Ulf Lagercrantz

A Norway spruce FLOWERING LOCUS T homolog is implicated in control of growth rhythm in conifers.

Growth in perennial plants possesses an annual cycle of active growth and dormancy that is controlled by environmental factors, mainly photoperiod and temperature. In conifers and other nonangiosperm species, the molecular mechanisms behind these responses are currently unknown. In Norway spruce (Picea abies L. Karst.) seedlings, growth cessation and bud set are induced by short days and plants from southern latitudes require at least 7 to 10 h of darkness, whereas plants from northern latitudes need only 2 to 3 h of darkness. Bud burst, on the other hand, is almost exclusively controlled by temperature. To test the possible role of Norway spruce FLOWERING LOCUS T (FT)-like genes in growth rhythm, we have studied expression patterns of four Norway spruce FT family genes in two populations with a divergent bud set response under various photoperiodic conditions. Our data show a significant and tight correlation between growth rhythm (both bud set and bud burst), and expression pattern of one of the four Norway spruce phosphatidylethanolamine-binding protein gene family members (PaFT4) over a variety of experimental conditions. This study strongly suggests that one Norway spruce homolog to the FT gene, which controls flowering in angiosperms, is also a key integrator of photoperiodic and thermal signals in the control of growth rhythms in gymnosperms. The data also indicate that the divergent adaptive bud set responses of northern and southern Norway spruce populations, both to photoperiod and light quality, are mediated through PaFT4. These results provide a major advance in our understanding of the molecular control of a major adaptive trait in conifers and a tool for further molecular studies of adaptive variation in plants.

Evolution of the PEBP gene family in plants: functional diversification in seed plant evolution

The phosphatidyl ethanolamine-binding protein (PEBP) gene family is present in all eukaryote kingdoms, with three subfamilies identified in angiosperms (FLOWERING LOCUS T [FT], MOTHER OF FT AND TFL1 [MFT], and TERMINAL FLOWER1 [TFL1] like). In angiosperms, PEBP genes have been shown to function both as promoters and suppressors of flowering and to control plant architecture. In this study, we focus on previously uncharacterized PEBP genes from gymnosperms. Extensive database searches suggest that gymnosperms possess only two types of PEBP genes, MFT-like and a group that occupies an intermediate phylogenetic position between the FT-like and TFL1-like (FT/TFL1-like). Overexpression of Picea abies PEBP genes in Arabidopsis (Arabidopsis thaliana) suggests that the FT/TFL1-like genes (PaFTL1 and PaFTL2) code for proteins with a TFL1-like function. However, PaFTL1 and PaFTL2 also show highly divergent expression patterns. While the expression of PaFTL2 is correlated with annual growth rhythm and mainly confined to needles and vegetative and reproductive buds, the expression of PaFTL1 is largely restricted to microsporophylls of male cones. The P. abies MFT-like genes (PaMFT1 and PaMFT2) show a predominant expression during embryo development, a pattern that is also found for many MFT-like genes from angiosperms. P. abies PEBP gene expression is primarily detected in tissues undergoing physiological changes related to growth arrest and dormancy. A first duplication event resulting in two families of plant PEBP genes (MFT-like and FT/TFL1-like) seems to coincide with the evolution of seed plants, in which independent control of bud and seed dormancy was required, and the second duplication resulting in the FT-like and TFL1-like clades probably coincided with the evolution of angiosperms.

Disentangling the Roles of History and Local Selection in Shaping Clinal Variation of Allele Frequencies and Gene Expression in Norway Spruce (Picea abies)

Understanding the genetic basis of local adaptation is challenging due to the subtle balance among conflicting evolutionary forces that are involved in its establishment and maintenance. One system with which to tease apart these difficulties is clines in adaptive characters. Here we analyzed genetic and phenotypic variation in bud set, a highly heritable and adaptive trait, among 18 populations of Norway spruce (Picea abies), arrayed along a latitudinal gradient ranging from 47°N to 68°N. We confirmed that variation in bud set is strongly clinal, using a subset of five populations. Genotypes for 137 single-nucleotide polymorphisms (SNPs) chosen from 18 candidate genes putatively affecting bud set and 308 control SNPs chosen from 264 random genes were analyzed for patterns of genetic structure and correlation to environment. Population genetic structure was low (FST = 0.05), but latitudinal patterns were apparent among Scandinavian populations. Hence, part of the observed clinal variation should be attributable to population demography. Conditional on patterns of genetic structure, there was enrichment of SNPs within candidate genes for correlations with latitude. Twenty-nine SNPs were also outliers with respect to FST. The enrichment for clinal variation at SNPs within candidate genes (i.e., SNPs in PaGI, PaPhyP, PaPhyN, PaPRR7, and PaFTL2) indicated that local selection in the 18 populations, and/or selection in the ancestral populations from which they were recently derived, shaped the observed cline. Validation of these genes using expression studies also revealed that PaFTL2 expression is significantly associated with latitude, thereby confirming the central role played by this gene in the control of phenology in plants.

