Phytohormone-Mediated Tumorigenesis by Plant Pathogenic Bacteria

Potential Role and Utilization of Plant Growth Promoting Microbes in Plant Tissue Culture

Plant growth promoting microbes (PGPMs) play major roles in diverse ecosystems, including atmospheric nitrogen fixation, water uptake, solubilization, and transport of minerals from the soil to the plant. Different PGPMs are proposed as biofertilizers, biostimulants, and/or biocontrol agents to improve plant growth and productivity and thereby to contribute to agricultural sustainability and food security. However, little information exists regarding the use of PGPMs in micropropagation such as the in vitro plant tissue culture. This review presents an overview of the importance of PGPMs and their potential application in plant micropropagation. Our analysis, based on published articles, reveals that the process of in vitro classical tissue culture techniques, under strictly aseptic conditions, deserves to be reviewed to allow vitroplants to benefit from the positive effect of PGPMs. Furthermore, exploiting the potential benefits of PGPMs will lead to lessen the cost production of vitroplants during micropropagation process and will make the technique of plant tissue culture more efficient. The last part of the review will indicate where research is needed in the future.

Vascularization is a general requirement for growth of plant and animal tumours

Solid-tumour growth in animals as in humans depends on angiogenesis. Tumours that fail to induce the formation of new blood vessels do not enlarge beyond a few millimetres in diameter. Plant tumours induced by Agrobacterium tumefaciens can reach diameters of more than 100 mm, thus raising the question of how they are sufficiently supplied with nutrients and water. Until recently, these rapidly growing tumours were considered unorganized or partly organized masses. However, in analogy to animal and human tumours, growth of leaf and stem tumours depends on neovascularization. Plant tumour cells induce the formation of a sophisticated vascular network consisting of water-conducting vessels and assimilate-transporting sieve elements. Similar to animal and human tumours that overexpress angiogenic growth factors, plant tumours overexpress the T-DNA-encoded vascularization-promoting growth factors auxin and cytokinin upon AGROBACTERIUM: infection. High auxin levels induce ethylene emission from the tumours, which has a strong impact on tumour and host stem, as well as on root structure and function. Ethylene apparently stimulates abscisic acid synthesis in the leaves above the tumour, which reduces transpiration and thus protects the host plant from rapid wilting. Hence, for the elucidation of phytohormone-dependent vascular development in plants, such tumours are regarded as an excellent model system. The comparison of analogous requirement of neovascularization for tumour growth in plants, as in animals and humans, is discussed in terms of interdisciplinary strategies of possible prevention and therapy.

Agrobacterium rhizogenes mutants that fail to bind to plant cells

Transposon insertion mutants of Agrobacterium rhizogenes were screened to obtain mutant bacteria that failed to bind to carrot suspension culture cells. A light microscope binding assay was used. The bacterial isolates that were reduced in binding to carrot cells were all avirulent on Bryophyllum diagremontiana leaves and on carrot root disks. The mutants did not appear to be altered in cellulose production. The composition of the medium affected the ability of the parent and mutant bacteria to bind to carrot cells. The parent strain bound to carrot cells in greatest numbers in low-ionic-strength media such as 4% sucrose but still showed significant binding in Murashige-Skoog tissue culture medium. All of the mutants showed reduced binding in 4% sucrose after 2 h of incubation with carrot cells. One mutant was delayed in binding in 4% sucrose. This mutant and one other mutant also showed reduced binding to carrot cells in Murashige-Skoog medium. To determine whether the Tn5 insertion was responsible for the mutant phenotype, DNA containing the Tn5 insertion was cloned from the mutant bacteria and used to introduce Tn5 into the parent strain in the same location as in the original mutant by marker exchange. The resulting transconjugants had the same avirulent, nonattaching phenotype as the original mutants, suggesting that the mutant phenotype was due to the Tn5 insertion. The cloned DNA containing the Tn5 insertion was also tested for homology to DNA of known genes that affect attachment of Agrobacterium tumefaciens to plant cells by DNA hybridization. No homology to chv, att, or pscA clones was observed. In addition, cloned chv, att, and pscA genes from A. tumefaciens were unable to complement the attachment-minus A. rhizogenes mutants. Thus, the A. rhizogenes nonattaching mutants appear to be different from the previously described A. tumefaciens mutants.

Cytokinin production by Agrobacterium and Pseudomonas spp

The production of cytokinins by plant-associated bacteria was examined by radioimmunoassay. Strains producing trans-zeatin were identified in the genera Agrobacterium and Pseudomonas. Agrobacterium tumefaciens strains containing nopaline tumor-inducing plasmids, A. tumefaciens Lippia isolates, and Agrobacterium rhizogenes strains produced trans-zeatin in culture at 0.5 to 44 micrograms/liter. Pseudomonas solanacearum and Pseudomonas syringae pv. savastanoi produced trans-zeatin at levels of up to 1 mg/liter. In vitro cytokinin biosynthetic activity was measured for representative strains and was found to correlate with trans-zeatin production. The genetic locus for trans-zeatin secretion (tzs) was cloned from four strains: A. tumefaciens T37, A. rhizogenes A4, P. solanacearum K60, and P. syringae pv. savastanoi 1006. Southern blot analysis showed substantial homology of the Agrobacterium tzs genes to each other but not to the two Pseudomonas genes.

Electric field mediated stable transformation of carrot protoplasts with naked DNA

We have developed an electroporation procedure for the transformation of carrot protoplasts with Ti-plasmid DNA from Agrobacterium tumefaciens. The uptake of pTiC58 into carrot protoplasts was mediated by high voltage electrical pulses at field strengths from 0.5 to 3.8 kV/cm. Protoplast regeneration, somatic embryogenesis and plantlet regeneration were unaffected by the electroporation conditions selected for DNA uptake. Uptake of plasmid pTiC58 resulted in hormone independent regeneration of carrot protoplasts. Transformed somatic embryos were detected in carrot cultures 45 days after electroporation. The transformed somatic embryos developed into teratomas which synthesized nopaline. Hybridization was obtained between a labeled T-DNA fragment from pTiC58 and DNA fragments from 4 month old teratomas regenerated from electro-transformed protoplasts. Based on the number of somatic embryos regenerated after electro-transformation, a frequency of 1.6×10(2) transformants/10(4) somatic embryos/μg pTiC58 DNA was obtained.