RNASE ACTIVITY PREVENTS THE GROWTH OF A FUNGAL PATHOGEN IN TOBACCO-LEAVES AND INCREASES UPON INDUCTION OF SYSTEMIC ACQUIRED-RESISTANCE WITHELICITIN

The hypersensitive response and systemic acquired resistance (SAR) can be induced in tobacco (Nicotiana tabacum L.) plants by cryptogein, an elicitin secreted by Phytophthora cryptogea. Stem application of cryp togein leads to the establishment of acquired resistance to subsequent leaf infection with Phytophthora parasitica var nicotianae, the agent of the tobacco black shank disease. We have studied early events that occur after the infection and show here that a tobacco gene encoding the extracellular S-like RNase NE is expressed in response to inoculat ion with the pathogenic fungus. Upon induction of SAR with cryptogein, the accumulation of NE transcripts coincided with a rapid induction o f RNase activity and with the increase in the activity of at least two different extracellular RNases. Moreover, exogenous application of RN ase activity in the extracellular space of leaves led to a reduction o f the fungus development by up to 90%, independently of any cryptogein treatment and in the absence of apparent necrosis. These results indi cate that the up-regulation of apoplastic RNase activity after inocula tion could contribute to the control of fungal invasion in plants indu ced to SAR with cryptogein.