Transcriptional variation, also called expression level polymorphism (ELP), contributes to intra-specific phenotypic variation in many organisms. Differentially expressed transcripts are typically enriched for stress-related genes, suggesting that differences in response to the environment are a particularly common point of divergence among gentoypes. Analysis of ELPs also has been suggested as a way to assess unintended consequences of transgene introduction; however, it is important that interpretation of transcriptional changes be performed within the context of potential fitness effects. In these studies we sought to examine differential gene expression in response to salinity for two widely used Arabidopsis thaliana ecotypes, Wassilewskija (Ws) and Columbia (Col), and a single gene mutation (glabrous, gl1-1) in the Col background (Col(gl)), in relation to genetic, phenotypic, and fitness differences.
Growth analyses were performed with seedlings germinated on culture media and growth chamber-grown plants carried through the full life cycle. Transcriptome analyses were performed with salt treated and control growth-chamber grown plants six days post initiation of salt stress. Ws plants had the least salt injury and highest dry matter accumulation and seed production in salt stressed conditions. ELPs among genoytypes and in response to 100 mM NaCl were enriched for genes associated with response to stress, including stress-associated transcription factors, heat shock and redox metabolism genes, and R genes. Application of salt resulted in many more transcripts up- or down-regulated in Col and Ws than in Col(gl). Many of the transcripts influenced by salt in Col were already altered in gl1-1 plants in the absence of salt, although Col(gl) plants did not show any detectable signs of stress, or effects on fecundity in the absence of salt treatment. The majority of salt-induced transcriptional changes that occurred in Ws also occurred in Col, suggesting common salt stress responses in these two ecotypes. Many more genes were affected by salt in Col than Ws, however, possibly reflecting the greater salt injury observed for Col. There was minimal overlap between the transcripts that differed for Ws and Col prior to salt treatment and those that were subsequently affected by salt stress. Thus, many genes conferring comparative salt stress tolerance in Ws likely differ from those whose expression levels are modified in response to salt stress. These studies demonstrate transcriptional variation among Arabidopsis genotypes in response to salt stress. Greater transcriptome differences did not necessarily correspond with greater genetic difference or phenotypic differences in morphology, fecundity, and resistance to salt stress. These results suggest that depending on circumstance, transcriptional changes can reflect response to injury, facilitate adaptive expression of fitness-associated traits, or allow for phenotypic buffering to minimize the impact of genetic changes.
Overall design: Three Arabidopsis genotypes were grown in the growth chamber in the absence and presence of salt stress. Plants from 20 days after sowing (6 days after salt treatment) were used for RNA extraction and hybridization on Affymetrix microarrays. There were two biological replicates for each genotype and salt treatment combination.
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