De novo super-early progeny in interspecific crosses Pisum sativum L. × P. fulvum Sibth. et Sm

Earliness in crop plants has a crucial role in avoiding the stress of drought and heat, which are the most important challenging stressors in crop production and are predicted to increase in the near future due to global warming. Furthermore, it provides a guarantee of vegetable production in the short growing season of agricultural lands in the northern hemisphere and at high altitudes. The growing human population needs super early plant cultivars for these agricultural lands to meet future global demands. This study examined de novo super-early progeny, referred to as much earlier than that of the earlier parent, which flowered in 13-17 days and pod setting in 18-29 days after germination, discovered in F2 and studied up to F5 derived from interspecific crosses between garden pea (P. sativum L.) and the most distant relative of pea (P. fulvum Sibth. et Sm.). De novo super-early progeny were found to be earlier by about one month than P. sativum and two months than P. fulvum under short day conditions in the F5 population. In respect of days to flowering and pod setting, de novo super-early progeny had a relatively high level of narrow sense heritability (h2 = 82% and 80%, respectively), indicating that the selections for earliness in segregating populations was effective for improvement of extreme early maturing varieties. De novo super-early progeny could be grown under heat stress conditions due to the escape ability. Vegetable types were not only high yielding but also free of any known undesirable traits from the wild species, such as pod dehiscence and non-uniform maturity. It could be considered complementary to "speed breeding", possibly obtaining more than six generations per year in a suitable climate chamber. Not only de novo super-early progeny but also transgressive segregation for agro-morphological traits can be created via interspecific crosses between P. sativum and P. fulvum, a precious unopened treasure in the second gene pool. Useful progeny obtained from crossing wild species with cultivated species reveal the importance of wild species.

Sugar transporters in Fabaceae, featuring SUT MST and SWEET families of the model plant Medicago truncatula and the agricultural crop Pisum sativum

Sugar transporters play a crucial role for plant productivity, as they coordinate sugar fluxes from source leaf towards sink organs (seed, fruit, root) and regulate the supply of carbon resources towards the microorganisms of the rhizosphere (bacteria and fungi). Thus, sugar fluxes mediated by SUT (sucrose transporters), MST (monosaccharide transporters) and SWEET (sugar will eventually be exported transporters) families are key determinants of crop yield and shape the microbial communities living in the soil. In this work, we performed a systematic search for sugar transporters in Fabaceae genomes, focusing on model and agronomical plants. Here, we update the inventory of sugar transporter families mining the latest version of the Medicago truncatula genome and identify for the first time SUT MST and SWEET families of the agricultural crop Pisum sativum. The sugar transporter families of these Fabaceae species comprise respectively 7 MtSUT 7 PsSUT, 72 MtMST 59 PsMST and 26 MtSWEET 22 PsSWEET. Our comprehensive phylogenetic analysis sets a milestone for the scientific community, as we propose a new and simple nomenclature to correctly name SUT MST and SWEET families. Then, we searched for transcriptomic data available for our gene repertoire. We show that several clusters of homologous genes are co-expressed in different organs, suggesting that orthologous sugar transporters may have a conserved function. We focused our analysis on gene candidates that may be involved in remobilizing resources during flowering, grain filling and in allocating carbon towards roots colonized by arbuscular mycorrhizal fungi and Rhizobia. Our findings open new perspectives for agroecological applications in legume crops, as for instance improving the yield and quality of seed productions and promoting the use of symbiotic microorganisms.

Cryptic divergences in the genus Pisum L. (peas), as revealed by phylogenetic analysis of plastid genomes

Organellar genomes may shed light on complicated patterns of plant evolution at inter- and intraspecies level. Primary structure of plastid genomes sequenced in this study and taken from public databases was characterised and compared in 22 diverse, mostly wild representatives of the genus Pisum (peas). Phylogenetic trees reconstructed via Bayesian approach on the basis of entire plastid genomes resembled those reconstructed on the basis of a nuclear gene His5 coding for a minor histone H1 subtype. They reveal Pisum fulvum as an early divergence of the genus but do not support other taxonomical subdivisions. The positions of three accessions, classified as P. sativum subsp. elatius (the wild subspecies of the common pea), appeared quite unexpected. On the entire plastid genome tree, two accessions, from the Black Sea area of Turkey and Georgia, clustered with representatives of another species, P. fulvum, while the other, from Greece, was the first divergence of the P. sativum branch. We suppose these unusual plastid genomes to be ancient lineages ascending to a 'missing link' between P. fulvum and P. sativum, represented by accession Pe 013 from Turkey. Accessions with common pea appearance but deeply diverged plastids could occur through occasional crossing of diverged pea lines in the past and biparental plastid inheritance, both events being possible in peas.

