1lnstitute of Horticulture, Kishinau, Moldova; 2Ail Russian Institute of Selection and Seeds, Moscow, Russia; 3Institute of Genetics AS, Kishinau, Moldova

Background and objectives
Virus-induced changes in plant metabolism are similar to ageing processes in many respects. In particular, the increase of oxidative enzyme activity removes the competitive ratio of free peroxidation and fermentative oxidation in the direction of free peroxidation. The large number of free peroxide radicals disturbs membrane integrity and function. Bio-antioxidants move the above ratio in the direction of fermentative oxidation, destroying the negative effect of free peroxidation. Plant secondary metabolites named steroidal glycosides (SG) have antioxidant properties. The object was to examine the potential of SG as inducers and factors of plant resistance to viruses.

Results and conclusions
Our investigation has been carried out in two model pathosystems: tobacco mosaic virus (TMV)-tomato; and plum pox virus (PPV)-plum. It consisted of four main steps. (i) Testing the antiviral effects of a number of steroidal glycosides against TMV, Prunus necrotic ring spot virus and PPV in vitro and in vivo. (ii) SG-action was studied in TMV-tomato pathosystem. Stimulation of plant-host defence reactions via treatment in the above pathosystem with steroidal glycosides has been shown in experiments. The appearance of three new proteins (Rf1=0.24; Rf2=0.40., Rf3=0.44 ) in tomato leaves and the increasing of ribonuclease activity by 2.5 times in response to SG-treatment of plants can be considered as plant defence reactions, because a considerable decrease of TMV-infectivity takes place at the same time. The SG-treatment of tomato seeds decreased the degree of plants affection with Macrosporium solani, Xanthomonas vesicatoria, TMV and wilt diseases in the field experiment. In conclusion, exogenic steroidal glycosides may induce the specific plant resistance to pathogens (including viruses). But these compounds are natural plant secondary metabolites. What is their function in the forming of natural plant resistance? This has been studied in PPV-plum pathosystem.

(iii) Different plum organs (buds, leaves, fruits, stones and bark) contain of steroidal glycosides. Changes in SG-content resulted in the infection with PPV: the SG-content index increased in leaves, but decreased in fruits. This effect has been observed during 3 years on varieties with different types of susceptibility to PPV. SG-participation in the PPVpathogenesis has been suggested. (iv) The connection between the SG-content index of plum buds and the degree of fruits affection with PPV-has been studied on eight plum varieties with different types of susceptibility to PPV by correlation analysis. A correlation between the two above characteristics has been established with a coefficient of 0.75: the higher the SG-content index of plum buds, the lower the degree of fruit infection with PPV, significant at the 5% level. This result has been confirmed by field experiments.

In conclusion, endogenic steroidal glycosides are factors of plants virus resistance.