The potential roles of strigolactones and brassinosteroids in the autoregulation of nodulation pathway

Background and Aims

The number of nodules formed on a legume root system is under the strict genetic control of the autoregulation of nodulation (AON) pathway. Plant hormones are thought to play a role in AON; however, the involvement of two hormones recently described as having a largely positive role in nodulation, strigolactones and brassinosteroids, has not been examined in the AON process.

Methods

A genetic approach was used to examine if strigolactones or brassinosteroids interact with the AON system in pea (Pisum sativum). Double mutants between shoot-acting (Psclv2, Psnark) and root-acting (Psrdn1) mutants of the AON pathway and strigolactone-deficient (Psccd8) or brassinosteroid-deficient (lk) mutants were generated and assessed for various aspects of nodulation. Strigolactone production by AON mutant roots was also investigated.

Key Results

Supernodulation of the roots was observed in both brassinosteroid- and strigolactone-deficient AON double-mutant plants. This is despite the fact that the shoots of these plants displayed classic strigolactone-deficient (increased shoot branching) or brassinosteroid-deficient (extreme dwarf) phenotypes. No consistent effect of disruption of the AON pathway on strigolactone production was found, but root-acting Psrdn1 mutants did produce significantly more strigolactones.

Conclusions

No evidence was found that strigolactones or brassinosteroids act downstream of the AON genes examined. While in pea the AON mutants are epistatic to brassinosteroid and strigolactone synthesis genes, we argue that these hormones are likely to act independently of the AON system, having a role in the promotion of nodule formation.     

Combined inoculation with Glomus intraradices and Rhizobium tropici CIAT899 increases phosphorus use efficiency for symbiotic nitrogen fixation in common bean (Phaseolus vulgaris L.)

This study compared the response of common bean (Phaseolus vulgaris L.) to arbuscular mycorrhizal fungi (AMF) and rhizobia strain inoculation. Two common bean genotypes i.e. CocoT and Flamingo varying in their effectiveness for nitrogen fixation were inoculated with Glomus intraradices and Rhizobium tropici CIAT899, and grown for 50 days in soil–sand substrate in glasshouse conditions. Inoculation of common bean plants with the AM fungi resulted in a significant increase in nodulation compared to plants without inoculation. The combined inoculation of AM fungi and rhizobia significantly increased various plant growth parameters compared to simple inoculated plants. In addition, the combined inoculation of AM fungi and rhizobia resulted in significantly higher nitrogen and phosphorus accumulation in the shoots of common bean plants and improved phosphorus use efficiency compared with their controls, which were not dually inoculated. It is concluded that inoculation with rhizobia and arbuscular mycorrhizal fungi could improve the efficiency in phosphorus use for symbiotic nitrogen fixation especially under phosphorus deficiency.     

Foliar Chlorosis in Symbiotic Host and Nonhost Plants Induced by Rhizobium tropici Type B Strains

Rhizobium tropici CIAT899 induced chlorosis in the leaves of its symbiotic hosts, common bean (Phaseolus vulgaris L.), siratro (Macroptilium atropurpureum Urb.), and Leucaena leucocephala (Lam.) de Wit. Chlorosis induction by strains CIAT899 and CT9005, an exopolysaccharide-deficient mutant of CIAT899, required carbon substrate. When the bacteria were added at planting in a solution of mannitol (50 g/liter), as few as 10³ cells of CIAT899 were sufficient to induce chlorosis in bean plants. All carbon sources tested, including organic acids and mono- and disaccharides, supported chlorosis induction. The addition of a carbon source did not affect the growth rate or the population density of CT9005 in the bean plant rhizosphere. Cell-free filtrates of cultures of CT9005 did not induce detectable chlorosis. All type B strains of R. tropici tested also induced chlorosis in common bean. Type A strains of R. tropici and all other species of bacteria tested did not induce chlorosis. Several lines of evidence indicated that nodulation was not required for chlorosis induction. Strain RSP900, a pSym-cured derivative of CIAT899, induced chlorosis in wild-type P. vulgaris. In addition, NOD125, a nodulation-defective line of common bean, developed chlorosis when inoculated with CIAT899, but did not develop nodules. CIAT899 consistently induced severe chlorosis in the leaves of the nonhost legumes alfalfa (Medicago sativa L.) and Berseem clover (Trifolium alexandrinum L.), and induced chlorosis in 29 to 58% of the plants tested of sunflower, cucumber, and tomato seedlings, but it did not induce chlorosis in the leaves of corn or wheat. Chlorosis induction in nonhost plants also required carbon substrate. The data are consistent with the hypothesis that R. tropici type B strains produce a chlorosis-inducing factor that affects a wide range of plant species.     