UV dose-dependent DNA elimination in asymmetric somatic hybrids between Brassica napus and Arabidopsis thaliana

Abstract To evaluate UV-irradiation as an alternative method for chromosome fragmentation in the production of asymmetric somatic hybrids, UV-irradiated protoplasts of Arabidopsis thaliana were fused with protoplasts of Brassica napus . The A. thaliana protoplasts were irradiated with doses between 780 and 28 080 J/m 2 ( λ =254 nm) and hybrid plants were obtained from experiments using doses of 780, 2340 and 4680 J/m 2 . In addition, a control fusion experiment without irradiation was performed. In total, 312 shoots were regenerated and of those 58 plants were analysed for presence of A. thaliana DNA, using 13 mapped A. thaliana RFLP markers (two to four per chromosome). Increased UV-irradiation dose resulted in a higher frequency of asymmetric hybrids and a higher dose was also associated with the generation of hybrids with a higher degree of asymmetry. RFLP data further indicated that irradiation primarily resulted in loss of chromosome fragments rather than whole chromosomes from A. thaliana . Fertility was studied in 119 hybrid plants by selfing as well as pollination by B. napus . The fertility after selfing increased with increasing UV-irradiation dose, while the results of back-crossing were not affected by the irradiation.

High-density linkage mapping and evolution of paralogs and orthologs in Salix and Populus

BackgroundSalix (willow) and Populus (poplar) are members of the Salicaceae family and they share many ecological as well as genetic and genomic characteristics. The interest of using willow for biomass production is growing, which has resulted in increased pressure on breeding of high yielding and resistant clones adapted to different environments. The main purpose of this work was to develop dense genetic linkage maps for mapping of traits related to yield and resistance in willow. We used the Populus trichocarpa genome to extract evenly spaced markers and mapped the orthologous loci in the willow genome. The marker positions in the two genomes were used to study genome evolution since the divergence of the two lineages some 45 mya.ResultsWe constructed two linkage maps covering the 19 linkage groups in willow. The most detailed consensus map, S1, contains 495 markers with a total genetic distance of 2477 cM and an average distance of 5.0 cM between the markers. The S3 consensus map contains 221 markers and has a total genetic distance of 1793 cM and an average distance of 8.1 cM between the markers. We found high degree of synteny and gene order conservation between willow and poplar. There is however evidence for two major interchromosomal rearrangements involving poplar LG I and XVI and willow LG Ib, suggesting a fission or a fusion in one of the lineages, as well as five intrachromosomal inversions. The number of silent substitutions were three times lower (median: 0.12) between orthologs than between paralogs (median: 0.37 - 0.41).ConclusionsThe relatively slow rates of genomic change between willow and poplar mean that the genomic resources in poplar will be most useful in genomic research in willow, such as identifying genes underlying QTLs of important traits. Our data suggest that the whole-genome duplication occurred long before the divergence of the two genera, events which have until now been regarded as contemporary. Estimated silent substitution rates were 1.28 × 10-9 and 1.68 × 10-9 per site and year, which are close to rates found in other perennials but much lower than rates in annuals.

Morphogenic and genetic stability in longterm embryogenic cultures and somatic embryos of Norway spruce (Picea abies {L.} Karst)

Embryogenic cultures were initiated from mature zygotic embryos of Picea abies. The somatic embryos in the embryogenic cultures were first stimulated to mature and then either to develop further into plantlets or to differentiate new embryogenic cultures. The procedure was repeated three times during two years. The ability to give rise to new embryogenic cultures or to develop into plantlets was similar for all somatic embryos irrespective of how long they had been cultured in vitro. The nuclear DNA content, measured in a flow cytometer, was estimated at 32 pg/G1 nuclei in seedings developed from zygotic embryos. Nuclei isolated from embryogenic cultures and from plantlets regenerated from somatic embryos had the same DNA content as those isolated from seedlings.

