RNase Activity Prevents the Growth of a Fungal Pathogen in Tobacco Leaves and Increases upon Induction of Systemic Acquired Resistance with Elicitin

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Abstract
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 cryptogein 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 inoculation with the pathogenic fungus. Upon induction of SAR with cryptogein, the accumulation of NE transcripts coincided with a rapid induction of RNase activity and with the increase in the activity of at least two different extracellular RNases. Moreover, exogenous application of RNase activity in the extracellular space of leaves led to a reduction of the fungus development by up to 90%, independently of any cryptogein treatment and in the absence of apparent necrosis. These results indicate that the up-regulation of apoplastic RNase activity after inoculation could contribute to the control of fungal invasion in plants induced to SAR with cryptogein.