Colonization of Tomato Plants by Two Agrocin-Producing Strains of Agrobacterium tumefaciens

Agrobacterium radiobacter K84 is an effective, commercially applied, biological control agent for the plant disease crown gall, yet little is known about the survival and dissemination of K84. To trace K84 in the environment, spontaneous antibiotic-resistant mutants were used. Growth rates and phenotypes of streptomycinor rifampin-resistant K84 were similar to those of the parental K84, except the rifampin-resistant mutant produced less agrocin 84 as determined by bioassay. K84 and a strain of Agrobacterium tumefaciens established populations averaging 105 CFU/g in the rhizosphere of cherry and persisted on roots for 2 years. K84 established rhizosphere populations between 104 and 106 CFU/g on cherry, ryegrass, and 11 other herbaceous plants. Populations of K84 declined substantially in fallow soil or water over a 16-week period. K84 was detected in the rhizosphere of ryegrass located up to 40 cm from an inoculum source, indicating lateral dissemination of K84 in soil. In gall tissue on cherry, K84 established populations of 105 CFU/g, about 10to 100-fold less than that of the pathogen. These data demonstrate that K84 persists for up to 2 years in a field environment as a rhizosphere inhabitant or in association with crown gall tissue. Agrobacterium radiobacter (Beijerinck and van Delden) Conn K84 is used commercially to control crown gall, a tumorigenic plant disease caused by a ubiquitous soil-borne pathogen, Agrobacterium tumefaciens (41, 42). Losses due to crown gall can be extensive, particularly in the nursery industry, where each galled plant must be culled. Wounds caused by the routine practice of root pruning may be colonized and infected byA. tumefaciens when seedlings are planted in nursery soils infested with the pathogen. During the infection process, A. tumefaciens transfers the T-DNA region of the tumor-inducing plasmid (pTi) into the plant cell (58). Incorporation of T-DNA into the plant genome results in the formation of a hyperplastic growth called a gall. The gall provides a nutrient-rich environment for further bacterial growth. Galls can weaken, reduce the aesthetic quality, and eventually kill the host plant. There are no effective chemical controls currently available for crown gall. Strain K84 has demonstrated remarkable and widespread success as an agent for the biological control of crown gall (40). K84 has been used commercially for more than a decade in many regions of the world, including Australia,

Section: Discussionsupporting

confidence: 86%

Fate of Agrobacterium radiobacter K84 in the environment

Stockwell

Moore

Loper

1993

“…Wild-type A. tumefaciens C58 colonized tomato roots in numbers comparable to the numbers observed for colonization of tomato roots by the biocontrol strains Agrobacterium rhizogenes K84 and J73 (15), for colonization of pea roots by A. tumefaciens A723 (8), and for colonization of many root systems by pseudomonads (3,4,11,29). All regions of the root except the root tip were found to be colonized.…”

Section: Discussionsupporting

confidence: 63%

Root Colonization by Agrobacterium tumefaciens Is Reduced in cel , attB , attD , and attR Mutants

Matthysse

McMahan

1998

Root colonization by Agrobacterium tumefaciens was measured by using tomato and Arabidopsis thaliana roots dipped in a bacterial suspension and planted in soil. Wild-type bacteria showed extensive growth on tomato roots; the number of bacteria increased from 103 bacteria/cm of root length at the time of inoculation to more than 107 bacteria/cm after 10 days. The numbers of cellulose-minus and nonattachingattB, attD, and attR mutant bacteria were less than 1/10,000th the number of wild-type bacteria recovered from tomato roots. On roots of A. thalianaecotype Landsberg erecta, the numbers of wild-type bacteria increased from about 30 to 8,000 bacteria/cm of root length after 8 days. The numbers of cellulose-minus and nonattaching mutant bacteria were 1/100th to 1/10th the number of wild-type bacteria recovered after 8 days. The attachment of A. tumefaciens to cut A. thaliana roots incubated in 0.4% sucrose and observed with a light microscope was also reduced with cel andatt mutants. These results suggest that cellulose synthesis and attachment genes play a role in the ability of the bacteria to colonize roots, as well as in bacterial pathogenesis.

“…K84 has proven to be a good colonizer of the root system in different hosts. Previous results obtained by different authors in colonization experiments showed that the K84 population was stable on tomato and almond roots at 106 CFU/g and 108 CFU/cm2 of root, respectively (25,38). Montclar and Nemaguard seedlings instead of rooted plants were used for our colonization experiment because it was possible to germinate them under sterile conditions and to have clean roots before treatment with K84 or K1026.…”

Section: Discussionmentioning

confidence: 99%

Biological Control of Agrobacterium tumefaciens , Colonization, and pAgK84 Transfer with Agrobacterium radiobacter K84 and the Tra ^- Mutant Strain K1026

Vicedo

Peñalver

Asíns

et al. 1993

The efficacies of Agrobacterium radiobacter K84 and K1026 in root colonization, crown gall control, and plasmid transfer were compared. Levels of root colonization by K84 and K1026 of Montclar and Nemaguard peach seedlings were similar during the 21 days of the experiment. Four strains of A. tumefaciens bv. 1 were used for soil inoculations in biological control experiments on GF677 and Adafuel peach × almond rootstocks; two were sensitive and two were resistant to agrocin 84. Both strains K84 and K1026 were very efficient in controlling the sensitive strains, but some tumors appeared with both treatments. In the biocontrol of resistant strains, no galls were observed in K1026-treated plants, but some K84-treated plants had galls. Recovery of agrobacteria from galls in experiments with sensitive and resistant strains showed that all of the isolates from the controls or K1026-treated plants and most of the isolates from K84-treated plants had the same characteristics as the inoculated strains. Nine isolates from the K84-treated plants growing in soil inoculated with one resistant strain were virulent and produced agrocin 84. These isolates had a plasmid that hybridized with a probe prepared with the Bam HI C fragment from pAgK84. These results show the efficiency of K1026 in biocontrol of agrocin 84-sensitive and -resistant strains of A. tumefaciens and suggest the use of K1026 as a safer organism than K84 for biological control of crown gall.