Secretion of Zeatin, Ribosylzeatin, and Ribosyl-1'' -Methylzeatin by Pseudomonas savastanoi: Plasmid-Coded Cytokinin Biosynthesis

A Novel Microbial Dysbiosis Index and Intestinal Microbiota-Associated Markers as Tools of Precision Medicine in Inflammatory Bowel Disease Paediatric Patients

Recent evidence indicates that the gut microbiota (GM) has a significant impact on the inflammatory bowel disease (IBD) progression. Our aim was to investigate the GM profiles, the Microbial Dysbiosis Index (MDI) and the intestinal microbiota-associated markers in relation to IBD clinical characteristics and disease state. We performed 16S rRNA metataxonomy on both stools and ileal biopsies, metabolic dysbiosis tests on urine and intestinal permeability and mucosal immunity activation tests on the stools of 35 IBD paediatric patients. On the GM profile, we assigned the MDI to each patient. In the statistical analyses, the MDI was correlated with clinical parameters and intestinal microbial-associated markers. In IBD patients with high MDI, Gemellaceae and Enterobacteriaceae were increased in stools, and Fusobacterium, Haemophilus and Veillonella were increased in ileal biopsies. Ruminococcaceae and WAL_1855D were enriched in active disease condition; the last one was also positively correlated to MDI. Furthermore, the MDI results correlated with PUCAI and Matts scores in ulcerative colitis patients (UC). Finally, in our patients, we detected metabolic dysbiosis, intestinal permeability and mucosal immunity activation. In conclusion, the MDI showed a strong association with both severity and activity of IBD and a positive correlation with clinical scores, especially in UC. Thus, this evidence could be a useful tool for the diagnosis and prognosis of IBD.

Genomic Assessment of the Contribution of the Wolbachia Endosymbiont of Eurosta solidaginis to Gall Induction

We explored the genome of the Wolbachia strain, wEsol, symbiotic with the plant-gall-inducing fly Eurosta solidaginis with the goal of determining if wEsol contributes to gall induction by its insect host. Gall induction by insects has been hypothesized to involve the secretion of the phytohormones cytokinin and auxin and/or proteinaceous effectors to stimulate cell division and growth in the host plant. We sequenced the metagenome of E. solidaginis and wEsol and assembled and annotated the genome of wEsol. The wEsol genome has an assembled length of 1.66 Mbp and contains 1878 protein-coding genes. The wEsol genome is replete with proteins encoded by mobile genetic elements and shows evidence of seven different prophages. We also detected evidence of multiple small insertions of wEsol genes into the genome of the host insect. Our characterization of the genome of wEsol indicates that it is compromised in the synthesis of dimethylallyl pyrophosphate (DMAPP) and S-adenosyl L-methionine (SAM), which are precursors required for the synthesis of cytokinins and methylthiolated cytokinins. wEsol is also incapable of synthesizing tryptophan, and its genome contains no enzymes in any of the known pathways for the synthesis of indole-3-acetic acid (IAA) from tryptophan. wEsol must steal DMAPP and L-methionine from its host and therefore is unlikely to provide cytokinin and auxin to its insect host for use in gall induction. Furthermore, in spite of its large repertoire of predicted Type IV secreted effector proteins, these effectors are more likely to contribute to the acquisition of nutrients and the manipulation of the host's cellular environment to contribute to growth and reproduction of wEsol than to aid E. solidaginis in manipulating its host plant. Combined with earlier work that shows that wEsol is absent from the salivary glands of E. solidaginis, our results suggest that wEsol does not contribute to gall induction by its host.

Abscisic Acid: A Potential Secreted Effector Synthesized by Phytophagous Insects for Host-Plant Manipulation

Abscisic acid (ABA) is an isoprenoid-derived plant signaling molecule involved in a wide variety of plant processes, including facets of growth and development as well as responses to abiotic and biotic stress. ABA had previously been reported in a wide variety of animals, including insects and humans. We used high-performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-(ESI)-MS/MS) to examine concentrations of ABA in 17 species of phytophagous insects, including gall- and non-gall-inducing species from all insect orders with species known to induce plant galls: Thysanoptera, Hemiptera, Lepidoptera, Coleoptera, Diptera, and Hymenoptera. We found ABA in insect species in all six orders, in both gall-inducing and non-gall-inducing species, with no tendency for gall-inducing insects to have higher concentrations. The concentrations of ABA in insects often markedly exceeded those typically found in plants, suggesting it is highly improbable that insects obtain all their ABA from their host plant via consumption and sequestration. As a follow-up, we used immunohistochemistry to determine that ABA localizes to the salivary glands in the larvae of the gall-inducing Eurosta solidaginis (Diptera: Tephritidae). The high concentrations of ABA, combined with its localization to salivary glands, suggest that insects are synthesizing and secreting ABA to manipulate their host plants. The pervasiveness of ABA among both gall- and non-gall-inducing insects and our current knowledge of the role of ABA in plant processes suggest that insects are using ABA to manipulate source-sink mechanisms of nutrient allocation or to suppress host-plant defenses. ABA joins the triumvirate of phytohormones, along with cytokinins (CKs) and indole-3-acetic acid (IAA), that are abundant, widespread, and localized to glandular organs in insects and used to manipulate host plants.

Multiple relaxases contribute to the horizontal transfer of the virulence plasmids from the tumorigenic bacterium Pseudomonas syringae pv. savastanoi NCPPB 3335

Pseudomonas syringae pv. savastanoi NCPPB 3335 is the causal agent of olive knot disease and contains three virulence plasmids: pPsv48A (pA), 80 kb; pPsv48B (pB), 45 kb, and pPsv48C (pC), 42 kb. Here we show that pB contains a complete MPF_T (previously type IVA secretion system) and a functional origin of conjugational transfer adjacent to a relaxase of the MOB_P family; pC also contains a functional oriT-MOB_P array, whereas pA contains an incomplete MPF_I (previously type IVB secretion system), but not a recognizable oriT. Plasmid transfer occurred on solid and in liquid media, and on leaf surfaces of a non-host plant (Phaseolus vulgaris) with high (pB) or moderate frequency (pC); pA was transferred only occasionally after cointegration with pB. We found three plasmid-borne and three chromosomal relaxase genes, although the chromosomal relaxases did not contribute to plasmid dissemination. The MOB_P relaxase genes of pB and pC were functionally interchangeable, although with differing efficiencies. We also identified a functional MOB_Q mobilization region in pC, which could only mobilize this plasmid. Plasmid pB could be efficiently transferred to strains of six phylogroups of P. syringae sensu lato, whereas pC could only be mobilized to two strains of phylogroup 3 (genomospecies 2). In two of the recipient strains, pB was stably maintained after 21 subcultures in liquid medium. The carriage of several relaxases by the native plasmids of P. syringae impacts their transfer frequency and, by providing functional diversity and redundancy, adds robustness to the conjugation system.