Use of DNA marker to select well-adapted <Emphasis Type="Italic">Phaseolus</Emphasis>-symbionts strains under acid conditions and high temperature

Soil acidity and high temperature contribute to the failure of nodulation in the common bean. It is therefore urgent to select strains with a high competitive ability under these stress conditions. Two Egyptian Rhizobium etli strains, EBRI 2 and EBRI 26, were examined against Rhizobium tropici CIAT 899G labeled with the gus (β-glucuronidase) reporter gene. EBRI 2 and EBRI 26 were less competitive than CIAT 899G under acid conditions with both the Egyptian cultivar Giza 3 and the Colombian cultivar Rab 39. However, EBRI 2 and EBRI 26 gave higher nodule occupancy (78% and 62.5, respectively) than the nodule occupancy (18.5% and 35%) obtained by CIAT 899G at 35°C with cultivar Giza 3. Soil acidity (pH 5.8) was less detrimental to the nodule occupancy of EBRI 2 than EBRI 26 when they tested in competition with CIAT 899G.     

An apigenin-induced decrease in K-antigen production by Sinorhizobium sp. NGR234 is y4gM- and nodD1-dependent

Cultured cells of Sinorhizobium sp. NGR234 produce an abundance of capsular polysaccharides, or K antigens; however, cells that are cultured in the presence of apigenin, a nod gene inducer, exhibited a significant reduction in K-antigen production. The flavonoid-induced modulation in capsule production appeared to be related to the phase-shift changes associated with bacteroid differentiation. Therefore, the polysaccharides were extracted from Sinorhizobium sp. NGR234 bacteroids recovered from Vigna unguiculata cv Red Caloona root nodules, and subsequent analyses showed that the bacteroid extracts were virtually devoid of K-antigen. Polysaccharide extracts from two nodulation mutants cultured in the presence of apigenin were then analyzed, and the results showed that the flavonoid-inducible decrease in K-antigen production is y4gM- and nodD1-dependent.     

Three regulatory nodD alleles of diverged flavonoid-specificity are involved in host-dependent nodulation by Rhizobium meliloti

Summary Rhizobium meliloti infective on Medicago, Melilotus and Trigonella plants has three copies of the nodulation regulatory gene nodD. Strains containing mutations in nodD1 exhibited a delayed and/or decreased nodulation on Melilotus albus (Ma), Medicago sativa (Ms), Medicago quasifalcata (Mqu) and Trigonella coerulea (Tc), while on Medicago truncatula (Mt) they nodulated similarly to the wild-type R. meliloti. Delayed nodulation was observed also when nodD2 mutants were inoculated onto Ms, Mt and Tc, but not on Ma and Mqu. A nodD3 mutant exhibited delayed nodulation on Ms and Ma. Using a nodC-lacZ fusion and cloned nodD genes on plasmids, high induction levels were detected in R. meliloti when nodD1 was present with seed exudates from Ms, Ma and Mqu, nodD2 with those from Ms and Mt, and nodD3 with those from Ms, Ma and Mqu. NOne of the nodD copies exhibited high levels of nodC-lacZ induction when present with seed exudate from Tc. Only nodD1 induced nodC-lacZ expression in conjunction with the flavone, luteolin. The plant hosts used in this study exude different flavonoids and correlation between nodulation and nodC-lacZ induction abilities of the host exudates was observed. We concluded that all the three nodD copies of R. meliloti have common nod-promoter activating but diverged flavonoid-recognizing abilities. Thus, the three nodD alleles contribute to the activation of nodulation genes in a host-dependent manner.     

