Noel Ellis

UNIFOLIATA regulates leaf and flower morphogenesis in pea

BACKGROUND The vegetative phenotype of the pea mutant unifoliata (uni) is a simplification of the wild-type compound leaf to a single leaflet. Mutant uni plants are also self-sterile and the flowers resemble known floral meristem and organ identity mutants. In Antirrhinum and Arabidopsis, mutations in the floral meristem identity gene FLORICAULA/LEAFY (FLO/LFY) affect flower development alone, whereas the tobacco FLO/LFY homologue, NFL, is expressed in vegetative tissues, suggesting that NFL specifies determinacy in the progenitor cells for both flowers and leaves. In this paper, we characterised the pea homologue of FLO/LFY. RESULTS The pea cDNA homologue of FLO/LFY, PEAFLO, mapped to the uni locus in recombinant-inbred mapping populations and markers based on PEAFLO cosegregated with uni in segregating sibling populations. The characterisation of two spontaneous uni mutant alleles, one containing a deletion and the other a point mutation in the PEAFLO coding sequences, predicted that PEAFLO corresponds to UNI and that the mutant vegetative phenotype was conferred by the defective PEAFLO gene. CONCLUSIONS The uni mutant demonstrates that there are shared regulatory processes in the morphogenesis of leaves and flowers and that floral meristem identity genes have an extended role in plant development. Pleiotropic regulatory genes such as UNI support the hypothesis that leaves and flowers derive from a common ancestral sporophyll-like structure. The regulation of indeterminancy during leaf and flower morphogenesis by UNI may reflect a primitive function for the gene in the pre-angiosperm era.

Conservation of Arabidopsis Flowering Genes in Model Legumes

The model plants Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) have provided a wealth of information about genes and genetic pathways controlling the flowering process, but little is known about the corresponding pathways in legumes. The garden pea (Pisum sativum) has been used for several decades as a model system for physiological genetics of flowering, but the lack of molecular information about pea flowering genes has prevented direct comparison with other systems. To address this problem, we have searched expressed sequence tag and genome sequence databases to identify flowering-gene-related sequences from Medicago truncatula, soybean (Glycine max), and Lotus japonicus, and isolated corresponding sequences from pea by degenerate-primer polymerase chain reaction and library screening. We found that the majority of Arabidopsis flowering genes are represented in pea and in legume sequence databases, although several gene families, including the MADS-box, CONSTANS, and FLOWERING LOCUS T/TERMINAL FLOWER1 families, appear to have undergone differential expansion, and several important Arabidopsis genes, including FRIGIDA and members of the FLOWERING LOCUS C clade, are conspicuously absent. In several cases, pea and Medicago orthologs are shown to map to conserved map positions, emphasizing the closely syntenic relationship between these two species. These results demonstrate the potential benefit of parallel model systems for an understanding of flowering phenology in crop and model legume species.

DETERMINATE and LATE FLOWERING Are Two TERMINAL FLOWER1/CENTRORADIALIS Homologs That Control Two Distinct Phases of Flowering Initiation and Development in Pea

Genes in the TERMINAL FLOWER1 (TFL1)/CENTRORADIALIS family are important key regulatory genes involved in the control of flowering time and floral architecture in several different plant species. To understand the functions of TFL1 homologs in pea, we isolated three TFL1 homologs, which we have designated PsTFL1a, PsTFL1b, and PsTFL1c. By genetic mapping and sequencing of mutant alleles, we demonstrate that PsTFL1a corresponds to the DETERMINATE (DET) gene and PsTFL1c corresponds to the LATE FLOWERING (LF) gene. DET acts to maintain the indeterminacy of the apical meristem during flowering, and consistent with this role, DET expression is limited to the shoot apex after floral initiation. LF delays the induction of flowering by lengthening the vegetative phase, and allelic variation at the LF locus is an important component of natural variation for flowering time in pea. The most severe class of alleles flowers early and carries either a deletion of the entire PsTFL1c gene or an amino acid substitution. Other natural and induced alleles for LF, with an intermediate flowering time phenotype, present no changes in the PsTFL1c amino acid sequence but affect LF transcript level in the shoot apex: low LF transcript levels are correlated with early flowering, and high LF transcript levels are correlated with late flowering. Thus, different TFL1 homologs control two distinct aspects of plant development in pea, whereas a single gene, TFL1, performs both functions in Arabidopsis. These results show that different species have evolved different strategies to control key developmental transitions and also that the genetic basis for natural variation in flowering time may differ among plant species.