Probing into the unique relationship between a soil bacterium, Pseudomonas putida AKMP7 and Arabidopsis thaliana: A case of "conditional pathogenesis"

Plant growth-promoting rhizobacteria (PGPR) are beneficial soil bacteria that colonise the rhizosphere and help plants in growth, development, and stress tolerance. While there is a significant body of research elucidating their benefits to plants, studies on the "abnormal" or "unexpected" behavior of these bacteria are almost non-existent. One such study from our laboratory has previously reported a unique situation in which a certain strain of drought and thermo-tolerant PGPR, namely, Pseudomonas putida AKMP7, becomes pathogenic towards Arabidopsis thaliana under drought conditions, but not under normal (well-watered) conditions. In this study, we have probed deeper into this phenomenon of "conditional pathogenesis". We found that, AKMP7 imparts an enhancement in plant growth under well-watered conditions, while, causing a deterioration in plant health under drought conditions. In an attempt to understand the underlying reasons for this phenomenon, we analysed the phytohormones released by Pseudomonas putida AKMP7 using LC-ESI-MS/MS technique. We identified that AKMP7 releases zeatin (a cytokinin), the auxin derivative -indole acetamide and amino acid-conjugates of auxin (indole-3-acetyl-L-alanine, indole-3-acetyl-L-phenylalanine and indole-3-acetyl-L-aspartate) in the growth medium. By treating the plants with commercially obtained forms of these phytohormones, individually or in combination with AKMP7, we identified that zeatin and auxin derivative indole acetamide can play a crucial role in the conditional pathogenesis exhibited by this bacterium on A. thaliana under drought conditions. Our work lays a foundation for further understanding the precise molecular mechanisms involved in this unique phenomenon of conditional/opportunistic pathogenesis.

Bacillus altitudinis HNH7 and Bacillus velezensis HNH9 promote plant growth through upregulation of growth-promoting genes in upland cotton

AIMS

The potential of endophytic Bacillus strains to improve plant growth and yield was evaluated.

METHODS AND RESULTS

Endophytic Bacillus altitudinis HNH7 and Bacillus velezensis HNH9 were evaluated for their growth-promoting traits. In an in vitro plate assay, HNH7 and HNH9 exhibited proteolytic, amylolytic, lipolytic, and cellulolytic activity. HNH7 and HNH9 were able to solubilize iron by producing siderophores but were unable to solubilize insoluble phosphate. PCR confirmed the presence of four growth-promoting genes viz. pvd, budA, asbA, and satA in the genome of HNH7, while HNH9 also possessed the same genes except for budA. In a greenhouse experiment, HNH7 and HNH9 promoted the growth of upland cotton plants by upregulating the expression of growth-linked genes, EXP6, ARF1, ARF18, IAA9, CKX6, and GID1b. However, the expression of genes involved in ethylene biosynthesis i.e., ERF and ERF17 was downregulated after treating the plants with HNH7 and HNH9 compared to the control. Furthermore, cotton plants treated with HNH7 and HNH9 exhibited a significantly higher rate of photosynthesis and stomatal conductance.

CONCLUSION

HNH7 and HNH9 showed a promising potential to promote the growth of cotton plants.

SIGNIFICANCE AND IMPACT OF STUDY

Research on plant growth-promoting Bacillus strains can lead to the formation of biofertilizers.

The Localization of Phytohormones within the Gall-inducing Insect Eurosta solidaginis (Diptera: Tephritidae)

The phytohormone production hypothesis suggests that organisms, including insects, induce galls by producing and secreting plant growth hormones. Auxins and cytokinins are classes of phytohormones that induce cell growth and cell division, which could contribute to the plant tissue proliferation which constitutes the covering gall. Bacteria, symbiotic with insects, may also play a part in gall induction by insects through the synthesis of phytohormones or other effectors. Past studies have shown that concentrations of cytokinins and auxins in gall-inducing insects are higher than in their host plants. However, these analyses have involved whole-body extractions. Using immunolocalization of cytokinin and auxin, in the gall inducing stage of Eurosta solidaginis, we found both phytohormones to localize almost exclusively to the salivary glands. Co-localization of phytohormone label with a nucleic acid stain in the salivary glands revealed the absence of Wolbachia sp., the bacterial symbiont of E. solidaginis, which suggests that phytohormone production is symbiont independent. Our findings are consistent with the hypothesis that phytohormones are synthesized in and secreted from the salivary glands of E. solidaginis into host-plant tissues for the purpose of manipulating the host plant.

Genes ptz and idi, Coding for Cytokinin Biosynthesis Enzymes, Are Essential for Tumorigenesis and In Planta Growth by P. syringae pv. savastanoi NCPPB 3335

The phytopathogenic bacterium Pseudomonas syringae pv. savastanoi elicits aerial tumors on olive plants and is also able to synthesize large amounts of auxins and cytokinins. The auxin indoleacetic acid was shown to be required for tumorigenesis, but there is only correlational evidence suggesting a role for cytokinins. The model strain NCPPB 3335 contains two plasmid-borne genes coding for cytokinin biosynthesis enzymes: ptz, for an isopentenyl transferase and idi, for an isopentenyl-diphosphate delta-isomerase. Phylogenetic analyses showed that carriage of ptz and idi is not strictly associated with tumorigenic bacteria, that both genes were linked when first acquired by P. syringae, and that a different allele of ptz has been independently acquired by P. syringae pv. savastanoi and closely related bacteria. We generated mutant derivatives of NCPPB 3335 cured of virulence plasmids or with site-specific deletions of genes ptz and/or idi and evaluated their virulence in lignified and micropropagated olive plants. Strains lacking ptz, idi, or both produced tumors with average volumes up to 29 times smaller and reached populations up to two orders of magnitude lower than those induced by strain NCPPB 3335; these phenotypes reverted by complementation with the cloned genes. Trans-zeatin was the most abundant cytokinin in culture filtrates of NCPPB 3335. Deletion of gene ptz abolished biosynthesis of trans-zeatin and dihydrozeatin, whereas a reduced but significant amount of isopentenyladenine was still detected in the medium, suggesting the existence of other genes contributing to cytokinin biosynthesis in P. syringae. Conversely, extracts from strains lacking gene idi contained significantly higher amounts of trans-zeatin than extracts from the wild-type strain but similar amounts of the other cytokinins. This suggests that Idi might promote tumorigenesis by ensuring the biosynthesis of the most active cytokinin forms, their correct balance in planta, or by regulating the expression of other virulence genes. Therefore, gene ptz, but not gene idi, is essential for the biosynthesis of high amounts of cytokinins in culture; however, both ptz and idi are individually essential for the adequate development of tumors on olive plants by Psv NCPPB 3335.

