Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Genetic and Physiological Responses to Heat Stress in Brassica napus

Given the current rise in global temperatures, heat stress has become a major abiotic challenge affecting the growth and development of various crops and reducing their productivity. Brassica napus, the second largest source of vegetable oil worldwide, experiences a drastic reduction in seed yield and quality in response to heat. This review outlines the latest research that explores the genetic and physiological impact of heat stress on different developmental stages of B. napus with a special attention to the reproductive stages of floral progression, organogenesis, and post flowering. Several studies have shown that extreme temperature fluctuations during these crucial periods have detrimental effects on the plant and often leading to impaired growth and reduced seed production. The underlying mechanisms of heat stress adaptations and associated key regulatory genes are discussed. Furthermore, an overview and the implications of the polyploidy nature of B. napus and the regulatory role of alternative splicing in forming a priming-induced heat-stress memory are presented. New insights into the dynamics of epigenetic modifications during heat stress are discussed. Interestingly, while such studies are scarce in B. napus, opposite trends in expression of key genetic and epigenetic components have been identified in different species and in cultivars within the same species under various abiotic stresses, suggesting a complex role of these genes and their regulation in heat stress tolerance mechanisms. Additionally, omics-based studies are discussed with emphasis on the transcriptome, proteome and metabolome of B. napus, to gain a systems level understanding of how heat stress alters its yield and quality traits. The combination of omics approaches has revealed crucial interactions and regulatory networks taking part in the complex machinery of heat stress tolerance. We identify key knowledge gaps regarding the impact of heat stress on B. napus during its yield determining reproductive stages, where in-depth analysis of this subject is still needed. A deeper knowledge of heat stress response components and mechanisms in tissue specific models would serve as a stepping-stone to gaining insights into the regulation of thermotolerance that takes place in this important crop species and support future breeding of heat tolerant crops.

Systematic Analysis of Hsf Family Genes in the Brassica napus Genome Reveals Novel Responses to Heat, Drought and High CO2 Stresses

Drought and heat stress are major causes of lost plant crop yield. In the future, high levels of CO₂, in combination of other abiotic stress factors, will become a novel source of stress. Little is known of the mechanisms involved in the acclimation responses of plants to this combination of abiotic stress factors, though it has been demonstrated that heat shock transcription factors (Hsfs) are involved in plant response to various abiotic stresses. In this study, we performed a genome-wide identification and a systematic analysis of genes in the Hsf gene family in Brassica napus. A total of 64 genes encoding Hsf proteins were identified and classified into 3 major classes: A, B and C. We found that, unlike in other eudicots, the A9 subclass is absent in rapeseed. Further gene structure analysis revealed a loss of the only intron in the DBD domain for BnaHsf63 and -64 within class C, which is evolutionarily conserved in all Hsf genes. Transcription profile results demonstrated that most BnaHsf family genes are upregulated by both drought and heat conditions, while some are responded to a high CO₂ treatment. According to the combined RNA-seq and qRT-PCR analysis, the A1E/A4A/A7 subclasses were upregulated by both drought and heat treatments. Members in class C seemed to be predominantly induced only by drought. Among BnaHsf genes, the A2/A3/B2 subclasses were regulated by all three abiotic stresses. Members in A2/B2 subclasses were upregulated by drought and heat treatments, but were downregulated under high CO₂ conditions. While the A3 subclass was upregulated by all the three abiotic stresses. Various stress-related cis-acting elements, enriched in promoter regions, were correlated with the transcriptional response of BnaHsfs to these abiotic stresses. Further study of these novel groups of multifunctional BnaHsf genes will improve our understanding of plant acclimation response to abiotic stresses, and may be useful for improving the abiotic stress resistance of crop varieties.