Selection of Competitive Strains of Rhizobium Nodulating Phaseolus Vulgaris and Adapted to Environmental Conditions in Egypt,Using the Gus-Reporter Gene Technique

Two Rhizobium etli strains, EBRI 2 and EBRI 26, isolated from Egypt were tested for nodulation competitiveness on beans using Rhizobium tropici CIAT 899G as the competing strain. The insertion of the gus-reporter transposon mTn5ssgusA30 did not alter the nodulation or nitrogen fixation capacity of mutant strain CIAT 899G compared to the wild type. At neutral pH, R. etli strains EBRI 2 and EBRI 26 were more competitive than CIAT 899G with the bean cultivar Saxa. These two strains gave nodule occupancies of 52.1 and 61.1% competing with equal cell numbers of CIAT 899G. Nodule occupancies from these two native strains increased with the bean cultivar Giza 6 from Egypt to 66 and 67.5%. Based on these results, cultivar Giza 6 was used to select the most competitive strains under stress of salinity or alkalinity as a major problem for a large part of Egyptian soils. Under stress of salinity (0.2% NaCl or 34.2 mM NaCl), the salt-sensitive strain EBRI 2 was more competitive than the salt-resistant strain EBRI 26. Strain EBRI 2 gave 87.4% but strain EBRI 26 gave 63.7% nodule occupancy against CIAT 899G. The same trend of results was observed under stress of alkalinity (pH 8). Strain EBRI 2 occupied 83% while Strain EBRI 26 occupied 53.2%.     

nodD Alleles of Sinorhizobium fredii USDA191 Differentially Influence Soybean Nodulation,nodC Expression,and Production of Exopolysaccharides

All Rhizobium strains examined to date have one or multiple alleles of nodD. At least one copy of nodD and the presence of flavonoid exudates are required for nod gene induction and nodulation. Sinorhizobium fredii USDA191 has two copies of nodD. In this study, we demonstrate that inactivation of either copy of nodD caused a reduction in basal levels of expression of nodC. Extra copies of nodD1 had no effect on the expression of nodC when compared with the wild type, but extra copies of nodD2 abolished the inducer requirement, thereby rendering nodC constitutive. A nodD1 mutant was unable to nodulate soybean cultivars Peking and McCall. Inactivation of nodD2 or addition of extra copies of nodD1 or nodD2 caused delayed nodulation on Peking, and reduced the number of nodules on McCall. Both nodD alleles of S. fredii USDA191 appear to be involved in regulation of exopolysaccharide production; however, nodD2 appears to be more important in this respect than nodD1. RID= ID= <E5>Correspondence to: </E5>H.B. Krishnan; <E5>email:</E5> KrishnanH&commat;missouri.edu Received: 13 September 2002 / Accepted: 7 October 2002     

Soil chemical properties and dieback of Quercus robur in Atlantic wet forests after a weather extreme

Background and aims

Burkholderia phymatum strain GR01 is a recently reported common bean (Phaseolus vulgaris) symbiont isolated from nodules of plants grown in semi-arid soils in Morocco. The osmotolerance of B. phymatum GR01N under free-living and in symbiotic association with P. vulgaris was investigated in this study.

Methods

The osmotolerance of B. phymatum GR01N was checked by growing cells in the presence of varying concentrations of NaCl or sucrose, and the cellular solutes were analyzed in cell extracts by ¹³C-nuclear magnetic resonance (NMR) spectroscopy. Nodule occupancy was checked in P. vulgaris grown in the presence of 25, 35 or 50 mM NaCl and inoculated with a mixture of B. phymatum GR01N and R. tropici CIAT899R cells. The effect of salt stress on nodule biomass, plant dry weight, plant nitrogen content and leghaemoglobin content of nodules was also analyzed in plants inoculated with either B. phymatum GR01N or R. tropici CIAT899R and grown in the presence of 25 or 35 mM NaCl.

