Genome-wide identification and expression analysis of heat shock factors in Plantago fengdouensis under abiotic stresses

Plantago fengdouensis is a rare plant with extremely limited distribution, it once thrived on the central islands of the Yangtze River, where it was alternately exposed to flooding and high temperatures annually. Researches on various plants have confirmed that heat shock factors (HSFs) are the important regulatory factors mediating plants' responses to abiotic stresses and also participating in regulating plant growth and development. However, there is a research gap regarding the HSF family in P. fengdouensis. Therefore, a total of 30 PfHSFs were identified in the genome of P. fengdouensis and classified into three groups (i.e., A, B and C). The DNA binding domain (DBD) in PfHSFs was an N-terminal domain composed of Motif 1, Motif 2 and Motif 3, and was a highly conserved structure that existed in all PfHSFs. There were 14 segmental duplications found in PfHSFs, and the PfHSFs had undergone strong purifying selection during evolution. There were four gene pairs, PfHSFA8a/PfHSFA8b, PfHSFA1c/PfHSFA1d, PfHSFB2b/PfHSFB2c and PfHSFA4c/PfHSFA4d, that consistently exhibited identical expression patterns across different tissues, waterlogging treatment and heat treatment. Many cis-elements related to light response and phytohormone response were detected upstream of the PfHSFs promoters. The accumulation of most PfHSF transcripts did not show significant tissue specificity, whereas PfHSFB3a was particularly abundant in stems and flowers. PfHSFA6e and PfHSFA6f were upregulated under waterlogging; whereas PfHSFA6a-PfHSFA6f were suppressed under heat stress. Moreover, PfHSFA3, PfHSF8a and PfHSF8b were significantly induced in the early stage of heat stress. The co-expression analysis revealed correlations among PfHSFs and suggested that PfHSFs might be involved in circadian rhythm regulation, growth stage transition and flowering. The obtained results indicated that PfHSFs played an indispensable role in the adaptation to the alternating conditions of waterlogging and high temperature in the amphibious lifestyle of P. fengdouensis.