Plant tumors: a hundred years of study

The review provides information on the mechanisms underlying the development of spontaneous and pathogen-induced tumors in higher plants. The activation of meristem-specific regulators in plant tumors of various origins suggests the meristem-like nature of abnormal plant hyperplasia. Plant tumor formation has more than a century of research history. The study of this phenomenon has led to a number of important discoveries, including the development of the Agrobacterium-mediated transformation technique and the discovery of horizontal gene transfer from bacteria to plants. There are two main groups of plant tumors: pathogen-induced tumors (e.g., tumors induced by bacteria, viruses, fungi, insects, etc.), and spontaneous ones, which are formed in the absence of any pathogen in plants with certain genotypes (e.g., interspecific hybrids, inbred lines, and mutants). The causes of the transition of plant cells to tumor growth are different from those in animals, and they include the disturbance of phytohormonal balance and the acquisition of meristematic characteristics by differentiated cells. The aim of this review is to discuss the mechanisms underlying the development of most known examples of plant tumors.

Cytokinins Are Abundant and Widespread Among Insect Species

Cytokinins (CKs) are a class of compounds that have long been thought to be exclusively plant growth regulators. Interestingly, some species of phytopathogenic bacteria and fungi have been shown to, and gall-inducing insects have been hypothesized to, produce CKs and use them to manipulate their host plants. We used high performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-MS/MS) to examine concentrations of a wide range of CKs in 17 species of phytophagous insects, including gall- and non-gall-inducing species from all six orders of Insecta that contain species known to induce galls: Thysanoptera, Hemiptera, Lepidoptera, Coleoptera, Diptera, and Hymenoptera. We found CKs in all six orders of insects, and they were not associated exclusively with gall-inducing species. We detected 24 different CK analytes, varying in their chemical structure and biological activity. Isoprenoid precursor nucleotide and riboside forms of trans-zeatin (tZ) and isopentenyladenine (iP) were most abundant and widespread across the surveyed insect species. Notably, the observed concentrations of CKs often markedly exceeded those reported in plants suggesting that insects are synthesizing CKs rather than obtaining them from the host plant via tissue consumption, compound sequestration, and bioaccumulation. These findings support insect-derived CKs as means for gall-inducing insects to manipulate their host plant to facilitate cell proliferation, and for both gall- and non-gall-inducing insects to modify nutrient flux and plant defenses during herbivory. Furthermore, wide distribution of CKs across phytophagous insects, including non-gall-inducing species, suggests that insect-borne CKs could be involved in manipulation of source-sink mechanisms of nutrient allocation to sustain the feeding site and altering plant defensive responses, rather than solely gall induction. Given the absence of any evidence for genes in the de novo CK biosynthesis pathway in insects, we postulate that the tRNA-ipt pathway is responsible for CK production. However, the unusually high concentrations of CKs in insects, and the tendency toward dominance of their CK profiles by tZ and iP suggest that the tRNA-ipt pathway functions differently and substantially more efficiently in insects than in plants.

The Cytokinin Complex Associated With Rhodococcus fascians: Which Compounds Are Critical for Virulence?

Virulent strains of Rhodococcus fascians cause a range of disease symptoms, many of which can be mimicked by application of cytokinin. Both virulent and avirulent strains produce a complex of cytokinins, most of which can be derived from tRNA degradation. To test the three current hypotheses regarding the involvement of cytokinins as virulence determinants, we used PCR to detect specific genes, previously associated with a linear virulence plasmid, including two methyl transferase genes (mt1 and mt2) and fas4 (dimethyl transferase), of multiple strains of R. fascians. We inoculated Pisum sativum (pea) seeds with virulent and avirulent strains of R. fascians, monitored the plants over time and compared these to mock-inoculated controls. We used RT-qPCR to monitor the expression of mt1, mt2, and fas4 in inoculated tissues and LC-MS/MS to obtain a comprehensive picture of the cytokinin complement of inoculated cotyledons, roots and shoots over time. The presence and expression of mt1 and mt2 was associated with those strains of R. fascians classed as virulent, and not those classed as avirulent. Expression of mt1, mt2, and fas4 peaked at 9 days post-inoculation (dpi) in cotyledons and at 15 dpi in shoots and roots developed from seeds inoculated with virulent strain 602. Pea plants inoculated with virulent and avirulent strains of R. fascians both contained cytokinins likely to have been derived from tRNA turnover including the 2-methylthio cytokinins and cis-zeatin-derivatives. Along with the isopentenyladenine-type cytokinins, the levels of these compounds did not correlate with virulence. Only the novel 1- and 2-methylated isopentenyladenine cytokinins were uniquely associated with infection by the virulent strains and are, therefore, the likely causative factors of the disease symptoms.

Knots Untie: Molecular Determinants Involved in Knot Formation Induced by Pseudomonas savastanoi in Woody Hosts

The study of the molecular basis of tree diseases is lately receiving a renewed attention, especially with the emerging perception that pathogens require specific pathogenicity and virulence factors to successfully colonize woody hosts. Pathosystems involving woody plants are notoriously difficult to study, although the use of model bacterial strains together with genetically homogeneous micropropagated plant material is providing a significant impetus to our understanding of the molecular determinants leading to disease. The gammaproteobacterium Pseudomonas savastanoi belongs to the intensively studied Pseudomonas syringae complex, and includes three pathogenic lineages causing tumorous overgrowths (knots) in diverse economically relevant trees and shrubs. As it occurs with many other bacteria, pathogenicity of P. savastanoi is dependent on a type III secretion system, which is accompanied by a core set of at least 20 effector genes shared among strains isolated from olive, oleander, and ash. The induction of knots of wild-type size requires that the pathogen maintains adequate levels of diverse metabolites, including the phytohormones indole-3-acetic acid and cytokinins, as well as cyclic-di-GMP, some of which can also regulate the expression of other pathogenicity and virulence genes and participate in bacterial competitiveness. In a remarkable example of social networking, quorum sensing molecules allow for the communication among P. savastanoi and other members of the knot microbiome, while at the same time are essential for tumor formation. Additionally, a distinguishing feature of bacteria from the P. syringae complex isolated from woody organs is the possession of a 15 kb genomic island (WHOP) carrying four operons and three other genes involved in degradation of phenolic compounds. Two of these operons mediate the catabolism of anthranilate and catechol and, together with another operon, are required for the induction of full-size tumors in woody hosts, but not in non-woody micropropagated plants. The use of transposon mutagenesis also uncovered a treasure trove of additional P. savastanoi genes affecting virulence and participating in diverse bacterial processes. Although there is still much to be learned on what makes a bacterium a successful pathogen of trees, we are already untying the knots.