Divergence and population traits in evolution of the genus Pisum L. as reconstructed using genes of two histone H1 subtypes showing different phylogenetic resolution

Two histone H1 subtype genes, His7 and His5, were sequenced in a set of 56 pea accessions. Phylogenetic reconstruction based on concatenated His5 and His7 sequences had three main clades. First clade corresponded to Pisum fulvum, the next divergence separated a clade inside Pisum sativum in the broad sense that did not correspond strictly to any proposed taxonomical subdivisions. According to our estimations, the earliest divergence separating P. fulvum occurred 1.7±0.4MYA. The other divergence with high bootstrap support that separated two P. sativum groups took place approximately 1.3±0.3MYA. Thus, the main divergences in the genus took place either in late Pliocene or in early Pleistocene, the time of onset of the profound climate cooling in the northern hemisphere. The ω=K(a)/K(s) ratio was 2.5 times higher for His5 sequences than for His7. Thus, His7 gene, coding for a unique subtype specific for actively growing tissues, might have evolved under stricter evolutionary constraints than His5, that codes for a minor H1 subtype with less specific expression pattern. For this reason phylogenetic reconstructions separately obtained from His5 sequences resolved tree topology much better than those obtained from His7 sequences. Computational estimation of population dynamic parameters in the genus Pisum L. from His5-His7 sequences using IMa2 software revealed a decrease of effective population size on the early stage of Pisum evolution.

Pea starch (Pisum sativum L.) with slow digestion property produced using β-amylase and transglucosidase

Starches extracted from wrinkled (WP) and smooth (SP) peas were treated using β-amylase (B) alone and also with a combination of β-amylase and transglucosidase (BT). After enzymatic treatment, the proportions of slowly digested starch in WP-B, WP-BT, SP-B and SP-BT samples were increased by 6%, 9%, 9% and 12%, respectively. Starches treated by a combination of β-amylase and transglucosidase exhibited a smaller amount of longer amylopectin chains, a larger amount of short amylopectin chains, and higher branching fraction. The branching fraction was significantly increased, with an increase of 8%, 10%, 13% and 14% for WP-B, WP-BT, SP-B and SP-BT, respectively. The maximum absorbance and iodine binding of enzyme-treated starches were reduced compared with their native starch parents. The C-type crystalline structure completely disappeared after enzymatic treatment. The results support previous findings that increases in the amount of shorter amylopectin chains and branch fraction are likely to contribute to the slow digestion of starch.

Fatty acid profiling of the seed oils of some varieties of field peas (Pisum sativum) by RP-LC/ESI-MS/MS: towards the development of an oilseed pea

Reversed-phase liquid chromatography coupled to negative-ion electrospray tandem mass spectrometry (RP-LC/ESI-MS/MS) was used to study the fatty acid profile from the oil of harvested field pea (Pisum sativum) varieties as part of a research project to develop this legume as a commercial oilseed for Canada. The seed oils from pea samples contained palmitic and stearic acids as major saturated fatty acids. Oleic, linoleic and linolenic acids were the major unsaturated fatty acids found. Small percentages of other long chain fatty acids were also detected. This profile suggests that the species of field pea investigated might have the potential to be used as raw materials to develop a future new oilseed crop for the food industry. Fatty acid extracts did not require further manipulation before final analysis by RP-LC/ESI-MS/MS, indicating the utility and relative simplicity of this technique for future screening studies.

