Sanguinarine is an isoquinoline alkaloid produced by Papaveraceae plants. Because sanguinarine has antimicrobial activity, it is believed to be related to the plants' chemical defense systems. However, its action against plants has not been well understood. A previous study reported that among 12 alkaloids, sanguinarine was the only compound which enhanced heat tolerance in Arabidopsis. Here we performed a promoter assay using a heat shock protein gene (HSP17.6C-CI) of Arabidopsis to assess the induction of heat shock responses by alkaloids. Although sanguinarine induced the heat shock response, the other 11 alkaloids did not. Sanguinarine promoted the production of HSP17.6C-CI protein, but berberine and papaverine, which are isoquinoline alkaloids as well as sanguinarine, did not promote it. It is known that geldanamycin, a small molecule chaperone inhibitor, activates the heat shock response in Arabidopsis. Although sanguinarine inhibited the chaperone activities of wheat germ extract much like geldanamycin, berberine and papaverine influenced the activities very little. These results suggest that sanguinarine may promote the heat shock response by regulating the chaperone activities in the way that geldanamycin does in plants.
The heat shock protein 90 (HSP90) inhibitor, geldanamycin, is a chemical inducer of the heat shock response (HSR) in Arabidopsis. Geldanamycin is thought to activate the heat shock signal by dissociating the HSP90-heat shock factor (HSF) complex. Recent studies have indicated that plant HSP70 is also associated with HSF, suggesting that inhibition of HSP70 may induce the HSR. However, no studies have been conducted to test this hypothesis. Here, we found that a specific HSP70 inhibitor VER-155008 activated the promoter of a small HSP gene (At1 g53540, HSP17.6C-CI) of Arabidopsis, which was shown to be activated by geldanamycin and other HSP90 inhibitors. The production of HSP17.6C-CI, HSP70 and HSP90.1 proteins in Arabidopsis was enhanced by the addition of VER-155008. The reduction of chlorophyll contents by heat shock was ameliorated by VER-155008. Chaperone analyses indicated that VER-155008 inhibited the chaperone activities of wheat germ extract and human HSP70/HSP40, respectively. These results suggest that the inhibition of HSP70 by VER-155008 enhanced the heat tolerance of Arabidopsis by inducing the HSR in the plant.
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