Induction of the heat shock response in Arabidopsis by chlorinated 1,4-naphthoquinones

It is known that some plants produce 1,4-naphthoquinones (1,4NQs) to inhibit the growth of other plants as allelochemicals. Here, we report that a chlorinated 1,4NQ enhanced the heat tolerance of Arabidopsis, and this enhancement was associated with induction of the heat shock response (HSR) in the plant. Nine 1,4NQs were subjected to the HSR assay by using the promoter of a small heat shock protein (HSP17.6C-CI) gene of Arabidopsis. The results indicated that chlorinated 1,4NQs, i.e., 2,3-dichloro-1,4-naphthoquinone (DNQ) and 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone (DDNQ), apparently showed HSR-inducing activities five to six times greater than a positive control, geldanamycin. However, nonchlorinated 1,4NQs such as lawsone, juglone, plumbagin, and menadione showed much lower activities than DNQ and DDNQ. The administration of DDNQ increased the accumulation of the HSP17.6C-CI and HSP90.1 transcripts as well as of the corresponding proteins in the Arabidopsis seedlings. DDNQ significantly ameliorated the reductions of fresh weight and chlorophyll contents of the plant due to heat. These results suggest that the chlorinated 1,4NQs are potent HSR inducers that can enhance the heat tolerance of plants.