Expression of tobacco ethylene receptor NTHK1 alters plant responses to salt stress.

Ethylene has been regarded as a stress hormone involved in many stress responses. However, ethylene receptors have not been studied for the roles they played under salt stress condition. Previously, we characterized an ethylene receptor gene NTHK1 from tobacco, and found that NTHK1 is salt-inducible. Here, we report a further investigation towards the function of NTHK1 in response to salt stress by using a transgenic approach. We found that NTHK1 promotes leaf growth in the transgenic tobacco seedlings but affects salt sensitivity in these transgenic seedlings under salt stress condition. Differential Na+/K+ ratio was observed in the control Xanthi and NTHK1-transgenic plants after salt stress treatment. We further found that the NTHK1 transgene is also salt-inducible in the transgenic plants, and the higher NTHK1 expression results in early inductions of the ACC (1-aminocyclopropane-1-carboxylic acid) oxidase gene NtACO3 and ethylene responsive factor (ERF) genes NtERF1 and NtERF4 under salt stress. However, NTHK1 suppresses the salt-inducible expression of the ACC synthase gene NtACS1. These results indicate that NTHK1 regulates salt stress responses by affecting ion accumulation and related gene expressions, and hence have significance in elucidation of ethylene receptor functions during stress signal transduction.

OsGLU1, a putative membrane-bound endo-1,4-beta-D-glucanase from rice, affects plant internode elongation.

A dwarf mutant glu was identified from screening of T-DNA tagged rice population. Genetic analysis of the T1 generation of glu revealed that a segregation ratio of wild-type:dwarf phenotype was 3:1, suggesting that the mutated phenotype was controlled by a single recessive nuclear locus. The mutated gene OsGLU1, identified by Tail-PCR, encodes a putative membrane-bound endo-1,4-beta-D-glucanase, which is highly conserved between mono- and dicotyledonous plants. Mutation of OsGLU1 resulted in a reduction in cell elongation, and a decrease in cellulose content but an increase in pectin content, suggesting that OsGLU1 affects the internode elongation and cell wall components of rice plants. Transgenic glu mutants harboring the OsGLU1 gene complemented the mutation and displayed the wild-type phenotype. In addition, OsGLU1 RNAi plants showed similar phenotype as the glu mutant has. These results indicate that OsGLU1 plays important roles in plant cell growth. Gibberellins and brassinosteroids induced OsGLU1 expression. In rice genome, endo-1,4-beta-D-glucanases form a multiple gene family with 15 members, and each may have a distinct expression pattern in different organs. These results indicate that endo-1,4-beta-D-glucanases may play diverse roles in growth and developmental process of rice plants.

Characterization of a novel cell cycle-related gene from Arabidopsis.

Cell division is a fundamental biological process sharing conserved features and controls in all eukaryotes. The cell cycle is usually divided into four phases: G1, S, G2, and M. Regulated gene expression is an important mechanism for controlling cell cycle progression and genes involved in cell division-related processes often show transcriptional regulation dependent on cell cycle position. In the present report, a novel cell cycle-related gene (AtCPR) from Arabidopsis thaliana was isolated and characterized. Sequence analysis revealed that the deduced amino acid sequence of AtCPR showed 53.2% identity with p38-2G4, a mouse G1-to-S cell cycle specifically modulated and proliferation-associated nuclear protein. Assay of expression of AtCPR in partially synchronized cells suggested that AtCPR mRNA was expressed in the G1-to-S phase. In the AtCPR transgenic plants, no apparent phenotypic change was observed. By fusing a GFP tag to the AtCPR protein, it was found that AtCPR was mainly located in the nucleus. However, AtCPR does not have any transcriptional activation ability. cDNA microarray analysis showed that a total of 17 and 30 genes were identified as up-regulated and down-regulated, respectively.

Characterization of a DRE-binding transcription factor from a halophyte Atriplex hortensis.

Environmental stresses, such as salinity, drought and cold, can induce the expression of a large amount of genes. Among these are many transcription factors that regulate the expression of downstream genes by specifically binding to cis-elements or forming transcriptional complexes with other proteins. In the present study, a DREB-like transcription factor gene, named AhDREB1, was isolated from a halophyte Atriplex hortensis. AhDREB1 encoded a protein containing a conserved EREBP/AP2 domain featuring the DREB family. In yeast one-hybrid analysis AhDREB1 protein was specifically bound to DRE elements and activated the expression of the reporter genes of HIS3 and LacZ. The AhDREB1 gene was expressed in roots, stems and leaves of A. hortensis. Salinity induced its expression in roots, but not in other organs. Overexpression of AhDREB1 in transgenic tobacco led to the accumulation of its putative downstream genes. The performance of the transgenic lines was also tested under stressed conditions and two lines were found to be stress-tolerant. These results suggest that the AhDREB1 protein functions as a DRE-binding transcription factor and play roles in the stress-tolerant response of A. hortensis.

AhCMO, regulated by stresses in Atriplex hortensis, can improve drought tolerance in transgenic tobacco.

Choline monooxygenase (CMO) catalyzes the committed step of glycine betaine (GlyBet) biosynthesis in many flowering plants. To investigate its effect on various stress tolerances in plant metabolic engineering, we isolated and characterized the CMO gene from Atriplex hortensis, a GlyBet natural accumulator, and introduced it into tobacco to examine the effect of GlyBet on plant drought and salt tolerance, respectively. In A. hortensis, the expression of AhCMO was induced 3-fold in the root and stem, as well as in the leaf, when plants were treated with 400 mM of NaCl, indicating that the acceleration of GlyBet biosynthesis under salt stress was achieved through the whole plant, including organs without chloroplasts. AhCMO transcription was also regulated by drought, ABA and circadian rhythm. Over-expression of AhCMO improved drought tolerance in transgenic tobacco when cultured in medium containing PEG-6000. The transgenic plants also have a better performance under salt stress.