Results

Burkholderia phymatum strain GR01N showed increased tolerance to osmotic stress under free-living conditions as compared to the reference strain R. tropici CIAT899R. Strain GR01N accumulated trehalose, mannitol and alanine in response to saline stress, suggesting their role in the observed osmoloterance. Under conditions of saline stress, P. vulgaris plants nodulated by B. phymatum GR01N showed increased plant dry weight and nitrogen fixation, when compared to those inoculated with R. tropici CIAT899R. Nodule competition assays revealed that B. phymatum GR01N had higher levels of nodule occupancy than R. tropici CIAT899R in P. vulgaris plants grown under saline conditions.

Conclusions

Burkholderia phymatum strain GR01N displays a remarkable osmotolerance under free-living and symbiotic conditions.     

Multiple copies of nodD in Rhizobium tropici CIAT899 and BR816.

Rhizobium tropici strains are able to nodulate a wide range of host plants: Phaseolus vulgaris, Leucaena spp., and Macroptilium atropurpureum. We studied the nodD regulatory gene for nodulation of two R. tropici strains: CIAT899, the reference R. tropici type IIb strain, and BR816, a heat-tolerant strain isolated from Leucaena leucocephala. A survey revealed several nodD-hybridizing DNA regions in both strains: five distinct regions in CIAT899 and four distinct regions in BR816. Induction experiments of a nodABC-uidA fusion in combination with different nodD-hybridizing fragments in the presence of root exudates of the different hosts indicate that one particular nodD copy contributes to nodulation gene induction far more than any other nodD copy present. The nucleotide sequences of both nodD genes are reported here and show significant homology to those of the nodD genes of other rhizobia and a Bradyrhizobium strain. A dendrogram based on the protein sequences of 15 different NodD proteins shows that the R. tropici NodD proteins are linked most closely to each other and then to the NodD of Rhizobium phaseoli 8002.     

Response of common bean lines to inoculation: comparison between the <Emphasis Type="Italic">Rhizobium tropici</Emphasis> CIAT899 and the native <Emphasis Type="Italic">Rhizobium etli</Emphasis> 12a3 and their persistence in Tunisian soils

Since Phaseolus vulgaris (L) is poorly nodulated in all regions of Tunisia where this crop is grown, the response of common-bean lines CocoT and Flamingo to inoculation with reference Rhizobium tropici CIAT 899 or native rhizobia, namely Sinorhizobium fredii 1a6, Rhizobium etli 12a3, and Rhizobium gallicum 8a3, was studied in a field station. Since R. etli 12a3 was found to be the most effective native rhizobium, it was subsequently compared with R. tropici CIAT 899 in a broader study in two stations over 3 years. A significant interaction between bean and rhizobia was observed for nodule number, shoot dry weight, grain yield, and contents of nitrogen and chlorophyll. The native rhizobia was more efficient than CIAT899 for Flamingo, though not for CocoT. The Enzyme-linked immunosorbent assay technique was used with polyclonal antibody to assess the occupancy in nodule and persistence in soil of the inoculated rhizobia. For both stations the nodule occupancy was 100% during the first year for each rhizobium, but during the next 2 years, between 7 and 15% of nodules were formed by the rhizobia inoculated in the neighboring plot. It is concluded that the first-year inoculation is sufficient to maintain an adequate rate of nodulation during three growth cycles, and that the native R etli can be recommended for the common-bean inoculation in similar soils of Tunisia.     

Genome-wide identification of the Phaseolus vulgaris sRNAome using small RNA and degradome sequencing

Background

MiRNAs and phasiRNAs are negative regulators of gene expression. These small RNAs have been extensively studied in plant model species but only 10 mature microRNAs are present in miRBase version 21, the most used miRNA database, and no phasiRNAs have been identified for the model legume Phaseolus vulgaris. Thanks to the recent availability of the first version of the common bean genome, degradome data and small RNA libraries, we are able to present here a catalog of the microRNAs and phasiRNAs for this organism and, particularly, we suggest new protagonists in the symbiotic nodulation events.

