Over-expression of LlHsfA2b, a lily heat shock transcription factor lacking trans-activation activity in yeast, can enhance tolerance to heat and oxidative stress in transgenic Arabidopsis seedlings

Heat shock transcription factor (Hsf) is a central regulator in the heat signaling transduction pathway in eukaryotes. In this study, we isolated an HsfA2 gene named LlHsfA2b from the leaves of lily (Lilium longiflorum ‘White Heaven’) using the rapid amplification of cDNA ends (RACE) technique. Multiple alignment and phylogenetic analyses showed that LlHsfA2b has critical domains of Hsf class A. Expression analyses revealed that LlHsfA2b could be induced by heat and H2O2, but not by NaCl, drought, or abscisic acid (ABA) treatments; moreover, the transcript of LlHsfA2b were induced by heat earlier than the one of LlHsfA2 in lily. Following transient expression of LlHsfA2b in onion epidermal cells, GFP-LlHsfA2b was observed in the cell nucleus. Different from all known HsfA2s, LlHsfA2b failed to display trans-activation activity in yeast cell. In transgenic Arabidopsis overexpressing LlHsfA2b, the putative downstream genes of AtHsfA1d/e and AtHsfA2 were activated slightly under unstressed conditions. The transgenic Arabidopsis seedlings displayed enhanced tolerance to heat and oxidative stresses. The transient reporter assay exhibited that LllHsfA2b had a trans-activation activity in tobacco mesophyll protoplasts. At the same time, yeast two-hybrid assay showed that the putative LhHsfA2b interacted with either AtHsfA1d or AtHsfA2. In summary, our data reveal that LlHsfA2b is a novel gene associated with tolerance to heat and oxidative stress in lily, and is thought to function probably by directly activating downstream genes or by dimerization or trimerization with other HsfA protein harboring activation activity.