Systematics, biogeography, and character evolution of the legume tribe Fabeae with special focus on the middle-Atlantic island lineages

BACKGROUND

Tribe Fabeae comprises about 380 legume species, including some of the most ancient and important crops like lentil, pea, and broad bean. Breeding efforts in legume crops rely on a detailed knowledge of closest wild relatives and geographic origin. Relationships within the tribe, however, are incompletely known and previous molecular results conflicted with the traditional morphology-based classification. Here we analyse the systematics, biogeography, and character evolution in the tribe based on plastid and nuclear DNA sequences.

RESULTS

Phylogenetic analyses including c. 70% of the species in the tribe show that the genera Vicia and Lathyrus in their current circumscription are not monophyletic: Pisum and Vavilovia are nested in Lathyrus, the genus Lens is nested in Vicia. A small, well-supported clade including Vicia hirsuta, V. sylvatica, and some Mediterranean endemics, is the sister group to all remaining species in the tribe. Fabeae originated in the East Mediterranean region in the Miocene (23-16 million years ago (Ma)) and spread at least 39 times into Eurasia, seven times to the Americas, twice to tropical Africa and four times to Macaronesia. Broad bean (V. faba) and its sister V. paucijuga originated in Asia and might be sister to V. oroboides. Lentil (Lens culinaris ssp. culinaris) is of Mediterranean origin and together with eight very close relatives forms a clade that is nested in the core Vicia, where it evolved c. 14 Ma. The Pisum clade is nested in Lathyrus in a grade with the Mediterranean L. gloeosperma, L. neurolobus, and L. nissolia. The extinct Azorean endemic V. dennesiana belongs in section Cracca and is nested among Mediterranean species. According to our ancestral character state reconstruction results, ancestors of Fabeae had a basic chromosome number of 2n=14, an annual life form, and evenly hairy, dorsiventrally compressed styles.

CONCLUSIONS

Fabeae evolved in the Eastern Mediterranean in the middle Miocene and spread from there across Eurasia, into Tropical Africa, and at least seven times to the Americas. The middle-Atlantic islands were colonized four times but apparently did not serve as stepping-stones for Atlantic crossings. Long-distance dispersal events are relatively common in Fabeae (seven per ten million years). Current generic and infrageneric circumscriptions in Fabeae do not reflect monophyletic groups and should be revised. Suggestions for generic level delimitation are offered.

Agronomical factors influencing the legumin/vicilin ratio in pea (Pisum sativum L.) seeds

BACKGROUND

Many research studies have investigated the impact of agronomical factors on the protein content of pea (Pisum sativum). This study aimed to establish if a correlation exists between protein content and legumin/vicilin (L/V) ratio in pea seeds and to identify agronomical factors that have an impact on this ratio.

RESULTS

The L/V ratio was positively correlated with protein content (r = 0.58, P ≤ 0.01), but no linear regression was applicable (adjusted R(2) = 0.31). Both variety and cultivation year had a highly significant effect on the ratio (P < 0.001). The interaction between these two factors was also highly significant (P < 0.001), some varieties being less sensitive to climatic conditions than others. Cultivation location had a highly significant effect (P < 0.01). There was no interaction between variety and location. Seeding density had a highly significant effect on the ratio (P < 0.01), with a saturation effect above 60 seeds m(-2).

CONCLUSION

Further studies should establish if a linear regression model can be applied to pea varieties independently. Varieties with a stable L/V ratio can prove to be useful in the food industry. Other agronomical factors (soil type and seeding density) should be considered for the production of pea seeds with a specific L/V ratio.

LHCII organization and thylakoid lipids affect the sensitivity of the photosynthetic apparatus to high-light treatment

Pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photosynthetic oxygen evolution were used to investigate the role of the different amount and organization of light-harvesting complexes of photosystem II (LHCII) in four pea species on the susceptibility of the photosynthetic apparatus to high-light treatment. In this work we analyzed the thylakoid membrane lipid composition of the studied pea plants. A relationship between the structural organization of LHCII proteins, the amount of the main lipid classes and the sensitivity of the photosynthetic apparatus to high-light treatment was found. The results reveal that the photosynthetic apparatus, enriched in oligomeric forms of LHCII concomitant with decreased amount of anionic lipids and increased content of the monogalactosyldiacylglycerol (MGDG), is less sensitive to high light. Our data also suggest that the degree of LHCII oligomerization, as well as the lipid composition do not influence the degree of recovery of the PSII photochemistry after excess light exposure.

