Adaptive divergence in transcriptome response to heat and acclimation inArabidopsis thalianaplants from contrasting climates

Abstract

Phenotypic variation in stress response has been widely observed within species. This variation is an adaptive response to local climates and is controlled by gene sequence variation and especially by variation in expression at the transcriptome level. Plants from contrasting climates are thus expected to have different patterns in gene expression. Acclimation, a pre-exposure to sub-lethal temperature before exposing to extreme high temperature, is an important adaptive mechanism of plant survival. We are interested to evaluate the gene expression difference to heat stress for plants from contrasting climates and the role of acclimation in altering their gene expression pattern. Natural Arabidopsis thaliana plants from low elevation mediterranean and high elevation montane climates were exposed to two heat treatments at the bolting stage: a) 45 oC: a direct exposure to 45oC heat; b) 38/45 oC: an exposure to 45oC heat after a 38oC acclimation treatment. Variation in overall gene expression patterns was investigated. We also explored gene expression patterns for Hsp/Hsf pathway and reactive oxygen species (ROS) pathway. In both heat treatments, high elevation plants had more differentially expressed (DE) genes than low elevation plants. In 45 oC, only Hsp/Hsf pathway was activated in low elevation plants; both Hsp/Hsf and ROS pathways were activated in high elevation plants. Small Hsps had the highest magnitude of change in low elevation plants while Hsp70 and Hsp90 showed the largest magnitude of fold in high elevation plants. In 38/45 oC, Hsp/Hsf and ROS pathways were activated in both low and high elevation plants. Low elevation plants showed up-regulation in all Hsps, especially small Hsps; high elevation plants showed down-regulation in all Hsps. Low elevation and high elevation also adopted different genes in the ROS pathway. We also observed genes that shifted expression in both low and high elevation plants but with opposite directions of change. This study indicates that low and high elevation plants have evolved adaptive divergence in heat stress response. The contrasting patterns of temperature variation in low and high elevation sites appears to have played a strong role in the evolution of divergent patterns to high temperature stress, both pre-acclimation and direct exposure gene expression responses.

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