The Sym35 Gene Required for Root Nodule Development in Pea Is an Ortholog of Nin from Lotus japonicus

Comparative phenotypic analysis of pea (Pisum sativum) sym35 mutants and Lotus japonicus nin mutants suggested a similar function for thePsSym35 and LjNin genes in early stages of root nodule formation. Both the pea and L.japonicus mutants are non-nodulating but normal in their arbuscular mycorrhizal association. Both are characterized by excessive root hair curling in response to the bacterial microsymbiont, lack of infection thread initiation, and absence of cortical cell divisions. To investigate the molecular basis for the similarity, we cloned and sequenced the PsNin gene, taking advantage of sequence information from the previously cloned LjNin gene. An RFLP analysis on recombinant inbred lines mapped PsNinto the same chromosome arm as the PsSym35 locus and direct evidence demonstrating that PsNin is thePsSym35 gene was subsequently obtained by cosegregation analysis and sequencing of three independent Pssym35mutant alleles. L. japonicus and pea root nodules develop through different organogenic pathways, so it was of interest to compare the expression of the two orthologous genes during nodule formation. Overall, a similar developmental regulation of thePsNin and LjNin genes was shown by the transcriptional activation in root nodules of L. japonicus and pea. In the indeterminate pea nodules,PsNin is highly expressed in the meristematic cells of zone I and in the cells of infection zone II, corroborating expression of LjNin in determinate nodule primordia. At the protein level, seven domains, including the putative DNA binding/dimerization RWP-RK motif and the PB1 heterodimerization domain, are conserved between the LjNIN and PsNIN proteins.

The genetic diversity and evolution of field pea (Pisum) studied by high throughput retrotransposon based insertion polymorphism (RBIP) marker analysis

BackgroundThe genetic diversity of crop species is the result of natural selection on the wild progenitor and human intervention by ancient and modern farmers and breeders. The genomes of modern cultivars, old cultivated landraces, ecotypes and wild relatives reflect the effects of these forces and provide insights into germplasm structural diversity, the geographical dimension to species diversity and the process of domestication of wild organisms. This issue is also of great practical importance for crop improvement because wild germplasm represents a rich potential source of useful under-exploited alleles or allele combinations. The aim of the present study was to analyse a major Pisum germplasm collection to gain a broad understanding of the diversity and evolution of Pisum and provide a new rational framework for designing germplasm core collections of the genus.Results3020 Pisum germplasm samples from the John Innes Pisum germplasm collection were genotyped for 45 retrotransposon based insertion polymorphism (RBIP) markers by the Tagged Array Marker (TAM) method. The data set was stored in a purpose-built Germinate relational database and analysed by both principal coordinate analysis and a nested application of the Structure program which yielded substantially similar but complementary views of the diversity of the genus Pisum. Structure revealed three Groups (1-3) corresponding approximately to landrace, cultivar and wild Pisum respectively, which were resolved by nested Structure analysis into 14 Sub-Groups, many of which correlate with taxonomic sub-divisions of Pisum, domestication related phenotypic traits and/or restricted geographical locations. Genetic distances calculated between these Sub-Groups are broadly supported by principal coordinate analysis and these, together with the trait and geographical data, were used to infer a detailed model for the domestication of Pisum.ConclusionsThese data provide a clear picture of the major distinct gene pools into which the genus Pisum is partitioned and their geographical distribution. The data strongly support the model of independent domestications for P. sativum ssp abyssinicum and P. sativum. The relationships between these two cultivated germplasms and the various sub-divisions of wild Pisum have been clarified and the most likely ancestral wild gene pools for domesticated P. sativum identified. Lastly, this study provides a framework for defining global Pisum germplasm which will be useful for designing core collections.

Genetic mapping in pea. 1. RAPD-based genetic linkage map of Pisum sativum

Abstract A genetic linkage map of Pisum sativum L. was constructed based primarily on RAPD markers that were carefully selected for their reproducibility and scored in a population of 139 recombinant inbred lines (RILs). The mapping population was derived from a cross between a protein-rich dry-seed cultivar ‘Térèse’ and an increased branching mutant (K586) obtained from the pea cultivar ‘Torsdag’. The map currently comprises nine linkage groups with two groups comprising only 6 markers (n=7 in pea) and covers 1139 cM. This RAPD-based map has been aligned with the map based on the (JI281×JI399) RILs population that currently includes 355 markers in seven linkage groups covering 1881 cM. The difference in map lengths is discussed. For this alignment 7 RFLPs, 23 RAPD markers, the morphological marker le and the PCR marker corresponding to the gene Uni were used as common markers and scored in both populations.