Plant Growth Promotion by Volatile Organic Compounds Produced by Bacillus subtilis SYST2

Bacterial volatiles play a significant role in promoting plant growth by regulating the synthesis or metabolism of phytohormones. In vitro and growth chamber experiments were conducted to investigate the effect of volatile organic compounds (VOCs) produced by the plant growth promoting rhizobacterium Bacillus subtilis strain SYST2 on hormone regulation and growth promotion in tomato plants. We observed a significant increase in plant biomass under both experimental conditions; we observed an increase in photosynthesis and in the endogenous contents of gibberellin, auxin, and cytokinin, while a decrease in ethylene levels was noted. VOCs emitted by SYST2 were identified through gas chromatography-mass spectrometry analysis. Of 11 VOCs tested in glass jars containing plants in test tubes, only two, albuterol and 1,3-propanediole, were found to promote plant growth. Furthermore, tomato plants showed differential expression of genes involved in auxin (SlIAA1. SlIAA3), gibberellin (GA20ox-1), cytokinin (SlCKX1), expansin (Exp2, Exp9. Exp 18), and ethylene (ACO1) biosynthesis or metabolism in roots and leaves in response to B. subtilis SYST2 VOCs. Our findings suggest that SYST2-derived VOCs promote plant growth by triggering growth hormone activity, and provide new insights into the mechanism of plant growth promotion by bacterial VOCs.

The effects of bacterial volatile emissions on plant abiotic stress tolerance

Plant growth-promoting rhizobacteria (PGPR) are beneficial plant symbionts that have been successfully used in agriculture to increase seedling emergence, plant weight, crop yield, and disease resistance. Some PGPR strains release volatile organic compounds (VOCs) that can directly and/or indirectly mediate increases in plant biomass, disease resistance, and abiotic stress tolerance. This mini-review focuses on the enhancement of plant abiotic stress tolerance by bacterial VOCs. The review considers how PGPR VOCs induce tolerance to salinity and drought stress and also how they improve sulfur and iron nutrition in plants. The potential complexities in evaluating the effects of PGPR VOCs are also discussed.

PsasM2I, a type II restriction-modification system in Pseudomonas savastanoi pv. savastanoi: differential distribution of carrier strains in the environment and the evolutionary history of homologous RM systems in the Pseudomonas syringae complex

A type II restriction-modification system was found in a native plasmid of Pseudomonas savastanoi pv. savastanoi MLLI2. Functional analysis of the methyltransferase showed that the enzyme acts by protecting the DNA sequence CTGCAG from cleavage. Restriction endonuclease expression in recombinant Escherichia coli cells resulted in mutations in the REase sequence or transposition of insertion sequence 1A in the coding sequence, preventing lethal gene expression. Population screening detected homologous RM systems in other P. savastanoi strains and in the Pseudomonas syringae complex. An epidemiological survey carried out by sampling olive and oleander knots in two Italian regions showed an uneven diffusion of carrier strains, whose presence could be related to a selective advantage in maintaining the RM system in particular environments or subpopulations. Moreover, carrier strains can coexist in the same orchards, plants, and knot tissues with non-carriers, revealing unexpected genetic variability on a very small spatial scale. Phylogenetic analysis of the RM system and housekeeping gene sequences in the P. syringae complex demonstrated the ancient acquisition of the RM systems. However, the evolutionary history of the gene complex also showed the involvement of horizontal gene transfer between related strains and recombination events.

Identification of volatiles produced by Cladosporium cladosporioides CL-1, a fungal biocontrol agent that promotes plant growth

Certain microbial Volatile Organic Compounds (VOCs) have been reported to enhance the growth and development of plants. The biocontrol fungi, Cladosporium cladosporioides CL-1 significantly improved the growth of tobacco seedlings in vitro when they were co-cultivated without physical contact. SPME Quadrupole GC/MS/MS revealed that CL-1 emited the volatiles α-pinene, (−)-trans-caryophyllene, tetrahydro-2,2,5,5-tetramethylfuran, dehydroaromadendrene, and (+)-sativene. Potential roles of these volatiles in plant growth and development are discussed. Even though there were several fungal VOCs reported in the past that could influence plant growth, their exact mechanisms of action are not fully known. Fungal VOC-mediated plant growth promotion requires in-depth study in order for this technology to be used in large scale for crops, especially those grown under greenhouse conditions.

Pseudomonas savastanoi pv. savastanoi: some like it knot

Pseudomonas savastanoi pv. savastanoi is the causal agent of olive (Olea europaea) knot disease and an unorthodox member of the P. syringae complex, causing aerial tumours instead of the foliar necroses and cankers characteristic of most members of this complex. Olive knot is present wherever olive is grown; although losses are difficult to assess, it is assumed that olive knot is one of the most important diseases of the olive crop. The last century witnessed a large number of scientific articles describing the biology, epidemiology and control of this pathogen. However, most P. savastanoi pv. savastanoi strains are highly recalcitrant to genetic manipulation, which has effectively prevented the pathogen from benefitting from the scientific progress in molecular biology that has elevated the foliar pathogens of the P. syringae complex to supermodels. A number of studies in recent years have made significant advances in the biology, ecology and genetics of P. savastanoi pv. savastanoi, paving the way for the molecular dissection of its interaction with other nonpathogenic bacteria and their woody hosts. The selection of a genetically pliable model strain was soon followed by the development of rapid methods for virulence assessment with micropropagated olive plants and the analysis of cellular interactions with the plant host. The generation of a draft genome of strain NCPPB 3335 and the closed sequence of its three native plasmids has allowed for functional and comparative genomic analyses for the identification of its pathogenicity gene complement. This includes 34 putative type III effector genes and genomic regions, shared with other pathogens of woody hosts, which encode metabolic pathways associated with the degradation of lignin-derived compounds. Now, the time is right to explore the molecular basis of the P. savastanoi pv. savastanoi-olive interaction and to obtain insights into why some pathovars like it necrotic and why some like it knot.

SYNONYMS

Pseudomonas syringae pv. savastanoi.

TAXONOMY

Kingdom Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Family Pseudomonadaceae; Genus Pseudomonas; included in genomospecies 2 together with at least P. amygdali, P. ficuserectae, P. meliae and 16 other pathovars from the P. syringae complex (aesculi, ciccaronei, dendropanacis, eriobotryae, glycinea, hibisci, mellea, mori, myricae, phaseolicola, photiniae, sesami, tabaci, ulmi and certain strains of lachrymans and morsprunorum); when a formal proposal is made for the unification of these bacteria, the species name P. amygdali would take priority over P. savastanoi.