Genetic control and QTL analysis of cotyledon bleaching resistance in green field pea (Pisum sativum L.)

Resistance to bleaching is an important factor for quality grading of Canadian green field pea and an important selection criterion in green pea improvement. This research was conducted to determine the genetic control of bleaching resistance in green peas using 90 recombinant inbred lines (RILs) derived from a cross between cultivars Orb and CDC Striker. These lines were evaluated under field conditions for two years in two locations in Saskatchewan, Canada. Harvested whole seeds and cotyledons were evaluated for greenness using the Hunter Lab colorimeter before and after exposure to a high light intensity accelerated bleaching treatment. The RILs were genotyped using amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) molecular markers. Heritability estimates for whole seed and cotyledon greenness were moderate (0.72 and 0.69, respectively) and increased when assessed after exposing whole seeds and cotyledons to accelerated bleaching conditions (0.83 and 0.82, respectively). The genetic linkage map constructed based on a total of 224 AFLP and SSR markers spanned over 890 cM of the pea genome. Multiple QTL mapping detected major QTLs on LG IV and LG V as well as location- and year-specific QTLs on LG II and LG III associated with green cotyledon bleaching resistance in field pea. The results demonstrated the importance of the seed coat in protecting the cotyledons from bleaching.

Systems biology for plant breeding: the example of flowering time in pea

As part of a breeding strategy applied to pea (Pisum sativum L.), we propose the use of modelling as a tool for studying flowering time. The pea, both a crop and a model species for developmental processes, represents a valuable tool for systems biology approaches. A preliminary computational model for flowering control was previously developed based on genetic and physiological approaches. This paper discusses possible improvements of the model based on recent molecular advances on the regulation of flowering in peas and the model species Arabidopsis thaliana. A combination of a genetic approach together with agroecophysiological models that are not based on genotype, built into a complete model for flowering time prediction is also proposed. This complete model should allow an accurate prediction of flower initiation and also provide an integrative tool that will be useful for various purposes, from genetic networks to crop models.

A continuum of genetic divergence from sympatric host races to species in the pea aphid complex

Sympatric populations of insects adapted to different host plants, i.e., host races, are good models to investigate how natural selection can promote speciation in the face of ongoing gene flow. However, host races are documented in very few model systems and their gradual evolution into good species, as assumed under a Darwinian view of species formation, lacks strong empirical support. We aim at resolving this uncertainty by investigating host specialization and gene flow among populations of the pea aphid complex, Acyrthosiphon pisum. Genetic markers and tests of host plant specificity indicate the existence of at least 11 well-distinguished sympatric populations associated with different host plants in Western Europe. Population assignment tests show variable migration and hybridization rates among sympatric populations, delineating 8 host races and 3 possible species. Notably, hybridization correlates negatively with genetic differentiation, forming a continuum of population divergence toward virtually complete speciation. The pea aphid complex thus illustrates how ecological divergence can be sustained among many hybridizing populations and how insect host races blend into species by gradual reduction of gene flow.

Construction and characterization of two bacterial artificial chromosome libraries of pea (Pisum sativum L.) for the isolation of economically important genes

Pea (Pisum sativum L.) has a genome of about 4 Gb that appears to share conserved synteny with model legumes having genomes of 0.2-0.4 Gb despite extensive intergenic expansion. Pea plant inventory (PI) accession 269818 has been used to introgress genetic diversity into the cultivated germplasm pool. The aim here was to develop pea bacterial artificial chromosome (BAC) libraries that would enable the isolation of genes involved in plant disease resistance or control of economically important traits. The BAC libraries encompassed about 3.2 haploid genome equivalents consisting of partially HindIII-digested DNA fragments with a mean size of 105 kb that were inserted in 1 of 2 vectors. The low-copy oriT-based T-DNA vector (pCLD04541) library contained 55 680 clones. The single-copy oriS-based vector (pIndigoBAC-5) library contained 65 280 clones. Colony hybridization of a universal chloroplast probe indicated that about 1% of clones in the libraries were of chloroplast origin. The presence of about 0.1% empty vectors was inferred by white/blue colony plate counts. The usefulness of the libraries was tested by 2 replicated methods. First, high-density filters were probed with low copy number sequences. Second, BAC plate-pool DNA was used successfully to PCR amplify 7 of 9 published pea resistance gene analogs (RGAs) and several other low copy number pea sequences. Individual BAC clones encoding specific sequences were identified. Therefore, the HindIII BAC libraries of pea, based on germplasm accession PI 269818, will be useful for the isolation of genes underlying disease resistance and other economically important traits.

