The LysM Receptor-Like Kinase LysM RLK1 Is Required to Activate Defense and Abiotic-Stress Responses Induced by Overexpression of Fungal Chitinases in Arabidopsis Plants

Application of crab shell chitin or pentamer chitin oligosaccharide to Arabidopsis seedlings increased toler- ance to salinity in wild-type but not in knockout mutants of the LysM Receptor-Like Kinasel （CERK1/LysM RLK1） gene, known to play a critical role in signaling defense responses induced by exogenous chitin. Arabidopsis plants overexpress- ing the endochitinase chit36 and hexoaminidase excyl genes from the fungus Trichoderma asperelleoides T203 showed increased tolerance to salinity, heavy-metal stresses, and Botrytis cinerea infection. Resistant lines, overexpressing fungal chitinases at different levels, were outcrossed to lysm rlkl mutants. Independent homozygous hybrids lost resistance to biotic and abiotic stresses, despite enhanced chitinase activity. Expression analysis of 270 stress-related genes, including those induced by reactive oxygen species （ROS） and chitin, revealed constant up-regulation （at least twofold） of 10 genes in the chitinase-overexpressing line and an additional 76 salt-induced genes whose expression was not elevated in the lysm rlkl knockout mutant or the hybrids harboring the mutation. These findings elucidate that chitin-induced signaling mediated by LysM RLK1 receptor is not limited to biotic stress response but also encompasses abiotic-stress signaling and can be conveyed by ectopic expression of chitinases in plants.

Mapping theArabidopsis Metabolic Landscape by Untargeted Metabolomics at Different Environmental Conditions

Metabolic genome-wide association studies （mGWAS）, whereupon metabolite levels are regarded as traits, can help unravel the genetic basis of metabolic networks. A total of 309Arabidopsis accessions were grown under two independent environmental conditions （control and stress） and subjected to untargeted LC-MS- based metabolomic profiling; levels of the obtained hydrophilic metabolites were used in GWAS. Our two- condition-based GWAS for more than 3000 semi-polar metabolites resulted in the detection of 123 highly resolved metabolite quantitative trait loci （p ≤ 1.0E-08）, 24.39% of which were environment-specific. Interestingly, differently from natural variation in Arabidopsis primary metabolites, which tends to be controlled by a large number of small-effect loci, we found several major large-effect loci alongside a vast number of small-effect loci controlling variation of secondary metabolites. The two-condition-based GWAS was fol- lowed by integration with network-derived metabolite-transcript correlations using a time-course stress experiment. Through this integrative approach, we selected 70 key candidate associations between struc- tural genes and metabolites, and experimentally validated eight novel associations, two of them showing differential genetic regulation in the two environments studied. We demonstrate the power of combining large-scale untargeted metabolomics-based GWAS with time-course-derived networks both performed under different ablotic environments for identifying metabollte-gene associations, providing novel global insights into the metabolic landscape of Arabidopsis.

Analysis of the Interface between Primary and Secondary Metabolism in Catharanthus roseus Cell Cultures Using ^13C-Stable Isotope Feeding and Coupled Mass Spectrometry

Dear Editor, The tropical plant Madagascar periwinkle Catharanthus roseus （L.） G.Don is a rich source of plant-derived medicinal ter-penoid indole alkaloids （TIAs）, including the anti-hypertensive ajmalicine, the sedative compound serpentine, and the anti-cancer drugs vinblastine and vincristine. However, the latter two compounds are produced in C. roseus plants only in very low amounts. Elicitors such as hormones （e.g. jasmonates or salicylic acid） activate plant natural defense responses, includ-ing increased secondary metabolite production （EI-Sayed and Verpoorte, 2007; Lackman et al.. 2011）.

Involvement of the Hexose Transporter Gene LeHT1 and of Sugars in Resistance of Tomato to Tomato Yellow Leaf Curl Virus

Dear Editor, Tomato yellow leaf curl virus （TYLCV） is a whitefly-trans- mitted geminivirus infecting tomato crops （Czosnek, 2007）. TYLCV-resistant （R） and susceptible （S） lines with the same genetic background have been bred using Solanum habro- chaites as the resistance source. The gene（s） conferring resist- ance are unknown. Previously, we demonstrated that the hexose transporter gene LeHT1 is up-regulated upon infec- tion in R plants and its silencing in R plants （RH） leads to the collapse of resistance （Eybishtz et al., 2010）. To uncover the role of LeHT1 in resistance, we （1） analyzed the transcriptome reprogramming in leaves of S, R, and RH plants using a home- designed microarray, before and 7 d after TYLCV inoculation （0, 7 dpi）, and （2） measured the concentration of sugars and their derivatives in S, R, and RH leaves at 1 and 7 dpi because LeHT1 is transporting both glucose and fructose （McCurdy et al., 2010）.