At the end of the day: a common molecular mechanism for photoperiod responses in plants?

Photoperiod or daylength affects a diverse set of traits in plants, including flowering and tuberization in annuals, as well as growth cessation and bud set in perennials. During the last 10-15 years, great progress has been made in the understanding of molecular mechanisms controlling photoperiodic induction of flowering, in particular in the model species Arabidopsis thaliana. An obvious question is to what extent the molecular mechanisms revealed in A. thaliana are also shared by other species and other traits controlled by photoperiod. The purpose of this review is to summarize data on the molecular mechanisms of photoperiod control in plants with a focus of annual growth rhythm in perennial plants.

Early evolution of the MFT-like gene family in plants.

Angiosperm genes sharing a conserved phosphatidylethanolamine-binding (PEPB) domain have been shown to be involved in the control of shoot meristem identity and flowering time. The family is divided into three subfamilies, FT-like, TFL1-like and MFT-like. This study is focused on the evolution of the MFT-like clade, suggested to be ancestral to the two other clades. We report that the bryophyte Physcomitrella patens and the lycopod Selaginella moellendorfii contain four and two MFT-like genes respectively. Neither species have any FT or TFL1-like genes. Furthermore, we have identified a new subclade of MFT-like genes in Angiosperms. Quantitative expression analysis of MFT-like genes in Physcomitrella patens reveals that the expression patterns are circadian and reaches maximum in gametangia and sporophytes. Our data suggest that the occurrence FT and TFL1-like genes, is associated with the evolution of seed plants. Expression data for Physcomitrella MFT-like genes implicates an involvement in the development of reproductive tissues in the moss.

Mapping the Brassica genome

The six cultivated species of Brassica furnish a wide range of crop types (including oilseed, vegetable and fodder crops) which seem quite different when observed under normal cultivation (Figure 1). However, Brassica species and a large number of other wild and cultivated species are all closely related (Figure 2) and genetic exchange through sexual crosses is possible across most of this very extensive gene pool. Traditionally, the investigation of genome organization in plants has employed cytology to study chromosomes and genetic markers to define linkage groups. Cytology is difficult in Brassica because the chromosomes are small, but the genus is very amenable to investigations using molecular-genetic markers because of the high degree of natural polymorphism. Gene homology and the general structure of the genome seems to be conserved between Brassica and related genera and modern marker technologies are freely interchangeable across this group. However, the collinearity of related chromosomes in different Brassica species has been disrupted frequently by chromosomal translocations. Thus Brassica species have quite distinct genetic maps, in contrast to cereal species where collinear homoeologous chromosomes are the general rule. The mapping of the Brassica genome will have a considerable impact on the breeding of Brassica crops. In particular, it will facilitate the transfer of beneficial genes between species and the rapid introgression of genes from wild plants into useful cultivars. These improvements in breeding should be translated into crops which are more easily adapted to suit the needs of new agronomic practices and the demands of a changing environment.

Does the core circadian clock in the moss Physcomitrella patens (Bryophyta) comprise a single loop

BackgroundThe endogenous circadian clock allows the organism to synchronize processes both to daily and seasonal changes. In plants, many metabolic processes such as photosynthesis, as well as photoperiodic responses, are under the control of a circadian clock. Comparative studies with the moss Physcomitrella patens provide the opportunity to study many aspects of land plant evolution. Here we present a comparative overview of clock-associated components and the circadian network in the moss P. patens.ResultsThe moss P. patens has a set of conserved circadian core components that share genetic relationship and gene expression patterns with clock genes of vascular plants. These genes include Myb-like transcription factors PpCCA1a and PpCCA1b, pseudo-response regulators PpPRR1-4, and regulatory elements PpELF3, PpLUX and possibly PpELF4. However, the moss lacks homologs of AtTOC1, AtGI and the AtZTL-family of genes, which can be found in all vascular plants studied here. These three genes constitute essential components of two of the three integrated feed-back loops in the current model of the Arabidopsis circadian clock mechanism. Consequently, our results suggest instead a single loop circadian clock in the moss. Possibly as a result of this, temperature compensation of core clock gene expression appears to be decreased in P. patens.ConclusionsThis study is the first comparative overview of the circadian clock mechanism in a basal land plant, the moss P. patens. Our results indicate that the moss clock mechanism may represent an ancestral state in contrast to the more complex and partly duplicated structure of subsequent land plants. These findings may provide insights into the understanding of the evolution of circadian network topology.