Abscisic acid and desiccation-dependent expression of a novel putative SNF5-type chromatin-remodeling gene in Pisum sativum

Snf5-like proteins are components of multiprotein chromatin remodeling complexes involved in the ATP-dependent alteration of DNA-histone contacts. Mostly described in yeast and animals, the only plant SNF5-like gene characterized so far has been BSH from Arabidopsis thaliana (L.) Heynh. We report the cloning and characterization of expression of a SNF5-like gene from pea (Pisum sativum L. cv. Lincoln), which has been designated PsSNF5. Southern analysis showed a single copy of the gene in the pea genome. The cDNA contained a 723bp open reading frame encoding a 240 amino acid protein of 27.4kDa with a potential nuclear localization signal. PsSNF5 protein sequence closely resembled BSH, with which it showed an overall amino acid identity of 78.5%. Two-hybrid experiments showed that PsSNF5 is functionally interchangeable with Arabidopsis BSH in the interactions with other components of the remodeling complex. Phylogenetic analysis demonstrated that PsSNF5 clustered with translated expressed sequence tags from other Leguminosae, hypothetically coding for new Snf5-like proteins. RT-PCR expression analysis demonstrated that the PsSNF5 gene is constitutively expressed in all the tissues examined, with minor differences in expression level in different tissues. Nevertheless, expression analysis revealed that PsSNF5 was up-regulated in the last stages of embryo development, when water content decreases. Moreover, abscisic acid and drought stress induced PsSNF5 accumulation in germinating embryos and vegetative tissues, suggesting that chromatin remodeling induced by PsSNF5-containing complexes might contribute to the response to that phytohormone.

Identification and detection of genetic relatedness among important varieties of pea (Pisum sativum L.) grown in India

Among the cool season legume crops grown in India and the Indian sub-continent, peas are very popular and preferred by the growers as well as consumers for various uses. The third largest area in pea cultivation is occupied by India after Canada and Russia. Among the important and popular varieties of peas that are grown in India, several are from exotic background. But very little work has been done to carry out the genetic diversity present in the widely adapted Indian pea varieties using DNA markers. Twenty-four most popular and widely adapted varieties were subjected to RAPD analysis to find out the genetic relatedness among them using 60 decamer primers. All the primers used in our study were found to be polymorphic and seven of them showed 100% polymorphism. Out of 579 amplified products, 433 showed polymorphism (74.8%). On an average, 9.65 bands were amplified per primer. Cluster analysis based on Jaccard's similarity coefficient using UPGMA grouped all the tall type varieties together, whereas, dwarf types formed two different clusters based upon their pedigree. The arithmetic mean heterozygosity (Hav) value and marker index (MI) was found to be 0.496 and 4.787, respectively, thus this indicated the efficiency of RAPD as a marker system. Moreover, the calculated value of probability of identical match by chance suggested that about 10(53) genotypes can be unambiguously distinguish by employing 60 RAPD primers.