Results

We identified a set of 185 mature miRNAs, including 121 previously unpublished sequences, encoded by 307 precursors and distributed in 98 families. Degradome data allowed us to identify a total of 181 targets for these miRNAs. We reveal two regulatory networks involving conserved miRNAs: those known to play crucial roles in the establishment of nodules, and novel miRNAs present only in common bean, suggesting a specific role for these sequences. In addition, we identified 125 loci that potentially produce phased small RNAs, with 47 of them having all the characteristics of being triggered by a total of 31 miRNAs, including 14 new miRNAs identified in this study.

Conclusions

We provide here a set of new small RNAs that contribute to the broader knowledge of the sRNAome of Phaseolus vulgaris. Thanks to the identification of the miRNA targets from degradome analysis and the construction of regulatory networks between the mature microRNAs, we present here the probable functional regulation associated with the sRNAome and, particularly, in N₂-fixing symbiotic nodules.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1639-5) contains supplementary material, which is available to authorized users.     

Symbiotic performance of common bean and soybean co-inoculated with rhizobia and <Emphasis Type="Italic">Chryseobacterium balustinum</Emphasis> Aur9 under moderate saline conditions

The effect of co-inoculating beans and soybeans with rhizobia and Chryseobacterium, a plant growth promoting bacteria (PGPR), was studied under conditions of mild saline stress. Chryseobacterium balustinum Aur9 was used with Rhizobium tropici CIAT899 or R. etli ISP42 to inoculate common bean (Phaseolus vulgaris L.), or jointly with Ensifer (Sinorhizobium) fredii SMH12 and HH103 to inoculate soybean (Glycine max (L.) Merrill). The effect of co-inoculation was studied by following nodule primordia initiation, nodulation kinetics and symbiotic performance in plants grown under moderate saline conditions (25 mM NaCl). In common bean, co-inoculation improved nodule primordia formation when compared with single inoculation (R. tropici CIAT899). However, co-inoculation did not provide benefits in the development of nodule primordia in soybean with E. fredii SMH12. The kinetic of nodulation in bean was also favored by double inocula resulting in a higher number of nodules. Long-term effects of co-inoculation on beans and soybeans depended on the rhizobial species used. In both, control and saline conditions, co-inoculation of R. tropici CIAT899 and C. balustinum Aur9 improved bean growth when compared with the single inoculation (CIAT899). However, the positive effect of double inocula on plant growth did not occur when using R. etli ISP42. Soybean plants receiving double inoculation (E. fredii SMH12 and C. balustinum Aur9) showed better symbiotic performance, mostly under saline stress, than with a single inoculation. The results indicate that co-inoculation with C. balustinum and rhizobia under mild saline conditions partially relieves the salt-stress effects, although do not always result advantageous for symbiotic N₂ fixation in legume plants.     

The Rhizobium leguminosarum nodulation gene nodF encodes a polypeptide similar to acyl-carrier protein and is regulated by nodD plus a factor in pea root exudate

The DNA sequence of ~3.5 kb of the nodulation (nod) region of the Rhizobium leguminosarum symbiotic plasmid pRL1JI was determined. Three open reading frames were identified; genes corresponding to these have been called nodD, nodE and nodF.nodD is adjacent to nodA and is transcribed in the opposite direction. The nodF and nodE genes are downstream of, and transcribed in the same direction as, nodD with 667 nucleotides between nodD and nodF and three nucleotides separating nodF and nodE. The induction of the nodFE operon requires the nodD gene product and a component present in plant root exudate. Regions of DNA sequence preceding nodF are similar to those preceding nodA; these sequences may be involved in the regulation of the expression of nodA and nodF. Analysis of nodD revealed an amino acid sequence similar to the predicted DNA-binding domain of known DNA-binding proteins. A protein comparison of the nodF protein showed it to be similar to the acyl-carrier protein from Escherichia coli and barley, especially around the pantothenate-binding region and on this basis it is thought that this protein may be involved in an acyl transfer reaction.     

