Arid regions represent nearly 30 % of the Earth's terrestrial surface, but their microbial biodiversity is not yet well characterized. The surface sands of deserts, a subset of arid regions, are generally subjected to large temperature fluctuations plus high UV light exposure and are low in organic matter. We examined surface sand samples from the Taklamaken (China, three samples) and Gobi (Mongolia, two samples) deserts, using pyrosequencing of PCR-amplified 16S V1/V2 rDNA sequences from total extracted DNA in order to gain an assessment of the bacterial population diversity. In total, 4,088 OTUs (using ≥97 % sequence similarity levels), with Chao1 estimates varying from 1,172 to 2,425 OTUs per sample, were discernable. These could be grouped into 102 families belonging to 15 phyla, with OTUs belonging to the Firmicutes, Proteobacteria, Bacteroidetes, and Actinobacteria phyla being the most abundant. The bacterial population composition was statistically different among the samples, though members from 30 genera were found to be common among the five samples. An increase in phylotype numbers with increasing C/N ratio was noted, suggesting a possible role in the bacterial richness of these desert sand environments. Our results imply an unexpectedly large bacterial diversity residing in the harsh environment of these two Asian deserts, worthy of further investigation.

Degradation of the quorum-sensing (QS) signals known as N-acylhomoserine lactones (AHL) by soil bacteria may be useful as a beneficial trait for protecting crops, such as potato plants, against the worldwide pathogen Pectobacterium. In this work, analytical chemistry and microbial and molecular approaches were combined to explore and compare biostimulation of native and introduced AHL-degrading Rhodococcus erythropolis populations in the rhizosphere of potato plants cultivated in farm greenhouses under hydroponic conditions. We first identified gamma-heptalactone (GHL) as a novel biostimulating agent that efficiently promotes plant root colonization by AHL-degrading R. erythropolis population. We also characterized an AHLdegrading biocontrol R. erythropolis isolate, R138, which was introduced in the potato rhizosphere. Moreover, root colonization by AHL-degrading bacteria receiving different combinations of GHL and R138 treatments was compared by using a cultivationbased approach (percentage of AHL-degrading bacteria), pyrosequencing of PCR-amplified rrs loci (total bacterial community), and quantitative PCR (qPCR) of the qsdA gene, which encodes an AHL lactonase in R. erythropolis. Higher densities of the AHLdegrading R. erythropolis population in the rhizosphere were observed when GHL treatment was associated with biocontrol strain R138. Under this condition, the introduced R. erythropolis population displaced the native R. erythropolis population. Finally, chemical analyses revealed that GHL, gamma-caprolactone (GCL), and their by-products, gamma-hydroxyheptanoic acid and gamma-hydroxycaproic acid, rapidly disappeared from the rhizosphere and did not accumulate in plant tissues. This integrative study highlights biostimulation as a potential innovative approach for improving root colonization by beneficial bacteria. N-Acylhomoserine lactones are intercellular signals used by numerous alpha-, beta-, and gammaproteobacteria to regulate gene expression at the population and community levels (18,38). The mechanism connecting cell population to gene expression via AHLs is termed quorum sensing (QS) (17). Pectobacterium carotovorum is a causative agent of blackleg and soft rot diseases in several crops, including potato plants and tubers. Over the 2004-2009 period, quality refusals oscillated between 2 and 5% and 3 and 8% of the total seed tuber production in France and Netherlands, respectively; blackleg disease, which is caused by Pectobacterium and Dickeya, represented 10 to 30% and 60 to 80% of the causes of refusal in the same countries. In Pectobacterium, production of virulence factors such as pectinolytic and cellulolytic enzymes and harpins is positively controlled by AHLs (23, 29). In Pectobacterium carotovorum, inhibition of AHL synthesis or degradation of the AHLs produced results in the absence of the expression of the QS-regulated genes and consequently in a decrease of the virulence symptoms on potato plants (23,31).During the past decade, several approaches have been proposed to disrupt QS regulatio...