Tendril-less regulates tendril formation in pea leaves

Tendrils are contact-sensitive, filamentous organs that permit climbing plants to tether to their taller neighbors. Tendrilled legume species are grown as field crops, where the tendrils contribute to the physical support of the crop prior to harvest. The homeotic tendril-less (tl) mutation in garden pea (Pisum sativum), identified almost a century ago, transforms tendrils into leaflets. In this study, we used a systematic marker screen of fast neutron–generated tl deletion mutants to identify Tl as a Class I homeodomain leucine zipper (HDZIP) transcription factor. We confirmed the tendril-less phenotype as loss of function by targeting induced local lesions in genomes (TILLING) in garden pea and by analysis of the tendril-less phenotype of the t mutant in sweet pea (Lathyrus odoratus). The conversion of tendrils into leaflets in both mutants demonstrates that the pea tendril is a modified leaflet, inhibited from completing laminar development by Tl. We provide evidence to show that lamina inhibition requires Unifoliata/LEAFY-mediated Tl expression in organs emerging in the distal region of the leaf primordium. Phylogenetic analyses show that Tl is an unusual Class I HDZIP protein and that tendrils evolved either once or twice in Papilionoid legumes. We suggest that tendrils arose in the Fabeae clade of Papilionoid legumes through acquisition of the Tl gene.

Phylogeny, phylogeography and genetic diversity of the Pisum genus

The tribe Fabeae (formerly Vicieae) contains some of humanitys most important grain legume crops, namely Lathyrus (grass pea/sweet pea/chickling vetches; about 160 species); Lens (lentils; 4 species); Pisum (peas; 3 species); Vicia (vetches; about 140 species); and the monotypic genus Vavilovia. Reconstructing the phylogenetic relationships within this group is essential for understanding the origin and diversification of these crops. Our study, based on molecular data, has positioned Pisum genetically between Vicia and Lathyrus and shows it to be closely allied to Vavilovia. A study of phylogeography, using a combination of plastid and nuclear markers, suggested that wild pea spread from its centre of origin, the Middle East, eastwards to the Caucasus, Iran and Afghanistan, and westwards to the Mediterranean. To allow for direct data comparison, we utilized model-based Bayesian Analysis of Population structure (BAPS) software on 4429 Pisum accessions from three large world germplasm collections that include both wild and domesticated pea analyzed by retrotransposon-based markers. An analysis of genetic diversity identified separate clusters containing wild material, distinguishing Pisum fulvum, P. elatius and P. abyssinicum, supporting the view of separate species or subspecies. Moreover, accessions of domesticated peas of Afghan, Ethiopian and Chinese origin were distinguished. In addition to revealing the genetic relationships, these results also provided insight into geographical and phylogenetic partitioning of genetic diversity. This study provides the framework for defining global Pisum germplasm diversity as well as suggesting a model for the domestication of the cultivated species. These findings, together with gene-based sequence analysis, show that although introgression from wild species has been common throughout pea domestication, much of the diversity still resides in wild material and could be used further in breeding. Moreover, although existing collections contain over 10,000 pea accessions, effort should be directed towards collecting more wild material in order to preserve the genetic diversity of the species.

Do Transcription Factors Play Special Roles in Adaptive Variation

Ever since the work by Jacob and Monod on the Lac operon, scientists have appreciated that the control of gene expression is one of the most important points of regulation in biology. Although many other layers of regulation exist, the possibility of control at the start point of production of RNA

Conserved genetic determinant of motor organ identity in Medicago truncatula and related legumes

Plants exhibit various kinds of movements that have fascinated scientists and the public for centuries. Physiological studies in plants with the so-called motor organ or pulvinus suggest that cells at opposite sides of the pulvinus mediate leaf or leaflet movements by swelling and shrinking. How motor organ identity is determined is unknown. Using a genetic approach, we isolated a mutant designated elongated petiolule1 (elp1) from Medicago truncatula that fails to fold its leaflets in the dark due to loss of motor organs. Map-based cloning indicated that ELP1 encodes a putative plant-specific LOB domain transcription factor. RNA in situ analysis revealed that ELP1 is expressed in primordial cells that give rise to the motor organ. Ectopic expression of ELP1 resulted in dwarf plants with petioles and rachises reduced in length, and the epidermal cells gained characteristics of motor organ epidermal cells. By identifying ELP1 orthologs from other legume species, namely pea (Pisum sativum) and Lotus japonicus, we show that this motor organ identity is regulated by a conserved molecular mechanism.