Functional regions of HpaXm as elicitors with specific heat tolerance induce the hypersensitive response or plant growth promotion in nonhost plants

<div><p>HpaXm produced by the cotton leaf blight bacterium <i>Xanthomonas citri</i> subsp. <i>malvacearum</i> is a novel harpin elicitor of the induced hypersensitive response (HR) in tobacco. We investigated whether fragments of HpaXm, compared with fragments of Hpa1Xoo, are sufficient for HR or plant growth promotion (PGP) elicitation using four synthetic peptides (HpaXm_35-51, HpaXm_10-39, Hpa1Xoo_36-52 and Hpa1Xoo_10-40). We also heated the fragments to determine the heat tolerance of the functional fragments. HpaXm_35-51 and Hpa1Xoo_36-52 induced hypersensitive response (HR). Bursts of reactive oxygen intermediates (ROI) induced by HpaXm_35-51 and Hpa1Xoo_36-52 were earlier and stronger than those induced by HpaXm and Hpa1Xoo. In plants treated with HpaXm_35-51 or Hpa1Xoo_36-52, the expression of the HR marker genes <i>Hin1 and Hsr203J</i> and the active oxygen metabolism related gene <i>AOX</i> were significantly upregulated. These findings suggest that the predicted α-helical structures of the HpaXm_35-51 and Hpa1Xoo_36-52 fragments are crucial for HR. PGP result by soaking seeds in unheated/heated HpaXm_10-39 or Hpa1Xoo_10-40 solution prior to transfer, which obviously enhances root growth and the aerial parts of plants. The PGP related gene <i>NtEXP6</i> was greatly enhanced when plants were sprayed with a solution of HpaXm_10-39 or Hpa1Xoo_10-40; heated fragment treatments induced higher levels of <i>NtEXP6</i> expression than unheated HpaXm fragments. In addition, HR marker genes induced by the heated fragments had lower expression levels than when induced with unheated HpaXm fragments. Moreover, the expression levels of HR marker genes and PGP related genes induced by treatment with Hpa1Xoo fragments before or after heating were the opposite of those induced by HpaXm fragments. Different functional fragments of harpin and different harpins with the same functional region have different degrees of heat tolerance. Therefore, the heat resistance of harpin is conservative, but the degree of heat tolerance of the functional fragments is specific.</p>