Ice-Cap: a method for growing Arabidopsis and tomato plants in 96-well plates for high-throughput genotyping

Plants as vectors for environmental prion transmission

Prions cause fatal neurodegenerative diseases and exhibit remarkable durability, which engenders a wide array of potential exposure scenarios. In chronic wasting disease of deer, elk, moose, and reindeer and in scrapie of sheep and goats, prions are transmitted via environmental routes and the ability of plants to accumulate and subsequently transmit prions has been hypothesized, but not previously demonstrated. Here, we establish the ability of several crop and other plant species to take up prions via their roots and translocate them to above-ground tissues from various growth media including soils. We demonstrate that plants can accumulate prions in above-ground tissues to levels sufficient to transmit disease after oral ingestion by mice. Our results suggest plants may serve as vectors for prion transmission in the environment-a finding with implications for wildlife conservation, agriculture, and public health.

A double-mutant collection targeting MAP kinase related genes in Arabidopsis for studying genetic interactions

Mitogen-activated protein kinase cascades are conserved in all eukaryotes. In Arabidopsis thaliana there are approximately 80 genes encoding MAP kinase kinase kinases (MAP3K), 10 genes encoding MAP kinase kinases (MAP2K), and 20 genes encoding MAP kinases (MAPK). Reverse genetic analysis has failed to reveal abnormal phenotypes for a majority of these genes. One strategy for uncovering gene function when single-mutant lines do not produce an informative phenotype is to perform a systematic genetic interaction screen whereby double-mutants are created from a large library of single-mutant lines. Here we describe a new collection of 275 double-mutant lines derived from a library of single-mutants targeting genes related to MAP kinase signaling. To facilitate this study, we developed a high-throughput double-mutant generating pipeline using a system for growing Arabidopsis seedlings in 96-well plates. A quantitative root growth assay was used to screen for evidence of genetic interactions in this double-mutant collection. Our screen revealed four genetic interactions, all of which caused synthetic enhancement of the root growth defects observed in a MAP kinase 4 (MPK4) single-mutant line. Seeds for this double-mutant collection are publicly available through the Arabidopsis Biological Resource Center. Scientists interested in diverse biological processes can now screen this double-mutant collection under a wide range of growth conditions in order to search for additional genetic interactions that may provide new insights into MAP kinase signaling.

iTILLING: personalized mutation screening

One powerful approach to studying gene function is to analyze the phenotype of an organism carrying a mutant allele of a gene of interest. In order to use this experimental approach, one must have the ability to easily isolate individual organisms carrying desired mutations. A widely used method for accomplishing this task in plants and other organisms is a procedure called TILLING. A traditional TILLING project has at its foundation an ordered mutant population produced by treating seeds with a chemical mutagen. From this mutagenized seed, thousands of individual mutant lines are produced, and corresponding DNA samples are collected. For several plant species, publicly accessible screening facilities have been established that perform mutant screens on a gene-by-gene basis in response to customer requests using PCR and heteroduplex detection methods. The iTILLING method described in this chapter represents an individualized version of the TILLING process. Performing a traditional TILLING experiment requires a large investment in time and resources to establish the well-ordered mutant population. By contrast, iTILLING is a low-investment alternative that provides the individual research lab with a practical solution to mutation screening. The main difference between the two approaches is that iTILLING is not based on the establishment of a durable, organized mutant population. Instead, a system for growing Arabidopsis seedlings in 96-well plates is used to produce an ephemeral mutant population for screening. Because the intention is not to develop a long-term resource, a considerable savings in time and money is realized when using iTILLING as compared to traditional TILLING. iTILLING is not intended to serve as a replacement to traditional TILLING. Rather, iTILLING provides a strategy by which custom mutagenesis screens can be performed by individual labs using unique genetic backgrounds that are of specific interest to that research group.

Deletion of a tandem gene family in Arabidopsis: increased MEKK2 abundance triggers autoimmunity when the MEKK1-MKK1/2-MPK4 signaling cascade is disrupted

An Arabidopsis thaliana mitogen-activated protein (MAP) kinase cascade composed of MEKK1, MKK1/MKK2, and MPK4 was previously described as a negative regulator of defense response. MEKK1 encodes a MAP kinase kinase kinase and is a member of a tandemly duplicated gene family with MEKK2 and MEKK3. Using T-DNA insertion lines, we isolated a novel deletion mutant disrupting this gene family and found it to be phenotypically wild-type, in contrast with the mekk1 dwarf phenotype. Follow-up genetic analyses indicated that MEKK2 is required for the mekk1, mkk1 mkk2, and mpk4 autoimmune phenotypes. We next analyzed a T-DNA insertion in the MEKK2 promoter region and found that although it does not reduce the basal expression of MEKK2, it does prevent the upregulation of MEKK2 that is observed in mpk4 plants. This mekk2 allele can rescue the mpk4 autoimmune phenotype in a dosage-dependent manner. We also found that expression of constitutively active MPK4 restored MEKK2 abundance to wild-type levels in mekk1 mutant plants. Finally, using mass spectrometry, we showed that MEKK2 protein levels mirror MEKK2 mRNA levels. Taken together, our results indicate that activated MPK4 is responsible for regulating MEKK2 RNA abundance. In turn, the abundance of MEKK2 appears to be under cellular surveillance such that a modest increase can trigger defense response activation.