Screening of field pea (Pisum sativum) selections for their reactions to root-knot nematode (Meloidogyne incognita)

Pot studies were carried out to evaluate the reactions of 23 selections of field pea (Pisum sativum) against Meloidogyne incognita (2000 freshly hatched juveniles (J2)/pot). Experiment was conducted under greenhouse conditions ((24.7+/-3) degrees C and (62+/-7)% RH) and terminated 45 d after inoculation. The roots of all the selections were assessed to determine root-knot indices (RKI) on a 0-4 scale. Out of 23 selections HFP-990713, Pant P-25, and HFP-0129 were resistant; Pant P-2005, NDP-2 and Pant P-42 were tolerant; LFP-305, HFP-8909, HFP-4, HUP-31, HFP-0128, Pant P-31, Pant P-40, LFP-363, and HFP-0118 were moderately resistant; HFP-0110, HUDP-28, HUDP-15, HUDP-27, HUP-30, HUP-2 and HUDP-26 were moderately susceptible; and only Ambika was susceptible to M. incognita. It was observed that reproduction of nematode was favored on tolerant and susceptible cultivars but inhibited on resistant ones. Strong negative correlation was observed between the total fresh and dry plant weights and the root-knot index. The selection Pant P-42 showed highest tolerance among all the selections tested and can be recommended for field trials, whereas, selection Ambika showed highest susceptibility and should be avoided.

Genetic diversity within Pisum sativum using protein- and PCR-based markers

A collection of 148 Pisum accessions, mostly from Western Europe, and including both primitive germplasm and cultivated types, was structured using 121 protein- and PCR-based markers. This molecular marker-based classification allowed us to trace back major lineages of pea breeding in Western Europe over the last decades, and to follow the main breeding objectives: increase of seed weight, introduction of the afila foliage type and white flowers, and improvement of frost tolerance for winter-sown peas. The classification was largely consistent with the available pedigree data, and clearly resolved the different main varietal types according to their end-uses (fodder, food and feed peas) from exotic types and wild forms. Fodder types were further separated into two sub-groups. Feed peas, corresponding to either spring-sown or winter-sown types, were also separated, with two apparently different gene pools for winter-sown peas. The garden pea group was the most difficult to structure, probably due to a continuum in breeding of feed peas from garden types. The classification also stressed the paradox between the narrowness of the genetic basis of recent cultivars and the very large diversity available within P. sativum. A sub-collection of 43 accessions representing 96% of the whole allelic variability is proposed as a starting point for the construction of a core collection.

Seed lectin from pisum arvense: isolation, biochemical characterization and amino acid sequence

A glucose/mannose lectin was purified by affinity chromatography from Pisum arvense seeds (PAL) and the 50 kDa molecular mass in solution determined by size exclusion chromatography. SDS-PAGE and electrospray ionization mass spectrometry showed two distinct polypeptide chains: alpha (Mr. 5591 Da) and beta (19986 Da). The lectin was extensively characterized in terms of its biochemical and biological aspects. The amino acid sequence was established by Edman degradation of overlapping peptides. PAL in solution behaves as a dimer and has its monomeric structure formed by two distinct polypeptide chains named alpha (Mr. 5591 Da) and beta (19986 Da) by Electrospray ionization (ESI) mass spectrometry. PAL possesses identical amino acid sequences to that of pea seed lectin but undoubtedly does not exhibit sequence heterogeneity. It is discussed that P. arvense should be considered as a synonym of P. sativum. Furthermore, like pea lectin, PAL discriminates biantennary fucosylated glycan, determined by surface plasmon resonance.

Transposable elements reveal the impact of introgression, rather than transposition, in Pisum diversity, evolution, and domestication

The genetic structure and evolutionary history of the genus Pisum were studied exploiting our germplasm collection to compare the contribution of different mechanisms to the generation of diversity. We used sequence-specific amplification polymorphism (SSAP) markers to assess insertion site polymorphism generated by a representative of each of the two major groups of LTR-containing retrotransposons, PDR1 (Ty1/copia-like) and Cyclops (Ty3/gypsy-like), together with Pis1, a member of the En/Spm transposon superfamily. The analysis of extended sets of the four main Pisum species, P. fulvum, P. elatius, P. abyssinicum, and P. sativum, together with the reference set, revealed a distinct pattern of the NJ (Neighbor-Joining) tree for each basic lineage, which reflects the different evolutionary history of each species. The SSAP markers showed that Pisum is exceptionally polymorphic for an inbreeding species. The patterns of phylogenetic relationships deduced from different transposable elements were in general agreement. The retrotransposon-derived markers gave a clearer separation of the main lineages than the Pis1 markers and were able to distinguish the truly wild form of P. elatius from the antecedents of P. sativum. There were more species-specific and unique PDR1 markers than Pis1 markers in P. fulvum and P. elatius, pointing to PDR1 activity during speciation and diversification, but the proportion of these markers is low. The overall genetic diversity of Pisum and the extreme polymorphism in all species, except P. abyssinicum, indicate a high contribution of recombination between multiple ancestral lineages compared to transposition within lineages. The two independently domesticated pea species, P. abyssinicum and P. sativum, arose in contrasting ways from the common processes of hybridization, introgression, and selection without associated transpositional activity.