MICROBIOLOGICAL PROPERTIES

Gram-negative rods, 0.4-0.8 × 1.0-3.0 μm, aerobic. Motile by one to four polar flagella, rather slow growing, optimal temperatures for growth of 25-30 °C; oxidase negative, arginine dihydrolase negative; elicits the hypersensitive response on tobacco; most isolates are fluorescent and levan negative, although some isolates are nonfluorescent and levan positive.

HOST RANGE

P. savastanoi pv. savastanoi causes tumours in cultivated and wild olive and ash (Fraxinus excelsior). Although strains from olive have been reported to infect oleander (Nerium oleander), this is generally not the case; however, strains of P. savastanoi pv. nerii can infect olive. Pathovars fraxini and nerii are differentiated from pathovar savastanoi mostly in their host range, and were not formally recognized until 1996. Literature before about 1996 generally names strains of the three pathovars as P. syringae ssp. savastanoi or P. savastanoi ssp. savastanoi, contributing to confusion on the host range and biological properties.

DISEASE SYMPTOMS

Symptoms of infected trees include hyperplastic growths (tumorous galls or knots) on the stems and branches of the host plant and, occasionally, on leaves and fruits.

EPIDEMIOLOGY

The pathogen can survive and multiply on aerial plant surfaces, as well as in knots, from where it can be dispersed by rain, wind, insects and human activities, entering the plant through wounds. Populations are very unevenly distributed in the plant, and suffer drastic fluctuations throughout the year, with maximum numbers of bacteria occurring during rainy and warm months. Populations of P. savastanoi pv. savastanoi are normally associated with nonpathogenic bacteria, both epiphytically and endophytically, and have been demonstrated to form mutualistic consortia with Erwinia toletana and Pantoea agglomerans, which could result in increased bacterial populations and disease symptoms.

DISEASE CONTROL

Based on preventive measures, mostly sanitary and cultural practices. Integrated control programmes benefit from regular applications of copper formulations, which should be maintained for at least a few years for maximum benefit. Olive cultivars vary in their susceptibility to olive knot, but there are no known cultivars with full resistance to the pathogen.

USEFUL WEBSITES

http://www.pseudomonas-syringae.org/; http://genome.ppws.vt.edu/cgi-bin/MLST/home.pl; ASAP access to the P. savastanoi pv. savastanoi NCPPB 3335 genome sequence https://asap.ahabs.wisc.edu/asap/logon.php.

Analysis of volatile organic compounds emitted by plant growth-promoting fungus Phoma sp. GS8-3 for growth promotion effects on tobacco

We extracted volatile organic compounds (VOCs) emitted by a plant growth-promoting fungus (PGPF) Phoma sp. GS8-3 by gas chromatography and identified them by mass spectrometry. All of the identified compounds belonged to C4-C8 hydrocarbons. Volatiles varied in number and quantity by the culture period of the fungus (in days). 2-Methyl-propanol and 3-methyl-butanol formed the main components of the volatile blends for all the culture periods of fungus. Growth-promoting effects of the identified synthetic compounds were analyzed individually and in blends using tobacco plants. We found that the mixture of volatiles extracted from 3-day-old culture showed significant growth promotion in tobacco in vitro. The volatile blend showed better growth promotion at lower than higher concentrations. Our results confirm the potential role of volatile organic compounds in the mechanism of growth enhancement by GS8-3.

The modulating effect of bacterial volatiles on plant growth: current knowledge and future challenges

Bacteria interact with plants in many different ways. In recent years, bacterial production of volatiles has emerged as a novel process by which bacteria modulate plant growth. Exposure to the volatiles produced by certain bacterial strains has been shown to lead to up to 5-fold increased plant biomass or to plant death. Despite these drastic growth alterations, the elucidation of the molecules responsible, of the mechanism of perception by the plant and of the specific metabolic changes induced in planta is still in its infancy. This review summarizes the current knowledge and highlights future lines of research that should increase our knowledge of the volatile-mediated dialogue between bacteria and plants.

Sequence and role in virulence of the three plasmid complement of the model tumor-inducing bacterium Pseudomonas savastanoi pv. savastanoi NCPPB 3335

Pseudomonas savastanoi pv. savastanoi NCPPB 3335 is a model for the study of the molecular basis of disease production and tumor formation in woody hosts, and its draft genome sequence has been recently obtained. Here we closed the sequence of the plasmid complement of this strain, composed of three circular molecules of 78,357 nt (pPsv48A), 45,220 nt (pPsv48B), and 42,103 nt (pPsv48C), all belonging to the pPT23A-like family of plasmids widely distributed in the P. syringae complex. A total of 152 coding sequences were predicted in the plasmid complement, of which 38 are hypothetical proteins and seven correspond to putative virulence genes. Plasmid pPsv48A contains an incomplete Type IVB secretion system, the type III secretion system (T3SS) effector gene hopAF1, gene ptz, involved in cytokinin biosynthesis, and three copies of a gene highly conserved in plant-associated proteobacteria, which is preceded by a hrp box motif. A complete Type IVA secretion system, a well conserved origin of transfer (oriT), and a homolog of the T3SS effector gene hopAO1 are present in pPsv48B, while pPsv48C contains a gene with significant homology to isopentenyl-diphosphate delta-isomerase, type 1. Several potential mobile elements were found on the three plasmids, including three types of MITE, a derivative of IS801, and a new transposon effector, ISPsy30. Although the replication regions of these three plasmids are phylogenetically closely related, their structure is diverse, suggesting that the plasmid architecture results from an active exchange of sequences. Artificial inoculations of olive plants with mutants cured of plasmids pPsv48A and pPsv48B showed that pPsv48A is necessary for full virulence and for the development of mature xylem vessels within the knots; we were unable to obtain mutants cured of pPsv48C, which contains five putative toxin-antitoxin genes.

