Integration of early disease-resistance phenotyping, histological characterization, and transcriptome sequencing reveals insights into downy mildew resistance in impatiens

Downy mildew(DM),caused by obligate parasitic oomycetes,is a destructive disease for a wide range of crops worldwide.Recent outbreaks of impatiens downy mildew(IDM)in many countries have caused huge economic losses.A system to reveal plant–pathogen interactions in the early stage of infection and quickly assess resistance/susceptibility of plants to DM is desired.In this study,we established an early and rapid system to achieve these goals using impatiens as a model.Thirty-two cultivars of Impatiens walleriana and I.hawkeri were evaluated for their responses to IDM at cotyledon,first/second pair of true leaf,and mature plant stages.All I.walleriana cultivars were highly susceptible to IDM.While all I.hawkeri cultivars were resistant to IDM starting at the first true leaf stage,many(14/16)were susceptible to IDM at the cotyledon stage.Two cultivars showed resistance even at the cotyledon stage.Histological characterization showed that the resistance mechanism of the I.hawkeri cultivars resembles that in grapevine and type II resistance in sunflower.By integrating full-length transcriptome sequencing(Iso-Seq)and RNA-Seq,we constructed the first reference transcriptome for Impatiens comprised of 48,758 sequences with an N50 length of 2060 bp.Comparative transcriptome and qRT-PCR analyses revealed strong candidate genes for IDM resistance,including three resistance genes orthologous to the sunflower gene RGC203,a potential candidate associated with DM resistance.Our approach of integrating early disease-resistance phenotyping,histological characterization,and transcriptome analysis lay a solid foundation to improve DM resistance in impatiens and may provide a model for other crops.

Differential response of tomato genotypes to Xanthomonas-specific pathogen-associated molecular patterns and correlation with bacterial spot (Xanthomonas perforans) resistance

Plants depend on innate immune responses to retard the initial spread of pathogens entering through stomata,hydathodes or injuries.These responses are triggered by conserved patterns in pathogen-encoded molecules known as pathogen-associated molecular patterns(PAMPs).Production of reactive oxygen species(ROS)is one of the first responses,and the resulting‘oxidative burst’is considered to be a first line of defense.In this study,we conducted association analyses between ROS production and bacterial spot(BS;Xanthomonas spp.)resistance in 63 genotypes of tomato(Solanum lycopersicum L.).A luminol-based assay was performed on leaf tissues that had been treated with a flagellin 22(flg22),flagellin 28 and a Xanthomonas-specific flg22(flg22-Xac)peptide,to measure PAMP-induced ROS production in each genotype.These genotypes were also assessed for BS disease response by inoculation with Xanthomonas perforans,race T4.Although there was no consistent relationship between peptides used and host response to the BS,there was a significant negative correlation(r=−0.25,P<0.05)between foliar disease severity and ROS production,when flg22-Xac was used.This response could potentially be used to identify the Xanthomonas-specific PRR allele in tomato,and eventually PAMP-triggered immunity loci could be mapped in a segregating population.This has potential significance in tomato improvement.