The genetics of domestication of yardlong bean, Vigna unguiculata (L.) Walp. ssp. unguiculata cv.-gr. sesquipedalis

Background and Aims

The genetics of domestication of yardlong bean [Vigna unguiculata (L.) Walp. ssp. unguiculata cv.-gr. sesquipedalis] is of particular interest because the genome of this legume has experienced divergent domestication. Initially, cowpea was domesticated from wild cowpea in Africa; in Asia a vegetable form of cowpea, yardlong bean, subsequently evolved from cowpea. Information on the genetics of domestication-related traits would be useful for yardlong bean and cowpea breeding programmes, as well as comparative genome study among members of the genus Vigna. The objectives of this study were to identify quantitative trait loci (QTLs) for domestication-related traits in yardlong bean and compare them with previously reported QTLs in closely related Vigna.

Methods

Two linkage maps were developed from BC₁F₁ and F₂ populations from the cross between yardlong bean (V. unguiculata ssp. unguiculata cv.-gr. sesquipedalis) accession JP81610 and wild cowpea (V. unguiculata ssp. unguiculata var. spontanea) accession TVnu457. Using these linkage maps, QTLs for 24 domestication-related traits were analysed and mapped. QTLs were detected for traits related to seed, pod, stem and leaf.

Key Results

Most traits were controlled by between one and 11 QTLs. QTLs for domestication-related traits show co-location on several narrow genomic regions on almost all linkage groups (LGs), but especially on LGs 3, 7, 8 and 11. Major QTLs for sizes of seed, pod, stem and leaf were principally located on LG7. Pleiotropy or close linkage of genes for the traits is suggested in these chromosome regions.

Conclusions

This is the first report of QTLs for domestication-related traits in yardlong bean. The results provide a foundation for marker-assisted selection of domestication-related QTLs in yardlong bean and enhance understanding of domestication in the genus Vigna.     

Multiple origins of the determinate growth habit in domesticated common bean (Phaseolus vulgaris)

Background and Aims

The actual number of domestications of a crop is one of the key questions in domestication studies. Answers to this question have generally been based on relationships between wild progenitors and domesticated descendants determined with anonymous molecular markers. In this study, this question was investigated by determining the number of instances a domestication phenotype had been selected in a crop species. One of the traits that appeared during domestication of common bean (Phaseolus vulgaris) is determinacy, in which stems end with a terminal inflorescence. It has been shown earlier that a homologue of the arabidopsis TFL1 gene – PvTFL1y – controls determinacy in a naturally occurring variation of common bean.

Methods

Sequence variation was analysed for PvTFL1y in a sample of 46 wild and domesticated accessions that included determinate and indeterminate accessions.

Key Results

Indeterminate types – wild and domesticated – showed only synonymous nucleotide substitutions. Determinate types – observed only among domesticated accessions – showed, in addition to synonymous substitutions, non-synonymous substitutions, indels, a putative intron-splicing failure, a retrotransposon insertion and a deletion of the entire locus. The retrotransposon insertion was observed in 70 % of determinate cultivars, in the Americas and elsewhere. Other determinate mutants had a more restricted distribution in the Americas only, either in the Andean or in the Mesoamerican gene pool of common bean.

Conclusions

Although each of the determinacy haplotypes probably does not represent distinct domestication events, they are consistent with the multiple (seven) domestication pattern in the genus Phaseolus. The predominance of determinacy in the Andean gene pool may reflect domestication of common bean prior to maize introduction in the Andes.     

Plant growth-promoting rhizobacteria for improving nodulation and nitrogen fixation in the common bean (Phaseolus vulgaris L.)

