The induction of abscisic-acid-mediated drought tolerance is independent of ethylene signaling in Arabidopsis plants responding to a harpin protein

Harpin proteins from plant pathogenic bacteria can activate distinct signaling pathways and cause multiple effects in plants. When Arabidopsis thaliana (Arabidopsis) plants are treated with the HrpN(Ea) harpin produced by Erwinia amylovora, a bacterial pathogen that causes fire blight in rosaceous plants, abscisic acid (ABA) is stimulated to mediate drought tolerance, and ethylene signaling is activated to regulate plant growth enhancement and insect resistance. It is unclear if ABA and ethylene signaling interacts in response to harpin proteins. Here we report that ethylene signaling is dispensable to the induction of ABA-mediated drought tolerance in Arabidopsis. In wild-type (WT) plants growing under drought stress conditions, ABA, but not ethylene, was required for HrpN(Ea) to promote cellular adaptive responses and decrease drought severity of plants. During the induction of drought tolerance in HrpN(Ea)-treated WT plants, expression of the ABA signaling gene ABI2 was induced coincidently with decreases in transcripts of ETR1, which encodes an ethylene receptor, and several other genes that are also involved in ABA and ethylene signal transduction pathways. In response to HrpN(Ea), the Arabidopsis etr1-1 mutant developed drought tolerance similarly as did WT, but the abi2-1 mutant did not, suggesting that sensing of ABA is essential, but sensing of ethylene is not. Consistently, the induction of drought tolerance was abolished by inhibiting WT to synthesize ABA, instead of ethylene. Our results suggest that HrpN(Ea) treatment enables plants to prioritize the ABA signal transduction pathway over ethylene signaling in accordance with the real-time requirement to survive under drought stress conditions.