Virus infection and other factors affecting ribonuclease activity of plant leaves.

Abstract

Abstract Rubbing of bean ( Phaseolus vulgaris L.) leaves, partial dehydration of cowpea [ Vigna sinensis (L.) Endl.] leaves, and inoculation of bean leaves with inactivated or viable pod mottle virus or of Datura stramonium L. leaves with tobacco mosaic virus resulted in increased ribonuclease activity of such leaves. The response after rubbing was more pronounced when the leaves were dusted with Carborundum prior to rubbing and when the medium contained Cu ++ or Zn ++ ions. The reaction was first noticeable approximately 2 hours after rubbing, reached its highest level 1 day after rubbing, and lasted for several days. Spraying of unwounded leaves with solutions of CuSO 4 also led to increased ribonuclease activity. Virus-infected leaves maintained higher levels of ribonuclease activity than healthy control leaves or leaves inoculated with inactivated virus. The effect of rubbing was not restricted to the upper epidermis, but extended into the mesophyll. Neither phosphatase nor phosphodiesterase activity significantly increased in bean or Datura leaves after rubbing of the leaves or after inoculation with virus. It is concluded that increased ribonuclease activity is not a specific response to virus infection, but occurs as a general shock reaction. In bean leaves with high ribonuclease activity, more lesions were produced and more pod mottle virus was synthesized than in leaves with lower ribonuclease activity. However, stimulation of ribonuclease activity by rubbing of bean leaves did not lead to increased pod mottle virus lesion formation or to stepped-up virus synthesis. The significance of these results is discussed with respect to a possible role of ribonuclease in the synthesis of viral nucleic acid.