A greenhouse experiment was performed to evaluate the effects of plant growth-promoting rhizobacteria (PGPR) on nodulation, biological nitrogen fixation (BNF) and growth of the common bean (Phaseolus vulgaris L. cv. Tenderlake). Single and dual inoculation treatments of bean with Rhizobium and/or PGPR were administered to detect possible changes in the levels of and interactions between the phytohormones IAA and cytokinin. Bean plants cv. Tenderlake were grown in pots containing Fluvic Neosol eutrophic (pH 6.5). Fourteen kilogram aliquots of soil contained in 15-l pots were autoclaved. Bean seeds were surface sterilized and inoculated with Rhizobium tropici (CIAT 899-standard strain) alone and in combination with one of the PGPR strains: Bacillus endophyticus (DSM 13796), B. pumilus (DSM 27), B. subtilis (DSM 704), Paenibacillus lautus (DSM 13411), P. macerans (DSM 24), P. polymyxa (DSM 36), P. polymyxa (Loutit L.) or Bacillus sp.(65E180). The experimental design was randomized block design with three replications. Beans co-inoculated with Rhizobium tropici (CIAT899) and Paenibacillus polymyxa (DSM 36) had higher leghemoglobin concentrations, nitrogenase activity and N₂ fixation efficiency and thereby formed associations of greater symbiotic efficiency. Inoculation with Rhizobium and P. polymyxa strain Loutit (L) stimulated nodulation as well as nitrogen fixation. PGPR also stimulated specific-nodulation (number of nodules per gram of root dry weight) increases that translated into higher levels of accumulated nitrogen. The activities of phytohormones depended on their content and interactions with Rhizobium tropici and Paenibacillus and/or Bacillus (PGPR) strains which affect the cytokinin in content in the common bean.     

Discrete differences between strains of different Rhizobium spp. for competitive nodule occupancy on beans

Rhizobium etli strain TAL182 and R. leguminosarum bv phaseoli strain 8002, both of which produce melanin pigment, were tested for their nodulation competitiveness on beans by paired inoculation with two strains which do not produce melanin: R. tropici strain CIAT899 and Rhizobium sp. strain TAL1145. An assay was developed to distinguish nodules formed by the melanin-producing and non-producing strains. Strain TAL182 had discrete competitive superiority over CIAT899 and TAL1145 for nodulation of beans. Nodulation competitiveness was not correlated with the ability to produce melanin pigment or the host range of the Rhizobium strains tested.The authors are with the Department of Plant Molecular Physiology, University of Hawaii, 3050 Maile Way, Gillmore 402, Honolulu, HI 96822, USA     

Carpeloidy in flower evolution and diversification: a comparative study in Carica papaya and Arabidopsis thaliana

Background and Aims

Bisexual flowers of Carica papaya range from highly regular flowers to morphs with various fusions of stamens to the ovary. Arabidopsis thaliana sup1 mutants have carpels replaced by chimeric carpel–stamen structures. Comparative analysis of stamen to carpel conversions in the two different plant systems was used to understand the stage and origin of carpeloidy when derived from stamen tissues, and consequently to understand how carpeloidy contributes to innovations in flower evolution.

Methods

Floral development of bisexual flowers of Carica was studied by scanning electron microscopy and was compared with teratological sup mutants of A. thaliana.

Key Results

In Carica development of bisexual flowers was similar to wild (unisexual) forms up to locule initiation. Feminization ranges from fusion of stamen tissue to the gynoecium to complete carpeloidy of antepetalous stamens. In A. thaliana, partial stamen feminization occurs exclusively at the flower apex, with normal stamens forming at the periphery. Such transformations take place relatively late in development, indicating strong developmental plasticity of most stamen tissues. These results are compared with evo-devo theories on flower bisexuality, as derived from unisexual ancestors. The Arabidopsis data highlight possible early evolutionary events in the acquisition of bisexuality by a patchy transformation of stamen parts into female parts linked to a flower axis-position effect. The Carica results highlight tissue-fusion mechanisms in angiosperms leading to carpeloidy once bisexual flowers have evolved.

Conclusions

We show two different developmental routes leading to stamen to carpel conversions by late re-specification. The process may be a fundamental aspect of flower development that is hidden in most instances by developmental homeostasis.