Proteome analysis of Arabidopsis seedlings exposed to bacterial volatiles

Plant root-associated bacteria (rhizobacteria) elicit plant basal immunity referred to as induced systemic resistance (ISR) against multiple pathogens. Among multi-bacterial determinants involving such ISR, the induction of ISR and promotion of growth by bacterial volatile compounds was previously reported. To exploit global de novo expression of plant proteins by bacterial volatiles, proteomic analysis was performed after exposure of Arabidopsis plants to the rhizobacterium Bacillus subtilis GB03. Ethylene biosynthesis enzymes were significantly up-regulated. Analysis by quantitative reverse transcriptase polymerase chain reaction confirmed that ethylene biosynthesis-related genes SAM-2, ACS4, ACS12, and ACO2 as well as ethylene response genes, ERF1, GST2, and CHIB were up-regulated by the exposure to bacterial volatiles. More interestingly, the emission of bacterial volatiles significantly up-regulated both key defense mechanisms mediated by jasmonic acid and salicylic acid signaling pathways. In addition, high accumulation of antioxidant proteins also provided evidence of decreased sensitivity to reactive oxygen species during the elicitation of ISR by bacterial volatiles. The present results suggest that the proteomic analysis of plant defense responses in bacterial volatile-mediated ISR can reveal the mechanisms of plant basal defenses orchestrated by endogenous ethylene production pathways and the generation of reactive oxygen species.

Bacillus megaterium strain XTBG34 promotes plant growth by producing 2-pentylfuran

Several chemical changes in soil are associated with plant growth-promoting rhizobacteria. An endosporeforming bacterium, strain XTBG34, was isolated from a Xishuangbanna Tropical Botanical Garden soil sample and identified as Bacillus megaterium. The strain's volatiles had remarkable plant growth promotion activity in Arabidopsis thaliana plants; after 15 days treatment, the fresh weight of plants inoculated with XTBG34 was almost 2-fold compared with those inoculated with DH5alpha. Head space volatile compounds produced by XTBG34, trapped with headspace solid phase microextraction and identified by gas chromatography-mass spectrometry, included aldehydes, alkanes, ketones and aroma components. Of the 11 compounds assayed for plant growth promotion activity in divided Petri plates, only 2-pentylfuran increased plant growth. We have therefore identified a new plant growth promotion volatile of B. megaterium XTBG34, which deserves further study in the mechanisms of interaction between plant growth-promoting rhizobacteria and plants.

Sustained growth promotion in Arabidopsis with long-term exposure to the beneficial soil bacterium Bacillus subtilis (GB03)

Volatile emissions from the commercial growth promoting soil bacterium Bacillus subtilis (GB03) are effective in augmenting short-term growth, photosynthetic capacity and salt tolerance in Petri-dish grown Arabidopsis seedlings. In contrast, the impact sustained GB03 volatile exposure on plant growth and development has yet to be examined. Here is provided physical and physiological data establishing that bacterial volatiles induce long-term growth promotion, elevated photosynthetic capacity and iron accumulation, as well as delayed albeit higher seed count compared with water-treated control plants. Plants were grown unrestricted in double Magenta boxes containing solid MS media for up to twelve weeks with GB03 volatiles introduced in separate containers within the chamber so that plant bacterial interactions were only by airborne transmission. These results establish that GB03 volatiles induce sustained beneficial effects on Arabidopsis growth including robust and extended vegetative growth followed by elevated seed set.

Soil bacteria elevate essential oil accumulation and emissions in sweet basil

Plant growth-promoting rhizobacteria ameliorate environmental conditions for plants by facilitating nutrient uptake and mitigating disease susceptibility. While volatile chemicals from certain soil microbes are sufficient to elicit growth and defense responses in Arabidopsis, whether such volatile signals can induce essential oil accumulation and chemical emissions has yet to be reported. Here, we provide biochemical evidence that the plant growth-promoting soil bacterium Bacillus subtilis GB03 releases volatile chemicals that elevate fresh weight essential oil accumulation and emissions along with plant size in the terpene-rich herb sweet basil (Ocimum basilicum). The two major essential oil components from sweet basil, alpha-terpineol and eugenol, increased ca. 2- and 10-fold, respectively, in plants exposed to GB03 volatiles or with root inoculation as compared to water controls. On a fresh and dry weight basis, shoot and root biomass increases of ca. 2-fold were observed with GB03 volatile exposure or GB03 media inoculation as compared with controls. In testing the efficacy of GB03 volatiles to trigger plant growth and secondary compound production, a physical partition separating roots from bacterial media was provided to preclude nonvolatile microbial elicitors from contributing to GB03-stimulated basil responses. These results demonstrate that volatile bacterial elicitors can concomitantly increase essential oil production and biomass in an herbaceous species rich in commercially valued essential oils.

Soil bacteria confer plant salt tolerance by tissue-specific regulation of the sodium transporter HKT1

Elevated sodium (Na(+)) decreases plant growth and, thereby, agricultural productivity. The ion transporter high-affinity K(+) transporter (HKT)1 controls Na(+) import in roots, yet dysfunction or overexpression of HKT1 fails to increase salt tolerance, raising questions as to HKT1's role in regulating Na(+) homeostasis. Here, we report that tissue-specific regulation of HKT1 by the soil bacterium Bacillus subtilis GB03 confers salt tolerance in Arabidopsis thaliana. Under salt stress (100 mM NaCl), GB03 concurrently down- and upregulates HKT1 expression in roots and shoots, respectively, resulting in lower Na(+) accumulation throughout the plant compared with controls. Consistent with HKT1 participation in GB03-induced salt tolerance, GB03 fails to rescue salt-stressed athkt1 mutants from stunted foliar growth and elevated total Na(+) whereas salt-stressed Na(+) export mutants sos3 show GB03-induced salt tolerance with enhanced shoot and root growth as well as reduced total Na(+). These results demonstrate that tissue-specific regulation of HKT1 is critical for managing Na(+) homeostasis in salt-stressed plants, as well as underscore the breadth and sophistication of plant-microbe interactions.

Global genomic analysis of Pseudomonas savastanoi pv. savastanoi plasmids

Pseudomonas savastanoi pv. savastanoi strains harbor native plasmids belonging to the pPT23A plasmid family (PFPs) which are detected in all pathovars of the related species Pseudomonas syringae examined and contribute to the ecological and pathogenic fitness of their host. However, there is a general lack of information about the gene content of P. savastanoi pv. savastanoi plasmids and their role in the interaction of this pathogen with olive plants. We designed a DNA macroarray containing 135 plasmid-borne P. syringae genes to conduct a global genetic analysis of 32 plasmids obtained from 10 P. savastanoi pv. savastanoi strains. Hybridization results revealed that the number of PFPs per strain varied from one to four. Additionally, most strains contained at least one plasmid (designated non-PFP) that did not hybridize to the repA gene of pPT23A. Only three PFPs contained genes involved in the biosynthesis of the virulence factor indole-3-acetic acid (iaaM, iaaH, and iaaL). In contrast, ptz, a gene involved in the biosynthesis of cytokinins, was found in five PFPs and one non-PFP. Genes encoding a type IV secretion system (T4SS), type IVA, were found in both PFPs and non-PFPs; however, type IVB genes were found only on PFPs. Nine plasmids encoded both T4SSs, whereas seven other plasmids carried none of these genes. Most PFPs and non-PFPs hybridized to at least one putative type III secretion system effector gene and to a variety of additional genes encoding known P. syringae virulence factors and one or more insertion sequence transposase genes. These results indicate that non-PFPs may contribute to the virulence and fitness of the P. savastanoi pv. savastanoi host. The overall gene content of P. savastanoi pv. savastanoi plasmids, with their repeated information, mosaic arrangement, and insertion sequences, suggests a possible role in adaptation to a changing environment.

