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The signals that control the transcription of osmoregulated genes are not understood satisfactorily. The "turgor control model" suggested that the primary osmoregulatory signal in Enterobacteriaceae is turgor loss, which induces the kdp K ؉ transport operon and activates the Trk K ؉ permease. The ensuing increase in cytoplasmic K ؉ concentration was proposed to be the signal that turns on all secondary responses, including the induction of the proU (proline-glycine betaine transport) operon. The "ionic strength model" proposed that the regulatory signal for all osmotically controlled responses is the increase in the cytoplasmic ionic strength or macromolecular crowding after an osmotic upshift. The assumption in the turgor control model that the induction of kdp is a primary response to osmotic shock predicts that this response should precede all secondary responses. Both models predict that the induction of all osmotically activated responses should be independent of the chemical nature of the solute used to impose osmotic stress. We tested these predictions by quantitative real-time reverse transcription-PCR analysis of the expression of six osmotically regulated genes in Salmonella enterica serovar Typhimurium. After shock with 0.3 M NaCl, proU was induced at 4 min, proP and rpoS were induced at 4 to 6 min, kdp was induced at 8 to 9 min, and otsB and ompC were induced at 10 to 12 min. After an equivalent osmotic shock with 0.6 M sucrose, proU was induced with kinetics similar to those seen with NaCl, but induction of kdp was reduced 150-fold in comparison to induction by NaCl. Our results are inconsistent with both the turgor control and the ionic strength control models.Organisms respond to changes in external osmolality by accumulating or releasing low-molecular-weight "compatible solutes" that maintain the proper balance between internal and external osmolality. One of the prominent responses of Enterobacteriaceae to high ion concentrations is transcriptional induction a Ͼ100-fold of two operons: proU (proVWX), which encodes a transport system for the osmoprotectant compounds glycine betaine and proline, and kdpABC, which specifies the components of a high-affinity K ϩ transport system. The signal sensing mechanisms of the osmotic control of transcription of the kdp and proU operons are uncertain. Two osmoregulatory models were proposed for Enterobacteriaceae. The "turgor control model," which has been very influential in shaping thinking about bacterial osmoregulation, suggests that the fundamental signal is the loss of turgor resulting from the efflux of water after an osmotic upshift (3, 13). This model postulates that the first response of the cells to turgor loss is increased uptake of K ϩ , accomplished by stimulation of the activities of the constitutive, low-affinity K ϩ transport systems Trk and Kup and by transcriptional induction of the kdpABC operon. The model further proposes that elevated K ϩ acts as a second messenger to turn on all other osmotically activated responses, including the induct...

Reliable detection and quantification of barley and cereal yellow dwarf viruses (YDVs) is a critical component in managing yellow dwarf diseases in small grain cereal crops. The method currently used is enzyme-linked immunosorbent assay (ELISA), using antisera against the coat proteins that are specific for each of the various YDVs. Recently, quantitative real-time reverse-transcription polymerase chain reaction (Q-RT-PCR) has been used to detect bacterial and viral pathogens and to study gene expression. We applied this technique to detect and quantify YDVs using primers specific for Barley yellow dwarf virus-PAV (BYDV-PAV) and Cereal yellow dwarf virus-RPV (CYDV-RPV) coat protein genes because of the higher sensitivity of RT-PCR and the advantage of using a real-time PCR instrument. This Q-RT-PCR was used to detect BYDV and CYDV, and to examine disease development in a resistant wheatgrass, a resistant wheat line, a susceptible wheat line, and a susceptible oat line. BYDV-PAV and CYDV-RPV were detected as early as 2 and 6 h, respectively, in susceptible oat compared with detection by ELISA at 4 and 10 days postinoculation. BYDV-PAV RNA accumulated more rapidly and to a higher level than CYDV-RPV RNA in both oat and wheat, which may account for PAV being more prevalent and causing more severe viral disease than CYDV. Q-RT-PCR is reproducible, sensitive, and has the potential to be used for examining yellow dwarf disease and as a rapid diagnostic tool for YDVs.

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