The molecular characterization and in situ expression pattern of pea SCARECROW gene

Certain mutants of shoot gravitropism were reported to be ascribed to the SCR and SHR loci in Arabidopsis thaliana. The SCR gene was known to regulate the development of endodermis cells that are responsible for sensing gravity in a shoot. With the aim of elucidating the molecular mechanism for gravitropic responses in pea seedlings, we have isolated a putative pea SCR ortholog from a shoot cDNA library. Analyses of the cDNA clones revealed the structure of a full-length ORF coding for 819 amino acid residues. A remarkable feature of pea SCR protein was the presence of asparagine stretches at the N-terminal transcriptional activation domain, which was distinct from the occurrence of glutamine or alanine stretches in the Arabidopsis or maize SCR. A Northern blot analysis revealed a single 3.2-kb pea SCR transcript in addition to a closely related 2.5-kb transcript. Our in situ hybridization data indicated that pea SCR mRNA accumulated in the shoot apical meristem, leaf primordia and a root single cell layer corresponding to the endodermis. The expression patterns were similar to those reported for A. thaliana and Zea mays, suggesting that SCR may be functionally conserved among plants and involved in the differentiation of the endodermis.

Two types of pea leghemoglobin genes showing different O2-binding affinities and distinct patterns of spatial expression in nodules

Five distinct cDNA clones for leghemoglobin (Lb) were isolated from pea (Pisum sativum) nodules. They were classified into two groups designated PsLbA and PsLbB according to sequence homology, O(2)-binding affinities of the recombinant proteins, and in situ localization of the mRNAs. The PsLbB group was comprised of four cDNA clones: PsLb120-1, -8, -29, and -34. They showed a high similarity of deduced amino acid sequences and O(2)-binding affinities of their recombinant proteins. Among them, the spatial expression pattern of PsLb120-1 was investigated in great detail, indicating that its transcripts were localized in the region from infection zone II to the distal part of nitrogen fixation zone III in effective nodules. PsLb5-10, which is the only cDNA clone of the PsLbA type, differed considerably from the PsLbB type in amino acid sequence, and the recombinant protein had a higher O(2)-binding affinity than those of the PsLbB type. The transcripts of PsLb5-10 were detected throughout the central tissue of effective nodules. However, in ineffective nodules on the pea mutant E135 (sym13), transcripts of PsLb5-10 were restricted to the distal portion of the central tissue as well as those of PsLb120-1. These findings indicate that the pea genome contains two types of Lb genes and suggest that they have different roles in the development of nitrogen-fixing symbiosis in pea nodules.

Pea Ty1-copia group retrotransposons: transpositional activity and use as markers to study genetic diversity in Pisum

The variation in transposition history of different Ty1-copia group LTR retrotransposons in the species lineages of the Pisum genus has been investigated. A heterogeneous population of Ty1-copia elements was isolated by degenerate PCR and two of these (Tps12 and Tps19) were selected on the basis of their copy number and sequence conservation between closely related species for further in-depth study of their transpositional history in Pisum species. The insertional polymorphism of these elements and the previously characterised PDR1 element was studied by sequence-specific amplification polymorphism (SSAP). Each of these elements reveals a unique transpositional history within 55 diverse Pisum accessions. Phylogenetic trees based on the SSAP data show that SSAP markers for individual elements are able to resolve different species lineages within the Pisum genus. Finally, the SSAP data from all of these retrotransposon markers were combined to reveal a detailed picture of the intra and interspecies relationships within Pisum.

