The Arabidopsis cer26 mutant, like the cer2 mutant, is specifically affected in the very long chain fatty acid elongation process

Plant aerial organs are covered by cuticular waxes, which form a hydrophobic crystal layer that mainly serves as a waterproof barrier. Cuticular wax is a complex mixture of very long chain lipids deriving from fatty acids, predominantly of chain lengths from 26 to 34 carbons, which result from acyl-CoA elongase activity. The biochemical mechanism of elongation is well characterized; however, little is known about the specific proteins involved in the elongation of compounds with more than 26 carbons available as precursors of wax synthesis. In this context, we characterized the three Arabidopsis genes of the CER2-like family: CER2, CER26 and CER26-like . Expression pattern analysis showed that the three genes are differentially expressed in an organ- and tissue-specific manner. Using individual TDNA insertion mutants, together with a cer2 cer26 double mutant, we characterized the specific impact of the inactivation of the different genes on cuticular waxes. In particular, whereas the cer2 mutation impaired the production of wax components longer than 28 carbons, the cer26 mutant was found to be affected in the production of wax components longer than 30 carbons. The analysis of the acyl-CoA pool in the respective transgenic lines confirmed that inactivation of both genes specifically affects the fatty acid elongation process beyond 26 carbons. Furthermore, ectopic expression of CER26 in transgenic plants demonstrates that CER26 facilitates the elongation of the very long chain fatty acids of 30 carbons or more, with high tissular and substrate specificity

Combinatorial interaction network of transcriptomic and phenotypic responses to nitrogen and hormones in the Arabidopsis thaliana root

Plants form the basis of the food webs that sustain animal life. Exogenous factors, such as nutrients and sunlight, and endogenous factors, such as hormones, cooperate to control both the growth and the development of plants. We assessed how Arabidopsis thaliana integrated nutrient and hormone signaling pathways to control root growth and development by investigating the effects of combinatorial treatment with the nutrients nitrate and ammonium; the hormones auxin, cytokinin, and abscisic acid; and all binary combinations of these factors. We monitored and integrated short-term genome-wide changes in gene expression over hours and longterm effects on root development and architecture over several days. Our analysis revealed trends in nutrient and hormonal signal cross-talk and feedback, including responses that exhibited logic gate behavior, which means that they were triggered only when specific combinations of signals were present. From the data, we developed a multivariate network model comprising the signaling molecules, the early gene expression modulation, and the subsequent changes in root phenotypes. This multivariate network model pinpoints several genes that play key roles in the control of root development and may help understand how eukaryotes manage multifactorial signaling inputs

Multiple mechanisms of nitrate sensing by Arabidopsis nitrate transceptor NRT1.1

International audienceIn Arabidopsis the plasma membrane nitrate transceptor (transporter/receptor) NRT1.1 governs many physiological and developmental responses to nitrate. Alongside facilitating nitrate uptake, NRT1.1 regulates the expression levels of many nitrate assimilation pathway genes, modulates root system architecture, relieves seed dormancy and protects plants from ammonium toxicity. Here, we assess the functional and phenotypic consequences of point mutations in two key residues of NRT1.1 (P492 and T101). We show that the point mutations differentially affect several of the NRT1.1-dependent responses to nitrate, namely the repression of lateral root development at low nitrate concentrations, and the short-term upregulation of the nitrate-uptake gene NRT2.1, and its longer-term downregulation, at high nitrate concentrations. We also show that these mutations have differential effects on genome-wide gene expression. Our findings indicate that NRT1.1 activates four separate signalling mechanisms, which have independent structural bases in the protein. In particular, we present evidence to suggest that the phosphorylated and non-phosphorylated forms of NRT1.1 at T101 have distinct signalling functions, and that the nitrate-dependent regulation of root development depends on the phosphorylated form. Our findings add to the evidence that NRT1.1 is able to trigger independent signalling pathways in Arabidopsis in response to different environmental conditions

The competitiveness to form nodules shapes the capacities of <em>Rhizobium leguminosarum</em> sv viciae communities to promote symbiosis with specific hosts

National audienceCultivated fabeae legumes (pea, fababean, lentil) develop root nodules resulting from the symbiotic interaction with Rhizobium leguminosarum sv. viciae (Rlv). Individual Rlv bacteria are able to associate with various potential hosts, but in soil they are in mixture and they display a wide range of competitiveness to form nodules (CFN). Because in Rlv, CFN and capacity to fix nitrogen are genetically independent, CFN limits the effectiveness of inoculation strategies as efficient bacteria are often outcompeted by poorly efficient Rlv bacteria of the soil community. We developed a strategy to identify bacterial genes controlling CFN. A worldwide collection of 240 Rlv isolates was obtained by combining bacteria described in GenBank with new isolates obtained worldwide by pea, fababean and lentil root nodule trapping. 100 genomes (22 already in GenBank) were sequenced. The extended Rlv complex species includes probably 16 genospecies and two main groups of symbiosis plasmids that can be horizontally transferred. We identified phylogenetic clades of Rlv displaying contrasted levels of CFN upon pea and fababean. A molecular barcode was designed on nodD gene to discriminate and quantify intraspecific variability of Rlv in root systems allowing estimate CFN in symbiotic pea, fababean or lentil associations with multiple Rlv potential partners. Several plasmid regions genetically associated with pea/fababean CFN phenotypes were identified. Candidate genes include specific nod genes as well as other genes with unknown function in symbiosis

The Arabidopsis NRT1.1 transporter participates in the signalling pathway triggering root colonisation of nitrate-rich patches.

Localized proliferation of lateral roots in NO-rich patches is a striking example of the nutrient-induced plasticity of root development. In Arabidopsis, NO stimulation of lateral root elongation is apparently under the control of a NO-signaling pathway involving the ANR1 transcription factor. ANR1 is thought to transduce the NO signal internally, but the upstream NO sensing system is unknown. Here, we show that mutants of the NRT1.1 nitrate transporter display a strongly decreased root colonization of NO-rich patches, resulting from reduced lateral root elongation. This phenotype is not due to lower specific NO uptake activity in the mutants and is not suppressed when the NO-rich patch is supplemented with an alternative N source but is associated with dramatically decreased ANR1 expression. These results show that NRT1.1 promotes localized root proliferation independently of any nutritional effect and indicate a role in the ANR1-dependent NO signaling pathway, either as a NO sensor or as a facilitator of NO influx into NO-sensing cells. Consistent with this model, the NRT1.1 and ANR1 promoters both directed reporter gene expression in root primordia and root tips. The inability of NRT1.1-deficient mutants to promote increased lateral root proliferation in the NO-rich zone impairs the efficient acquisition of NO and leads to slower plant growth. We conclude that NRT1.1, which is localized at the forefront of soil exploration by the roots, is a key component of the NO-sensing system that enables the plant to detect and exploit NO-rich soil patches