Adaptive divergence in transcriptome response to heat and acclimation in Arabidopsis thaliana plants from contrasting climates

22 Phenotypic variation in stress response has been widely observed within species. This 23 variation is an adaptive response to local climates and is controlled by gene sequence 24 variation and especially by variation in expression at the transcriptome level. Plants 25 from contrasting climates are thus expected to have different patterns in gene 26 expression. Acclimation, a pre-exposure to sub-lethal temperature before exposing to 27 extreme high temperature, is an important adaptive mechanism of plant survival. We 28 are interested to evaluate the gene expression difference to heat stress for plants from 29 contrasting climates and the role of acclimation in altering their gene expression pattern. 30 Natural Arabidopsis thaliana plants from low elevation mediterranean and high elevation 31 montane climates were exposed to two heat treatments at the bolting stage: a) 45 C: a 32 direct exposure to 45C heat; b) 38/45 C: an exposure to 45C heat after a 38C 33 acclimation treatment. Variation in overall gene expression patterns was investigated. 34 We also explored gene expression patterns for Hsp/Hsf pathway and reactive oxygen 35 species (ROS) pathway. In both heat treatments, high elevation plants had more 36 differentially expressed (DE) genes than low elevation plants. In 45 C, only Hsp/Hsf 37 pathway was activated in low elevation plants; both Hsp/Hsf and ROS pathways were 38 activated in high elevation plants. Small Hsps had the highest magnitude of change in 39 low elevation plants while Hsp70 and Hsp90 showed the largest magnitude of fold in 40 high elevation plants. In 38/45 C, Hsp/Hsf and ROS pathways were activated in both 41 low and high elevation plants. Low elevation plants showed up-regulation in all Hsps, 42 especially small Hsps; high elevation plants showed down-regulation in all Hsps. Low 43 elevation and high elevation also adopted different genes in the ROS pathway. We also 44 observed genes that shifted expression in both low and high elevation plants but with 45 opposite directions of change. This study indicates that low and high elevation plants 46 have evolved adaptive divergence in heat stress response. The contrasting patterns of 47 temperature variation in low and high elevation sites appears to have played a strong 48 role in the evolution of divergent patterns to high temperature stress, both pre49 acclimation and direct exposure gene expression responses. 50