Soluble expression and purification of a functional harpin protein in Escherichia coli

The use of harpins in practical agricultural applications may enhance plant growth and induce disease resistance. However, few investigations focused on the optimal expression and purification of harpin. In this work, harpin protein fused with a thioredoxin tag and a hexahistidine tag was expressed in Escherichia coli BL21 (DE3) cells as a soluble form under the induction of 0.5 mmol/L isopropyl β-D-1-thiogalactopyranoside. The purity of the recombinant harpin was greater than 90% after one-step nickel-nitrilotriacetic acid affinity chromatography. The yield of purified TRX-harpin protein reached 17.1 mg per 100 mL of cell culture. TRX-harpin is thermostable and could trigger the hypersensitive response effect in tobacco, with an efficient dose as low as 30 μg/mL. The root lengths of TRX-harpin treated tobacco and wheat plants were nearly 1.6-fold and 1.8-fold longer, respectively, than plants treated with the empty vector preparation. Thus, using a N-terminal TRX-tagged fusion is an economic way to produce bioactive harpin.