Abstract
Genome-wide dissection of the heat stress response (HSR) is necessary to overcome problems in crop production caused by global warming. To identify HSR genes, we profiled gene expression in two Chinese cabbage inbred lines with different thermotolerances, Chiifu and Kenshin. Many genes exhibited >2-fold changes in expression upon exposure to 0.5– 4 h at 45°C (high temperature, HT): 5.2% (2,142 genes) in Chiifu and 3.7% (1,535 genes) in Kenshin. The most enriched GO (Gene Ontology) items included ‘response to heat’, ‘response to reactive oxygen species (ROS)’, ‘response to temperature stimulus’, ‘response to abiotic stimulus’, and ‘MAPKKK cascade’. In both lines, the genes most highly induced by HT encoded small heat shock proteins (Hsps) and heat shock factor (Hsf)-like proteins such as HsfB2A (Bra029292), whereas high-molecular weight Hsps were constitutively expressed. Other upstream HSR components were also up-regulated: ROS-scavenging genes like glutathione peroxidase 2 (BrGPX2, Bra022853), protein kinases, and phosphatases. Among heat stress (HS) marker genes in Arabidopsis, only exportin 1A (XPO1A) (Bra008580, Bra006382) can be applied to B. rapa for basal thermotolerance (BT) and short-term acquired thermotolerance (SAT) gene. CYP707A3 (Bra025083, Bra021965), which is involved in the dehydration response in Arabidopsis, was associated with membrane leakage in both lines following HS. Although many transcription factors (TF) genes, including DREB2A (Bra005852), were involved in HS tolerance in both lines, Bra024224 (MYB41) and Bra021735 (a bZIP/AIR1 [Anthocyanin-Impaired-Response-1]) were specific to Kenshin. Several candidate TFs involved in thermotolerance were confirmed as HSR genes by real-time PCR, and these assignments were further supported by promoter analysis. Although some of our findings are similar to those obtained using other plant species, clear differences in Brassica rapa reveal a distinct HSR in this species. Our data could also provide a springboard for developing molecular markers of HS and for engineering HS tolerant B. rapa.
Highlights
Heat stress (HS) and high temperature (HT) disturb cellular homeostasis and can interfere with various plant metabolic and physiological processes, including photosynthesis, growth, development, pollen fertility, and productivity in both crop species and model plants [1,2,3]
We identified differentially expressed genes (DEGs), expressed genes (SEGs), heat stress response (HSR) genes, HS marker genes, and membrane leakage-related (MLR) genes, and we discuss these genes in the context of HS
Our results indicate that electrolyte leakage could be a useful marker for temperature sensitivity in B. rapa
Summary
Heat stress (HS) and high temperature (HT) disturb cellular homeostasis and can interfere with various plant metabolic and physiological processes, including photosynthesis, growth, development, pollen fertility, and productivity in both crop species and model plants [1,2,3]. Plants can perceive HS in various ways, and they tolerate or acclimate to heat via activation of signaling pathways, expression of stress proteins, and production of defensive materials. The activation of HSR genes, such as heat stress transcription factors (Hsfs), heat shock proteins (Hsps), and some other transcription factors (TFs), are very important for heat tolerance [5,6,7]. A histone sensor in the nucleus and unfolded protein sensors present in the ER (endoplasmic reticulum) and cytosol play important roles in the perception process. These signals are transmitted to downstream components, such as calmodulins, mitogen-activated protein kinases, Hsp, and Hsfs. Chaperones for protein homeostasis, osmolytes, and secondary metabolites responsible for ROS detoxification and osmoprotection are important for thermotolerance [9]
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