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DOES THE BACTERIAL BIOCONTROL AGENT MA 342 ENTER A NONCULTURABLE STATE ON DRY-STORED BARLEY SEEDS?< /b>
DIRK JAN VAN DER GAAG, OLA ARWIDSSON, MARGARETA HOKEBERG and BERNDT GERHARDSON

Plant Pathology and Biocontrol Unit, Swedish University of Agricultural Sciences, P. 0. Box 7035, SE-750 07 Uppsala, Sweden


Background and objectives
The Pseudomonu strain MA 342 is an effective biocontrol agent against several seed-bome diseases in cereals [1]. The strain is used as a seed-inoculant and is as effective as conventional fungicides. MA342 has shown consistent performance in field trials in Sweden since 1991 and in several other countries since 1994. In 1997 the bacterium was commercialized in Sweden under the trade name Cedomon. Seeds treated with the bacterium can he stored for at least two years without loosing the biocontrol effect. It is hypothesized that the bacteria cater a dormant state on dry-stored seeds in which they can survive under dry conditions. MA 342 was tagged with the green fluorescent protein and a gene for kanamycin resistance allowing the enumeration of total number of cells (= green cells under blue light), and the number of culwmble cells [2]. Green cells are assumed to be living cells. In the present study the numbers of culturable MA 342 ccus on barley seeds are compared with the total numbers of cells after bacterisation. lle population dynamics of MA 342 is investigated after sowing bacterised seeds to assess the number of active cells needed to suppress disease.


Results and conclusions
Similar numbers of green cells, about 10 7 per seed, were present on barley seeds stored for 20 h or 3 months after bacterisation at room temperature. From the 3-months-stored seeds less than 100 cells per seed were culturable on TSA whereas 10 6 cells were culturable from seeds which had been stored for only 20 h after bacterisation. These results suggest that the bacterial cells enter a nonculturable stage on dry-stored seeds. The number of culturable cells on the 3-months-stored seeds remained low during the first three days after sowing and increased significantly between day 3 and 5 at 6oC. The total number of (green) cells was more or less stable at about 10 7 per seed. The increase in culturable cells on the sown seeds may be both due to multiplication of the low number of culturable cells present at time of sowing and to resuscitation of cells from a viable but nonculturable state. Data will be presented on the population dynamics of MA 342 on barley seeds stored for different time periods after bacterisation with different amount of cells. The number of bacterial cells will be related to disease suppression in the barley - Drechslera teres pathosystem.


References
1. H,5keberg M, Gerhardson B, Johnson L, 1997. European Journal of Plant Pathology 103, 25-33. 2. Tombollini R, Unge A, Davey ME, De Bruijn FJ and Jansson JK, 1997. FEMS Microbiology Ecology 22: 17-28.