In the complex family of heat stress transcription factors, HsfA1 has a unique role as master regulator of thermotolerance in tomato

Abstract

We generated transgenic tomato plants with altered expression of heat stress transcription factor HsfA1. Plants with 10-fold overexpression of HsfA1 (OE plants) were characterized by a single HsfA1 transgene cassette, whereas plants harboring a tandem inverted repeat of the cassette showed cosuppression (CS plants) by posttranscriptional silencing of the HsfA1 gene connected with formation of small interfering RNAs. Under normal growth conditions, major developmental parameters were similar for wild-type (WT), OE, and CS plants. However, CS plants and fruits were extremely sensitive to elevated temperatures, because heat stress-induced synthesis of chaperones and Hsfs was strongly reduced or lacking. Despite the complexity of the plant Hsf family with at least 17 members in tomato, HsfA1 has a unique function as master regulator for induced thermotolerance. Using transient reporter assays with mesophyll protoplasts from WT tomato, we demonstrated that plasmid-encoded HsfA1 and HsfA2 were well expressed. However, in CS protoplasts the cosuppression phenomenon was faithfully reproduced. Only transformation with HsfA2 expression plasmid led to normal expression of the transcription factor and reporter gene activation, whereas even high amounts of HsfA1 expression plasmids were silenced. Thermotolerance in CS protoplasts was restored by plasmid-borne HsfA2, resulting in expression of chaperones, thermoprotection of firefly luciferase, and assembly of heat stress granules. (Pelham 1982;Pelham and Bienz 1982;Nover 1987). Similar to other transcription factors, Hsfs have a modular structure with an N-terminal DNA-binding domain characterized by a central helix-turn-helix motif, an adjacent oligomerization domain with a bipartite heptad pattern of hydrophobic amino acid residues (HR-A/B region), and sequence motifs essential for nuclear import and export (NLS, NES;Wu 1995;Morimoto 1998;Heerklotz et al. 2001;Nover et al. 2001). In many cases, the C-terminal activation domains are characterized by short peptide motifs (AHA motifs) shown to be crucial for the activator function (Döring et al. 2000).Sequencing of the Arabidopsis genome revealed a unique complexity of the Hsf family with 21 members, in contrast to yeast and Drosophila with one Hsf and vertebrates with four Hsfs (Wu 1995;Nover et al. 2001). From analyses of expressed sequence tag (EST) libraries, it is evident that the size of the Hsf family is comparable also in other plants, with 17 Hsfs thus far identified from tomato ESTs . By structural characteristics and phylogenetic comparison, plant Hsfs were assigned to three classes. In Arabidopsis, there are 15

HsfA2 Controls the Activity of Developmentally and Stress-Regulated Heat Stress Protection Mechanisms in Tomato Male Reproductive Tissues

“…2D). Hsp101 protein has been also previously shown to accumulate in leaves of A1CS plants, suggesting efficient translation and Hsp101 protein accumulation even at reduced transcript levels (Mishra et al, 2002). The reduced accumulation of Hsps in heat-stressed leaves of A2AS lines compared to the wild type indicates that HsfA2 plays an important role as coactivator during induction of the HSR.…”

“…We generated transgenic plants expressing a cDNA cassette of tomato HsfA2 either in the sense (A2S) or in antisense orientation (A2AS) under the control of the Cauliflower mosaic virus (CaMV) 35S promoter as previously described for HsfA1a (Mishra et al, 2002). We selected three independent transgenic lines (A2AS-3, -7, and -15) with significantly reduced HsfA2 levels as confirmed by transcript analysis using qRT-PCR in seedlings (Fig.…”

“…Tomato HsfA2 requires interaction with HsfA1a for efficient nuclear retention and transcriptional activity (Mishra et al, 2002;Scharf et al, 1998). Overexpression of Arabidopsis HsfA2 in the quadruple HsfA1a/b/d/e knockout mutant restored thermotolerance, indicating that Arabidopsis HsfA2 is functional in the absence of the master regulator (Liu and Charng, 2013).…”

The Heat-Inducible Transcription Factor HsfA2 Enhances Anoxia Tolerance in Arabidopsis

“…In Arabidopsis, 21 Hsfs have been identified (Nover et al, 2001). Interestingly, it seems that in Arabidopsis, different from tomato (Solanum lycopersicum; Mishra et al, 2002), there is not a unique master regulator for the Hsf-mediated heat shock response. Different Hsfs have been identified as potential crucial element for Arabidopsis response not only to heat stress (Busch et al, 2005;Schramm et al, 2006Schramm et al, , 2008Charng et al, 2007) but also to oxidative stress (Li et al, 2005;Nishizawa et al, 2008), hydrogen peroxide (H 2 O 2 ), high light (Nishizawa et al, 2006), and salt stress as well (Ogawa et al, 2007).…”

Genetic engineering for heat tolerance in plants

“…In another independent study, hsf3 gene over-expressed in A. thaliana resulted in increased thermotolerance (Panchuk et al, 2002). Mishra et al (2002) over-expressed tomato hsfA1 gene in tomato plants. In this study, overexpressing lines showed increased thermotolerance while transgenic lines in which transgene was silenced due to cosuppression were thermosensitive.…”