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Effects of different combinations of short days and dark treatment and different light intensities during the short days on in vitro tuberization of three potato cultivars were examined. Tuberization in Desiree, Cleopatra and Gracia was induced by short-day-treatment after 4 weeks culture under long days. To pre- serve natural endogenous hormonal balance of in vitro plantlets growth regulators were not added to the medium. The shorter the duration of short-days period the higher light intensity necessary for earlier tuber initiation. There was also a synchronizing effect of the high light intensity on tuber initiation but it depended on photoperiod-treatments and genotypes. The higher the light intensity during induction peri- od the less favourable the effect of light applied after the induction period.

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Endogenous carbohydrate (fructose, glucose and sucrose) fractions were measured in calli of potato genotypes with different field tolerance to drought. Under in vitro stress conditions induced by 0.8 M mannitol, sucrose level of calli increased extremely in medium-tolerant (by 424.5%) and sensitive (by 302.7%) genotypes whilst the rate of increase was 12–18-times lower in the drought tolerant variety. Results suggest the applicability of sucrose as biochemical marker for distinguish drought tolerant genotypes from great population in callus culture.

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The effect of mannitol on different genotypes of potato was studied in callus and plantlet culture. In vitro responses of five potato genotypes with well-known field behaviour to water deficit were analysed. After a 4-week-long cultivation on media containing mannitol up to 0.8 M, different morpho-physiological parameters were determined and statistically analysed. The useful concentration of mannitol for in vitro screening the osmotic tolerance of different genotypes depended on the type of culture; it was 0.4 M in plantlet-test and 0.8 M in callus-test. In callus-test the relative increase of callus mass was a useful parameter for determination of osmotic tolerance of genotypes at cellular level. In plantlet culture, stress index calculated from the rate of surviving in vitro shoots, number and length of roots per surviving explant and the rate of rooted explants were applicable to determine three groups according to the tolerant, medium tolerant and sensitive categories in agreement with the field behaviour of these genotypes. Under in vitro stress conditions we were able to distinguish the examined genotypes with different drought tolerance.

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Acta Biologica Hungarica
Authors: Zsuzsanna Kiss, Judit Dobránszki, Ildikó Hudák, Zsuzsanna Birkó, G. Vargha, and S. Biró

The Gram-positive soil-borne streptomycetes exhibit a complex life cycle that is controlled by extracellular regulatory molecules. One interesting autoregulator is the protein factor C, originally isolated from the culture fluid of S. albidoflavus 45H. Southern hybridizations and database searches revealed that although homologues of factor C are not present in most Streptomyces strains, an exception is the plant pathogenic S. scabies , which causes common scab disease on potato. In S. scabies and related strains pathogenicity involves a large pathogenicity island that carries thaxtomin biosynthetic genes, the nec1 necrogenic factor and other putative virulence genes. Extracellular enzymes, including extracellular esterases, that attack the surface of the tubers and disintegrate the external protective layer are also known to be involved in pathogenicity. In S. albidoflavus 45H, factor C coordinates the expression of many secreted hydrolases. To find out whether esterase is also regulated by factor C, we made a factor C null mutant of strain 45H. The mutant showed a bald phenotype and was impaired in pathogenicity and esterase activity. This is a first indication that extracellular regulatory factors may play a role in the development of potato scab.

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