Jasmonates differentially affect interconnected signal-transduction pathways of Pieris rapae-induced defenses in Arabidopsis thaliana

The jasmonic acid （JA） pathway is the main signal-transduction pathway induced by insect folivory. Mutant plants affected in the jasmonate pathway （18：0 and/or 16：0-oxylipin routes） were studied to assess the effects of JA and its oxylipin intermediates 12-oxophytodienoic acid （OPDA） and dinor-OPDA （dnOPDA） on interconnected signal- transduction pathways that underlie induced defenses in Arabidopsis. Our data show that the oxylipin jasmonates dnOPDA, OPDA and JA have different roles in defense signaling induced after feeding by the chewing-biting caterpillar Pieris rapae. Jasmonic acid, and not OPDA or dnOPDA, is the major signaling compound required for the induction of the defense-related genes LOX2 （Lipo：~vgenase 2）, OPR3 （12-Oxophytodienoate reductase 3）, ACX1 （Acyl-CoA oxidase 1） and PAL1 （Phenvlalanine ammonia-lyase 1）. Monitoring PAL1 transcript levels clearly showed that accumulation of JA upon P rapae feeding results in the induction of the salicylic acid pathway. Furthermore, JA is the major signaling compound required for the P rapae-induced expression of the defense-related gene HPL1 （Hydroperoxide lyase 1）. The jasmonate dnOPDA influences the induction of the HPk- branch as well, yet its effect is antagonistic to the effect of JA. Our data show that these jasmonates may be used to fine-tune Arabidopsis＇ herbivore-induced responses in terms of the HPL-branch from the oxylipin pathway.

Transcriptional responses of Brassica nigra to feeding by specialist insects of different feeding guilds

Plants show phenotypic changes when challenged with herbivorous insects. The mechanisms underlying these changes include the activation of transcriptional responses, which are dependent on the attacking insect. Most transcriptomic studies on crucifer-insect interactions have focused on the model plant Arabidopsis thaliana, a species that faces low herbivore pressure in nature. Here, we study the transcriptional responses of plants from a wild black mustard （Brassica nigra） population to herbivores of different feeding guilds using an A. thaliana-based whole-genome microarray that has previously been shown to be suitable for transcriptomic analyses in Brassica. Transcriptional responses of B. nigra after infestation with either Pieris rapae caterpillars or Brevicoryne brassicae aphids are analyzed and compared. Additionally, the insect-induced expression changes of some individual genes are analyzed through quantitative real-time polymerase chain reaction. The results show that feeding by both insect species results in the accumulation of transcripts encoding proteins involved in the detoxification of reactive oxygen species, defensive proteins and glucosinolates and this is correlated with experimental evidence in the literature on such biochemical effects. Although genes encoding proteins involved in similar processes are regulated by both insects, there was little overlap in the induction or repression of individual genes. Furthermore, P. rapae and B. brassicae seem to affect different phytohormone signaling pathways. In conclusion, our results indicate that B. nigra activates several defense-related genes in response to P rapae or B. brassicae feeding, but that the response is dependent on the attacking insect species.

Quantitative resistance against Bemisia tabaci in Solanum pennellii:Genetics and metabolomics

The whitefly Bemisia tabaci is a serious threat in tomato cultivation worldwide as all varieties grown today are highly susceptible to this devastating herbivorous insect.Many accessions of the tomato wild relative Solanum pennellii show a high resistance towards B. tabaci. A mapping approach was used to elucidate the genetic background of whiteflyresistance related traits and associated biochemical traits in this species. Minor quantitative trait loci（QTLs） for whitefly adult survival（AS） and oviposition rate（OR） were identified and some were confirmed in an F2BC1 population, where they showed increased percentages of explained variance（more than 30%）. Bulked segregant analyses on pools of whiteflyresistant and-susceptible F2 plants enabled the identification of metabolites that correlate either with resistance or susceptibility. Genetic mapping of these metabolites showed that a large number of them co-localize with whiteflyresistance QTLs. Some of these whitefly-resistance QTLs are hotspots for metabolite QTLs. Although a large number of metabolite QTLs correlated to whitefly resistance or susceptibility, most of them are yet unknown compounds and further studies are needed to identify the metabolic pathways and genes involved. The results indicate a direct genetic correlation between biochemical-based resistance characteristics and reduced whitefly incidence in S. pennellii.