A salt-responsive receptor-like kinase gene regulated by the ethylene signaling pathway encodes a plasma membrane serine/threonine kinase.

NTHK1 is a salt-inducible ethylene receptor gene in tobacco. Transgenic tobacco plants for this gene show reduced ethylene sensitivity. Using cDNA microarray analysis, we were able to identify those genes that have different expression levels between NTHK1 transgenic plants and wild-type plants under salt stress conditions. One of these, AtLecRK2, which encodes a receptor-like kinase with an extracellular lectin-like domain, was characterized in detail in the present study. AtLecRK2 contains a signal peptide, an extracellular lectin-like domain, a single transmembrane domain and a cytoplasmic protein kinase domain. AtLecRK2 is transcribed in the root, flower and leaf but not in the stem. In wild-type Arabidopsis, salt stress induced the transcription level of AtLecRK2, whereas in the transgenic NTHK1 Arabidopsis induction of the AtLecRK2 transcript was inhibited and retarded. AtLecRK2 was constitutively overexpressed in the ethylene-overproducer mutant, eto1-1, and could be induced by ethylene. However, in the ethylene-insensitive mutant, ein2-1, the salt-induced expression pattern of AtLecRK2 was the same as that in wild-type plants. The results demonstrate that the induction of AtLecRK2 in response to salt stress is regulated by the ethylene signaling pathway. The induction was inhibited by the ethylene receptor, NTHK1, while it was independent of EIN2. The kinase activity of AtLecRK2 was also studied. We found that that AtLecRK2 can be autophosphorylated and has serine/threonine kinase activities. The subcellular localization of AtLecRK2-GFP in onion epidermal cells indicates that AtLecRK2 is localized on the plasma membrane.

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