia and an extensive set of rhizobial strains isolated from these hosts. These strains had been previously assigned to specific phylotypes on the basis of molecular analyses. In the first of two inoculation experiments, the growth responses of each host species was evaluated with rhizobial strains isolated from that species. The second experiment assessed performance across genera and the extent of host specificity using a subset of these strains.ResultsWhile host growth responses to their own (sympatric) isolates varied considerably, rhizobial phylotype was a significant predictor of symbiotic performance, indicating that bacterial species designations on the basis of molecular markers have ecological importance. Hosts responded in qualitatively different ways to sympatric and allopatric strains of rhizobia, ranging from species with a clear preference for their own strains, to those that were broad generalists, through to species that grew significantly better with allopatric strains.ConclusionTheory has focused on trade-offs between the provision of benefits and symbiont competitive ability that might explain the persistence of less beneficial strains. However, differences in performance among co-occurring host species could also drive such patterns. Our results thus highlight the likely importance of plant community structure in maintaining variation in symbiotic effectiveness.

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Summary1. Extensive clearing of native vegetation in Australia has contributed to major environmental problems, including land degradation, dryland salinity, soil erosion and loss of biodiversity. Re-establishing cover with deep-rooted perennial species is a major focus for conservation and sustainable land management, particularly with regard to hydrological control of recharge and saline discharge areas. However, considerable expense is involved in large-scale revegetation programmes and cost effectiveness is a real concern. 2. Low-cost revegetation approaches are needed that require little maintenance yet can substantially enhance reliable establishment and growth of native trees and shrubs. We evaluated results from direct-seeding field trials that examined the benefits of using native Australian Acacia species inoculated with effective strains of nitrogen-fixing rootnodule bacteria to revegetate degraded landscapes. 3. On average, inoculation led to a 118% increase in establishment of acacia seedlings, indicating that the use of elite strains of native bacteria can substantially reduce seed requirements. This is a major benefit given the expense of collecting sufficient native seed and the impacts of this activity on remnant population viability. 4. Particularly at sites experiencing harsher climatic conditions, subsequent survival of inoculated seedlings was significantly greater than for uninoculated controls. Moreover, inoculated acacias grew 10-58% faster than uninoculated controls during the critical early phase of establishment, although this varied among species and sites. 5. Synthesis and applications . Inoculation of Acacia species or other native leguminous shrubs and trees with elite strains of native rhizobia as part of direct-seeding techniques has the potential to increase the scope, rate and success of land restoration world-wide. Re-establishment of important plant-soil interactions in degraded soils can contribute significantly to the development of biodiverse self-regenerating native ecosystems in agricultural landscapes.