Rhizobacterial volatile emissions regulate auxin homeostasis and cell expansion in Arabidopsis

Certain plant growth-promoting rhizobacteria (PGPR), in the absence of physical contact with a plant stimulate growth via volatile organic compound (VOC) emissions, through largely unknown mechanisms. To probe how PGPR VOCs trigger growth in plants, RNA transcript levels of Arabidopsis seedlings exposed to Bacillus subtilus (strain GB03) were examined using oligonucleotide microarrays. In screening over 26,000 protein-coded transcripts, a group of approximately 600 differentially expressed genes related to cell wall modifications, primary and secondary metabolism, stress responses, hormone regulation and other expressed proteins were identified. Transcriptional and histochemical data indicate that VOCs from the PGPR strain GB03 trigger growth promotion in Arabidopsis by regulating auxin homeostasis. Specifically, gene expression for auxin synthesis was up regulated in aerial regions of GB03-exposed plants; auxin accumulation decreased in leaves and increased in roots with GB03 exposure as revealed in a transgenic DR5::GUS Arabidopsis line, suggesting activation of basipetal auxin transport. Application of the auxin transport inhibitor 1-naphthylphthalamic acid (NPA) restricted auxin accumulation to sites of synthesis thereby preventing GB03-mediated decreases in shoot auxin levels as well as thwarting GB03-mediated growth promotion. In addition, microarray data revealed coordinated regulation of cell wall loosening enzymes that implicated cell expansion with GB03 exposure, which was confirmed by comparative cytological measurements. The discovery that bacterial VOCs, devoid of auxin or other known plant hormones regulate auxin homeostasis and cell expansion provides a new paradigm as to how rhizobacteria promote plant growth.

Streptomyces turgidiscabies possesses a functional cytokinin biosynthetic pathway and produces leafy galls

Streptomyces turgidiscabies, a cause of potato scab, possesses a mobilizable pathogenicity island containing multiple virulence genes and a cytokinin biosynthetic pathway. These biosynthetic genes are homologous and collinear with the fas operon in Rhodococcus fascians. Reverse-transcriptase polymerase chain reaction of S. turgidiscabies demonstrated that all six genes were transcribed in oat bran broth with and without glucose, though transcription was partially repressed by glucose. The supernatant of S. turgidiscabies cultures had cytokinin activity in callus initiation and differentiation assays. Arabidopsis and tobacco plants inoculated with a thaxtomin-deficient mutant (deltanos) produced leafy galls, indistinguishable from those produced by R. fascians. Deletion of the ipt gene in the pathway eliminated gall phenotype. Other symptoms on tobacco included production of hairy roots and de novo meristems.

Bacterial volatiles promote growth in Arabidopsis

Several chemical changes in soil are associated with plant growth-promoting rhizobacteria (PGPR). Some bacterial strains directly regulate plant physiology by mimicking synthesis of plant hormones, whereas others increase mineral and nitrogen availability in the soil as a way to augment growth. Identification of bacterial chemical messengers that trigger growth promotion has been limited in part by the understanding of how plants respond to external stimuli. With an increasing appreciation of how volatile organic compounds signal plants and serve in plant defense, investigations into the role of volatile components in plant-bacterial systems now can follow. Here, we present chemical and plant-growth data showing that some PGPR release a blend of volatile components that promote growth of Arabidopsis thaliana. In particular, the volatile components 2,3-butanediol and acetoin were released exclusively from two bacterial strains that trigger the greatest level of growth promotion. Furthermore, pharmacological applications of 2,3-butanediol enhanced plant growth whereas bacterial mutants blocked in 2,3-butanediol and acetoin synthesis were devoid in this growth-promotion capacity. The demonstration that PGPR strains release different volatile blends and that plant growth is stimulated by differences in these volatile blends establishes an additional function for volatile organic compounds as signaling molecules mediating plant-microbe interactions.

Cytokinin production by plant growth promoting rhizobacteria and selected mutants

One of the proposed mechanisms by which rhizobacteria enhance plant growth is through the production of plant growth regulators. Five plant growth promoting rhizobacterial (PGPR) strains produced the cytokinin dihydrozeatin riboside (DHZR) in pure culture. Cytokinin production by Pseudomonas fluorescens G20-18, a rifampicin-resistant mutant (RIF), and two TnphoA-derived mutants (CNT1, CNT2), with reduced capacity to synthesize cytokinins, was further characterized in pure culture using immunoassay and thin layer chromatography. G20-18 produced higher amounts of three cytokinins, isopentenyl adenosine (IPA), trans-zeatin ribose (ZR), and DHZR than the three mutants during stationary phase. IPA was the major metabolite produced, but the proportion of ZR and DHZR accumulated by CNT1 and CNT2 increased with time. No differences were observed between strain G20-18 and the mutants in the amounts of indole acetic acid synthesized, nor were gibberellins detected in supernatants of any of the strains. Addition of 10(-5) M adenine increased cytokinin production in 96- and 168-h cultures of strain G20-18 by approximately 67%. G20-18 and the mutants CNT1 and CNT2 may be useful for determination of the role of cytokinin production in plant growth promotion by PGPR.