Assignment of linkage groups to pea chromosomes after karyotyping and gene mapping by fluorescent in situ hybridization

Chromosomes of the pea (Pisum sativum L.) were submitted to fluorescent in situ hybridization (FISH) with probes specific for the oligonucleotides (AG)12, (AC)12, (GAA)10, and (GATA)7 and for the genes encoding 25S rRNA, 5S rRNA and the storage proteins legumin A, K and vicilin. A fourth 5S rRNA gene locus, apparently specific for an accession of the cultivar Grüne Victoria, was newly detected. This allowed all seven chromosome pairs to be distinguished by FISH signals of rRNA genes. The same was possible using a combination of oligonucleotide probes or of oligonucleotides and rRNA gene-specific probes in multicolour FISH. Rehybridization with the 5S rRNA gene-specific probe allowed us to assign vicilin genes to the short arm of chromosome 5, the single legumin A locus to the long arm of chromosome 3 and the legumin B-type genes (exemplified by legumin K) to one locus on the short arm of chromosome 6. Correlation of these data with an updated version of the pea genetic map allowed the assignment of most linkage groups to defined chromosomes. It only remains to be established which of linkage groups IV and VII corresponds to the satellited chromosomes 4 or 7, respectively.

Intron position as an evolutionary marker of thioredoxins and thioredoxin domains

In contrast to prokaryotes, which typically possess one thioredoxin gene per genome, three different thioredoxin types have been described in higher plants. All are encoded by nuclear genes, but thioredoxins m and f are chloroplastic while thioredoxins h have no transit peptide and are probably cytoplasmic. We have cloned and sequenced Arabidopsis thaliana genomic fragments encoding the five previously described thioredoxins h, as well as a sixth gene encoding a new thioredoxin h. In spite of the high divergence of the sequences, five of them possess two introns at positions identical to the previously sequenced tobacco thioredoxin h gene, while a single one has only the first intron. The recently published sequence of Chlamydomonas thioredoxin h shows three introns, two at the same positions as in higher plants. This strongly suggests a common origin for all cytoplasmic thioredoxins of plants and green algae. In addition, we have cloned and sequenced pea DNA genomic fragments encoding thioredoxins m and f. The thioredoxin m sequence shows only one intron between the regions encoding the transit peptide and the mature protein, supporting the prokaryotic origin of this sequence and suggesting that its association with the transit peptide has been facilitated by exon shuffling. In contrast, the thioredoxin f sequence shows two introns, one at the same position as an intron in various plant and animal thioredoxins and the second at the same position as an intron in thioredoxin domains of disulfide isomerases. This strongly supports the hypothesis of a eukaryotic origin for chloroplastic thioredoxin f.

Resistance to nodulation of cv. Afghanistan peas is overcome by nodX, which mediates an O-acetylation of the Rhizobium leguminosarum lipo-oligosaccharide nodulation factor

Only some strains of Rhizobium leguminosarum biovar viciae can efficiently nodulate varieties of peas such as cv. Afghanistan, which carry a recessive allele that blocks efficient nodulation by most western isolates of R.I. viciae. One strain (TOM) which can nodulate cv. Afghanistan peas has a gene (nodX) that is required to overcome the nodulation resistance. Strain TOM makes significantly lower amounts of lipo-oligosaccharide nodulation factors than other strains of R.I. viciae and this effect appears to be due to lower levels of nod gene induction. These nodulation factors are similar to those from other R.I. viciae strains in that they consist of an oligomer of four or five beta 1-4-linked N-acetylglucosamine residues in which the terminal non-reducing glucosamine carries an O-acetyl group and a C18:4 or C18:1 N-acyl group. However, one of the nodulation factors made by strain TOM differs from the factors made by other strains of R.I. viciae in that it carries an O-acetyl group on the C-6 of the reducing N-acetylglucosamine residue. This acetylation is NodX-dependent and the pentameric nodulation factor is acetylated on the reducing N-acetylglucosamine residue whereas the tetrameric nodulation factor is not. Although the nodL gene product is also an O-acetyl transferase (it O-acetylates the C-6 of the terminal non-reducing glucosamine), there is very little similarity between the amino acid sequences of these two acetyl transferases.