Rhizosphere microbial communities associated to rose replant disease:links to plant growth and root metabolites

Growth depression of Rosa plants at sites previously used to cultivate the same or closely related species is a typical symptom of rose replant disease(RRD).Currently,limited information is available on the causes and the etiology of RRD compared to apple replant disease(ARD).Thus,this study aimed at analyzing growth characteristics,root morphology,and root metabolites,as well as microbial communities in the rhizosphere of the susceptible rootstock Rosa corymbifera‘Laxa’grown in RRD-affected soil from two sites(Heidgraben and Sangerhausen),either untreated or disinfected byγ-irradiation.In a greenhouse bioassay,plants developed significantly more biomass in theγ-irradiated than in the untreated soils of both sites.Several plant metabolites detected in R.corymbifera‘Laxa’roots were site-and treatment-dependent.Although aloesin was recorded in significantly higher concentrations in untreated than inγ-irradiated soils from Heidgraben,the concentrations of phenylalanine were significantly lower in roots from untreated soil of both sites.Rhizosphere microbial communities of 8-week-old plants were studied by sequencing of 16S rRNA,ITS,and cox gene fragments amplified from total community DNA.Supported by microscopic observations,sequences affiliated to the bacterial genus Streptomyces and the fungal genus Nectria were identified as potential causal agents of RRD in the soils investigated.The relative abundance of oomycetes belonging to the genus Pythiogeton showed a negative correlation to the growth of the plants.Overall,the RRD symptoms,the effects of soil treatments on the composition of the rhizosphere microbial community revealed striking similarities to findings related to ARD.

Effects of saline irrigation on soil salt accumulation and grain yield in the winter wheat-summer maize double cropping system in the low plain of North China

In the dominant winter wheat （WW）-summer maize （SM） double cropping system in the low plain located in the North China, limited access to fresh water, especially during dry season, constitutes a major obstacle to realize high crop productivity. Using the vast water resources of the saline upper aquifer for irrigation during WW jointing stage, may help to bridge the peak of dry season and relieve the tight water situation in the region. A field experiment was conducted during 2009-2012 to investigate the effects of saline irrigation during WW jointing stage on soil salt accumulation and productivity of WW and SM. The experiment treatments comprised no irrigation （T1）, fresh water irrigation （T2）, slightly saline water irrigation （T3：2.8 dS m-l）, and strongly saline water irrigation （T4：8.2 dS m-1） at WW jointing stage. With regard to WW yields and aggregated annual WW-SM yields, clear benefits of saline water irrigation （T3 ＆ T4） compared to no irrigation （T1）, as well as insignificant yield losses compared to fresh water irrigation （T2） occurred in all three experiment years. However, the increased soil salinity in eady SM season in consequence of saline irrigation exerted a negative effect on SM photosynthesis and final yield in two of three experiment years. To avoid the negative aftereffects of saline irrigation, sufficient fresh water irrigation during SM sowing phase （i.e., increase from 60 to 90 mm） is recommended to guarantee good growth conditions during the sensitive early growing period of SM. The risk of long-term accumulation of salts as a result of saline irrigation during the peak of dry season is considered low, due to deep leaching of salts during regularly occurring wet years, as demonstrated in the 2012 experiment year. Thus, applying saline water irrigation at jointing stage of WW and fresh water at sowing of SM is most promising to realize high yield and fresh irrigation water saving.

A dual positional specific lipoxygenase functions in the generation of flavor compounds during climacteric ripening of apple

Lipoxygenase(LOX)is an important contributor to the formation of aroma-active C6 aldehydes in apple(Malus×domestica)fruit upon tissue disruption but little is known about its role in autonomously produced aroma volatiles from intact tissue.We explored the expression of 22 putative LOX genes in apple throughout ripening,but only six LOXs were expressed in a ripening-dependent manner.Recombinant LOX1:Md:1a,LOX1:Md:1c,LOX2:Md:2a and LOX2:Md:2b proteins showed 13/9-LOX,9-LOX,13/9-LOX and 13-LOX activity with linoleic acid,respectively.While products of LOX1:Md:1c and LOX2:Md:2b were S-configured,LOX1:Md:1a and LOX2:Md:2a formed 13(R)-hydroperoxides as major products.Site-directed mutagenesis of Gly567 to an alanine converted the dual positional specific LOX1:Md:1a to an enzyme with a high specificity for 9(S)-hydroperoxide formation.The high expression level of the corresponding MdLOX1a gene in stored apple fruit,the genetic association with a quantitative trait locus for fruit ester and the remarkable agreement in regio-and stereoselectivity of the LOX1:Md:1a reaction with the overall LOX activity found in mature apple fruits,suggest a major physiological function of LOX1:Md:1a during climacteric ripening of apples.While LOX1:Md:1c,LOX2:Md:2a and LOX2:Md:2b may contribute to aldehyde production in immature fruit upon cell disruption our results furnish additional evidence that LOX1:Md:1a probably regulates the availability of precursors for ester production in intact fruit tissue.

The carrot monoterpene synthase gene cluster on chromosome 4 harbours genes encoding flavour-associated sabinene synthases

In plants,low molecular weight terpenes produced by terpene synthases(TPS)contribute to multiple ecologically and economically important traits.The present study investigates a carrot terpene synthase gene cluster on chromosome 4 associated with volatile monoterpene production.Two carrot mutants,yellow and cola,which are contrasting in the content of low molecular weight terpenes,were crossed to develop an F2 mapping population.The mapping analysis revealed overlapping QTLs on chromosome 4 for sabinene,α-thujene,α-terpinene,γ-terpinene,terpinen-4-ol and 4-carene.The genomic region of this locus includes a cluster of five terpene synthase genes(DcTPS04,DcTPS26,DcTPS27,DcTPS54 and DcTPS55).DcTPS04 and DcTPS54 displayed genotype-and tissue-specific variation in gene expression.Based on the QTL mapping results and the gene expression patterns,DcTPS04 and DcTPS54 were selected for functional characterization.In vitro enzyme assays showed that DcTPS54 is a single-product enzyme catalysing the formation of sabinene,whereas DcTPS04 is a multiple-product terpene synthase producingα-terpineol as a major product and four additional products including sabinene,β-limonene,β-pinene and myrcene.Furthermore,we developed a functional molecular marker that could discriminate carrot genotypes with different sabinene content in a set of 85 accessions.