A cDNA clone of a soybean gene encoding a proline-rich protein (PRP) was characterized and designated SbPRP ( Soy bean Proline- rich Protein). The SbPRP protein is a putative bimodular protein of 126 amino acids with a proline-rich domain and a hydrophobic cysteine-rich domain plus a signal peptide at the N terminal. Southern analysis indicates the presence of a single copy of the SbPRP gene in the soybean genome. The SbPRP gene expression was investigated and the results demonstrate that it accumulates in leaves and epicotyls of soybean seedlings, but not in cotyledons, hypocotyls and roots. The SbPRP mRNA was also expressed in response to salicylic acid and virus infection. In addition, the SbPRP gene transcription was regulated by circadian rhythm, salt stress, drought stress and plant hormones. These results indicate that the SbPRP gene might play a role in plant responses to multiple internal and external factors.

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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.