The Role of Arabidopsis thaliana Clade A1 Heat Shock Transcription Factors in Growth and Development

Arabidopsis thaliana clade A1 heat shock transcription factors function as the master regulators of heat shock and other abiotic stresses. Members of this clade (HSFA1a/b/d/e) are highly redundant but have individual and collective functions. Transgenic overexpression of one of its members, HSFA1b, in Arabidopsis was shown to have altered developmental traits as well as increased stress and disease tolerance. In this study, plants overexpressing HSFA1b had smaller rosettes in seedlings and mature plants, longer inflorescence and early flowering. A number of candidate developmental genes identified to be regulated by HSFA1b could not be satisfactorily confirmed using the HSFA1 quadruple knockout due to the mutant being a hybrid of 2 accessions (WS-0 and Col-0). Therefore, a new quadruple mutant was generated using the CRISPR/Cas9 gene editing system to knockout HSFA1a from an existing homozygous triple mutant (bdeKO) in a single accession (Col-0). The new quadruple mutant (QK2) was impaired in physiological responses in relation to abiotic stress along with other developmental defects in normal and ambient temperature. Furthermore, an interaction between HSFA1b (or HSFA1s) and ARGONAUTE proteins was established suggesting another pathway by which HSFA1s could regulate developmental genes under both normal and stress conditions via the action of microRNAs. Alongside confirming HSFA1b-regulated developmental genes, the QK2 was also instrumental in identifying microRNAs precursors that could be regulated by the clade. Together, results show how HSFA1 regulates growth and development during normal and stress conditions.