Synthesis of phytohormones by plant-associated bacteria

The plant hormones, auxins and cytokinins, are involved in several stages of plant growth and development such as cell elongation, cell division, tissue differentiation, and apical dominance. The biosynthesis and the underlying mechanism of auxins and cytokinins action are subjects of intense investigation. Not only plants but also microorganisms can synthesize auxins and cytokinins. The role of phytohormone biosynthesis by microorganisms is not fully elucidated: in several cases of pathogenic fungi and bacteria these compounds are involved in pathogenesis on plants; auxin and cytokinin production may also be involved in root growth stimulation by beneficial bacteria and associative symbiosis. The genetic mechanism of auxin biosynthesis and regulation by Pseudomonas, Agrobacterium, Rhizobium, Bradyrhizobium, and Azospirillum, are well studied; in these bacteria several physiological effects have been correlated to the bacterial phytohormones biosynthesis. The pathogenic bacteria Pseudomonas and Agrobacterium produce indole-3-acetic acid via the indole-3-acetamide pathway, for which the genes are plasmid borne. However, they do possess also the indole-3-pyruvic acid pathway, which is chromosomally encoded. In addition, they have genes that can conjugate free auxins or hydrolyze conjugated forms of auxins and cytokinins. In Agrobacterium there are also several genes, located near the auxin and cytokinin biosynthetic genes, that are involved in the regulation of auxins and cytokinins sensibility of the transformed plant tissue. Symbiotic bacteria Rhizobium and Bradyrhizobium synthesize indole-3-acetic acid via indole-3-pyruvic acid; also the genetic determinants for the indole-3-acetamide pathway have been detected, but their activity has not been demonstrated. In the plant growth-promoting bacterium Azospirillum, as in Agrobacterium and Pseudomonas, both the indole-3-pyruvic acid and the indole-3-acetamide pathways are present, although in Azospirillum the indole-3-pyruvic acid pathway is of major significance. In addition, biochemical evidence for a tryptophan-independent indole-3-acetic acid pathway in Azospirillum has been presented.

Immunoaffinity co-purification of cytokinins and analysis by high-performance liquid chromatography with ultraviolet-spectrum detection

A rapid methodology for the simultaneous analysis of a large number of cytokinins is presented. The cross-reactivity of a mixture of polyclonal antibodies against zeatin riboside and isopentenyladenosine was exploited in a protocol that can be used for immunoaffinity purification of 23 additional cytokinins. Ligands include the cytokinin bases zeatin, dihydrozeatin, isopentenyladenine, benzyl-adenine and kinetin, and their corresponding nucleoside, nucleoside-5'-monophosphate, and 9-glucoside derivatives, as well as cis-zeatin, cis-zeatin riboside, the 2-methylthiol derivatives of isopentenyladenosine and zeatin riboside, and benzyl-adenine-3-glucoside. Mixtures of cytokinins could be retained with high recoveries of all the components. Immunoaffinity purification of extracts of Arabidopsis thaliana (L.) Heynh. and Solarium tuberosum L. gave fractions clean enough, as verified by gas chromatographymass spectrometry (GC-MS), to allow analysis of endogenous cytokinins using a single high-performance liquid chromatography (HPLC) step with on-line UV-spectrum detection. The detection limit was 4-6 pmol. The procedure described forms a routine assaying technique that is faster and simpler, yet yields better qualitative and quantitative information than the commonly used procedure of immunoassaying of HPLC fractions.

Rapid identification of cytokinins by an immunological method

A method for rapid identification of bacterial cytokinins has been developed in which cultures are fed [(3)H]adenine, the cytokinins (including (3)H-labeled cytokinins) are isolated by immunoaffinity chromatography, and analyzed by HPLC with on-line scintillation counting. Analysis of Agrobacterium tumefaciens strains showed that some produced primarily trans-zeatin, whereas others produced primarily trans-zeatin riboside. Pseudomonas syringae pv savastanoi produced mixtures of trans-zeatin, dihydrozeatin, 1''-methyl-trans-zeatin riboside, and other unknown cytokinin-like substances. Corynebacterium fascians, produced cis-zeatin, isopentenyladenine and isopentenyladenosine. The technique is designed for qualitative rather than quantitative studies and allows ready identification of bacterial cytokinins. It may also have utility in the study of plant cytokinins if adequate incorporation of label into cytokinin precursor pools can be achieved.

Cytokinin Production by Bradyrhizobium japonicum

Although there is considerable circumstantial evidence for the involvement of cytokinins in legume nodulation, the cytokinins produced by rhizobia have not been well characterized. Bradyrhizobium japonicum 61A68, a bacterium which nodulates soybean (Glycine max [L.] Merr.), was grown in defined medium. Cytokinins were purified from the culture medium by Amberlite XAD-2 chromatography and fractionated by column chromatography on Sephadex LH-20 in 35% ethanol. Pooled fractions from the Sephadex column were analyzed for cytokinin activity with the tobacco callus bioassay. Cytokinin activity was observed in fractions corresponding to the elution volumes of zeatin, ribosylzeatin, and methylthiozeatin. No activity corresponding to the elution volumes of isopentenyladenine or its riboside was found. Total cytokinin activity in the B. japonicum culture filtrate was equivalent to approximately 1 microgram of kinetin per liter. Transfer RNA was isolated from B. japonicum cells by phenol extraction, followed by potassium acetate extraction, cetyltrimethylammonium bromide precipitation, and DEAE cellulose chromatography. Transfer RNA was enzymically hydrolyzed to nucleosides. High performance liquid chromatographic analysis of cytokinin nucleosides showed peaks corresponding to the retention times of trans-ribosylzeatin, methylthioribosylzeatin, isopentenyladenosine, and methylthioisopentenyladenosine. Analysis of the tRNA hydrolysate by Sephadex LH-20 chromatography and tobacco bioassay showed cytokinin activity in fractions corresponding to ribosylzeatin, methylthioribosylzeatin, and isopentenyladenosine. The presence of the trans isomer of ribosylzeatin was also determined by enzyme immunoassay.

Common evolutionary origin of the central portions of the Ri TL-DNA of Agrobacterium rhizogenes and the Ti T-DNAs of Agrobacterium tumefaciens

Analysis of published sequences for Ri TL-DNA (root-inducing left-hand transferred DNA) of Agrobacterium rhizogenes revealed several unsuspected structural features. First, Ri TL-DNA genes are redundant. Using redundancy as a criterion, three regions (left, middle and right) were discerned. The left one, ORFs (open reading frames) 1-7, contains no detectable redundancy. In the middle region a highly diverged gene family was detected in ORFs 8, 11, 12, 13 and 14. The right region contains an apparently recent duplication (ORF 15 =18+17). We interpret the phenomenon of redundancy, particularly in the central region that encodes the transformed phenotype, to be an adaptation that ensures function in a variety of host species. Comparison of Ri TL-DNA and Ti T-DNAs from Agrobacterium tumefaciens revealed common structures, unpredicted by previous nucleic acid hybridization studies. Ri TL-DNA ORF 8 is a diverged Ti T-DNA tms1. Both Agrobacterium genes consist of a member of the diverged gene family detected in the central part of the Ri TL-DNA, but fused to a sequence similar to iaaM of Pseudomonas savastonoi. Other members of this gene family were found scattered throughout Ti T-DNA. We argue that the central region of Ri and the part of Ti T-DNA including ORFs 5-10 evolved from a common ancestor. We present the hypothesis that the gene family encodes functions that alter developmental plasticity in higher plants.