Local signalling pathways regulate the Arabidopsis root developmental response to Mesorhizobium loti inoculation.

Numerous reports have shown that various rhizobia can interact with non-host plant species, improving mineral nutrition and promoting plant growth. To further investigate the effects of such non-host interactions on root development and functions, we inoculated Arabidopsis thaliana with the model nitrogen fixing rhizobacterium Mesorhizobium loti (strain MAFF303099). In vitro, we show that root colonization by M. loti remains epiphytic and that M. loti cells preferentially grow at sites where primary and secondary roots intersect. Besides resulting in an increase in shoot biomass production, colonization leads to transient inhibition of primary root growth, strong promotion of root hair elongation and increased apoplasmic acidification in periphery cells of a sizeable part of the root system. Using auxin mutants, axr1-3 and aux1-100, we show that a plant auxin pathway plays a major role in inhibiting root growth but not in promoting root hair elongation, indicating that root developmental responses involve several distinct pathways. Finally, using a split root device, we demonstrate that root colonization by M. loti, as well as by the bona fide plant growth promoting rhizobacteria Azospirillum brasilense and Pseudomonas, affect root development via local transduction pathways restricted to the colonised regions of the root system.

Quartz-enhanced photoacoustic spectroscopy sensor for ethylene detection with a 3.32 µm distributed feedback laser diode.

An antimonide distributed feedback quantum wells diode laser operating at 3.32 µm at near room temperature in the continuous wave regime has been used to perform ethylene detection based on quartz enhanced photoacoustic spectroscopy. An absorption line centered at 3007.52 cm(-1) was investigated and a normalized noise equivalent absorption coefficient (1σ) of 3.09 10(-7) cm(-1) W Hz(-1/2) was obtained. The linearity and the stability of the detection have been evaluated. Biological samples' respiration has been measured to validate the feasibility of the detection setup in an agronomic environment, especially on ripening apples.

A strong constitutive positive element is essential for the ammonium-regulated expression of a soybean gene encoding cytosolic glutamine synthetase.

In order to identify important promoter elements controlling the ammonium-regulated expression of the soybean gene GS15 encoding cytosolic glutamine synthetase, a series of 5' promoter deletions were fused to the GUS reporter gene. To allow the detection of positive and negative regulatory elements, a series of 3' deletions were fused to a -90 CaMV 35S promoter fragment placed upstream of the GUS gene. Both types of construct were introduced into Lotus corniculatus plants and soybean roots via Agrobacterium rhizogenes-mediated transformation. Both spectrophotometric enzymatic analysis and histochemical localization of GUS activity in roots, root nodules and shoots of transgenic plants revealed that a strong constitutive positive element (SCPE) of 400 bp, located in the promoter distal region is indispensable for the ammonium-regulated expression of GS15. Interestingly, this SCPE was able to direct constitutive expression in both a legume and non-legume background to a level similar to that driven by the CaMV 35S full-length promoter. In addition, results showed that separate proximal elements, located in the first 727 bp relative to the transcription start site, are essential for root- and root nodule-specific expression. This proximal region contains an AAAGAT and two TATTTAT consensus sequences characteristic of nodulin or nodule-enhanced gene promoters. A putative silencer region containing the same TATTTAT consensus sequence was identified between the SCPE and the organ-specific elements. The presence of positive, negative and organ-specific elements together with the three TATTTAT consensus sequences within the promoter strongly suggest that these multiple promoter fragments act in a cooperative manner, depending on the spatial conformation of the DNA for trans-acting factor accessibility.

Dephosphorylation activates the purified plant plasma membrane H+-ATPase--possible function of phosphothreonine residues in a mechanism not involving the regulatory C-terminal domain of the enzyme.

The plasma membrane H+-ATPase was purified from tobacco cells (line BY-2). After solubilization by lysophosphatidylcholine followed by separation on a glycerol gradient, a fraction with a high specific activity of 9 micromol ATP x min(-1) x mg protein(-1) was obtained, in which the H+-ATPase polypeptide represented at least 80% of the protein. The incubation of this fraction in the presence of alkaline phosphatase increased H+-ATPase activity by 40%, in a manner consistent with dephosphorylation of the enzyme itself. The hydrolytic activity of the solubilized enzyme and its proton translocating activity, after reconstitution into proteoliposomes, were stimulated to the same extent. Alkaline phosphatase treatment was also accompanied by a 92% decrease in the H+-ATPase phosphothreonine content, whereas the phosphoserine residues were almost unaffected. The dephosphorylation induced a slight decrease of the affinity of the enzyme towards ATP. The purified enzyme was not activated by lysophosphatidylcholine addition nor by trypsin-mediated proteolysis, two treatments reported to release the inhibitory control by the C-terminal domain of the H+-ATPase and to increase the affinity of the enzyme towards ATP. Based on these results, the regulatory phosphorylation evoked by alkaline phosphatase most likely differs from the autoinhibitory control of the H+-ATPase by its C-terminal domain.