Heat shock transcription factor 3 regulates plant immune response through modulation of salicylic acid accumulation and signalling in cassava

As the terminal components of signal transduction, heat stress transcription factors (Hsfs) mediate the activation of multiple genes responsive to various stresses. However, the information and functional analysis are very limited in non-model plants, especially in cassava (Manihot esculenta), one of the most important crops in tropical areas. In this study, 32 MeHsfs were identified from the cassava genome; the evolutionary tree, gene structures and motifs were also analysed. Gene expression analysis found that MeHsfs were commonly regulated by Xanthomonas axonopodis pv. manihotis (Xam). Amongst these MeHsfs, MeHsf3 was specifically located in the cell nucleus and showed transcriptionally activated activity on heat stress elements (HSEs). Through transient expression in Nicotiana benthamiana leaves and virus-induced gene silencing (VIGS) in cassava, we identified the essential role of MeHsf3 in plant disease resistance, by regulating the transcripts of Enhanced Disease Susceptibility 1 (EDS1) and pathogen-related gene 4 (PR4). Notably, as regulators of defence susceptibility, MeEDS1 and MePR4 were identified as direct targets of MeHsf3. Moreover, the disease sensitivity of MeHsf3- and MeEDS1-silenced plants could be restored by exogenous salicylic acid (SA) treatment. Taken together, this study highlights the involvement of MeHsf3 in defence resistance through the transcriptional activation of MeEDS1 and MePR4.