SummaryAn NAC-type transcription factor gene AtNAC2 was identified from Arabidopsis thaliana when expression patterns of the genes from a microarray analysis were examined. The AtNAC2 expression was induced by salt stress and this induction was reduced in magnitude in the transgenic Arabidopsis plants overexpressing tobacco ethylene receptor gene NTHK1. AtNAC2 is localized in the nucleus and has transcriptional activation activity. It can form a homodimer in yeast. AtNAC2 was highly expressed in roots and flowers, but less expressed in other organs examined. In addition to the salt induction, the AtNAC2 can also be induced by abscisic acid (ABA), ACC and NAA. The salt induction was enhanced in the ethylene overproducer mutant eto1-1, but suppressed in the ethylene-insensitive mutants etr1-1 and ein2-1, and in the auxin-insensitive mutant tir1-1 when compared with that in wild-type plants. However, the salt induction of AtNAC2 was not significantly affected in the ABA-insensitive mutants abi2-1, abi3-1 and abi4-1. These results indicate that the salt response of AtNAC2 requires ethylene signaling and auxin signaling pathways but does not require ABI2, ABI3 and ABI4, intermediates of the ABA signaling pathway. Overexpression of AtNAC2 in transgenic Arabidopsis plants resulted in promotion of lateral root development. AtNAC2 also promoted or inhibited downstream gene expressions. These results indicate that AtNAC2 may be a transcription factor incorporating the environmental and endogenous stimuli into the process of plant lateral root development.

SummaryA histidine kinase-based signaling system has been proposed to function in ethylene signal transduction pathway of plants and one ethylene receptor has been found to possess His kinase activity. Here we demonstrate that a His kinase-like ethylene receptor homologue NTHK1 from tobacco has serine/threonine (Ser/Thr) kinase activity, but no His kinase activity. Evidence obtained by analyzing acid/base stability, phosphoamino acid and substrate specificity of the phosphorylated kinase domain, supports this conclusion. In addition, mutation of the presumptive phosphorylation site His (H378) to Gln did not affect the kinase activity whereas deletion of the ATP-binding domain eliminated it, indicating that the conserved His (H378) is not required for the kinase activity and this activity is intrinsic to the NTHK1-KD. Moreover, confocal analysis of NTHK1 expression in insect cells and plant cells suggested the plasma membrane localization of the NTHK1 protein. Thus, NTHK1 may represent a distinct Ser/Thr kinase-type ethylene receptor and function in an alternative mechanism for ethylene signal transduction.

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We report 37.4 fs pulse generation from a ring-cavity Er:fiber laser at a repetition rate of 225 MHz. The spectral bandwidth of the pulse is 135 nm. The single-pulse energy is 0.31 nJ.

Ethylene plays important roles in plant growth, development, and stress responses. Two ethylene receptors, ETR1 from Arabidopsis and NTHK1 from tobacco (Nicotiana tabacum), have been found to have His kinase (HK) activity and Ser/Thr kinase activity, respectively, although both show similarity to bacterial two-component HK. Here, we report the characterization of another ethylene receptor homolog gene, NTHK2, from tobacco. This gene also encodes a HK-like protein and is induced by dehydration and CaCl 2 but not significantly affected by NaCl and abscisic acid treatments. The biochemical properties of the yeast (Schizosaccharomyces pombe)-expressed NTHK2 domains were further characterized. We found that NTHK2 possessed Ser/Thr kinase activity in the presence of Mn 21 and had HK activity in the presence of Ca 21. Several lines of evidence supported this conclusion, including hydrolytic stability, phosphoamino acid analysis, mutation, deletion, and substrate analysis. These properties have implications in elucidation of the complexity of the ethylene signal transduction pathway and understanding of ethylene functions in plants.Ethylene plays important roles in multiple aspects of plant growth, development, and stress responses (Abeles et al., 1992). In the presence of ethylene, plant seedlings exhibit triple responses in the dark, including inhibited root and hypocotyl elongation, radial expansion of the hypocotyl, and exaggerated growth of the apical hook. Taking advantage of the triple responses, a number of Arabidopsis mutants were genetically screened, and a series of components have been identified to be involved in the ethylene signal transduction pathway (Bleecker and Kende, 2000;Schaller and Kieber, 2002;Wang et al., 2002;Guo and Ecker, 2004). These components include ethylene receptors (Chang et al., 1993;Hua et al., 1995Sakai et al., 1998), the Raf-like Ser/Thr kinase (STK) CTR1 (Kieber et al., 1993), the membrane protein EIN2 (Alonso et al., 1999), and transcription factors EIN3 and ERF family (Chao et al., 1997;Solano et al., 1998). Additional components in the ethylene-response pathway have been found (Alonso et al., 2003), and an alternative pathway has also been suggested (Moshkov et al., 2003).In Arabidopsis, five ethylene receptor genes, namely ETR1, ERS1, ETR2, EIN4, and ERS2, have been isolated, and all of the encoded proteins show similarity to bacterial two-component His kinases (HK;Chang et al., 1993;Hua et al., 1995Sakai et al., 1998). The two-component signal transduction systems allow bacteria to adapt to changing environmental conditions. Based on the amino acid sequence similarity and structural features, the five Arabidopsis ethylene receptors can be classified into two subfamilies (Bleecker and Kende, 2000;Schaller and Kieber, 2002;Wang et al., 2002). Subfamily I contains ETR1 and ERS1, both of which have three transmembrane domains. Subfamily II contains ETR2, EIN4, and ERS2, and all the three proteins have a putative signal peptide in addition to the three conserved transmembrane doma...

A putative ethylene receptor gene NTHK1 encodes a protein with a putative signal peptide, three transmembrane segments, a putative histidine kinase domain and a putative receiver domain. The receiver domain was expressed in an Escherichia coli expression system, purified and used to generate polyclonal antibodies for immunohistochemistry analysis. The spatial expression of the NTHK1 protein was then investigated. We found that NTHK1 was abundant during flower and ovule development. It was also expressed in glandular hairs, stem, and in leaves that had been wounded. The NTHK1 gene was further introduced into the tobacco plant and we found that, in different transgenic lines, the NTHK1 gene was transcribed to various degrees. Upon ACC treatment, the etiolated transgenic seedlings showed reduced ethylene sensitivity when compared with the control, indicating that NTHK1 is a functional ethylene receptor in plants.

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