-Effects Condition the Induction of a Major Unfolded Protein Response Factor, , in Response to Heat Stress in Maize.

Adverse environmental conditions such as heat and salt stress create endoplasmic reticulum (ER) stress in maize and set off the unfolded protein response (UPR). A key feature of the UPR is the upregulation of and the splicing of its messenger RNA. We conducted an association analysis of a recombinant inbred line (RIL) derived from a cross of a tropical founder line, CML52 with a standard temperate line, B73. We found a major QTL conditioning heat-induced expression located to the gene. Based on the premise that the QTL might be associated with the promoter, we evaluated various maize inbred lines for their ability to upregulate the expression of in response to heat stress. In general, tropical lines with promoter regions similar to CML52 were more robust in upregulating in response to heat stress. This finding was confirmed by comparing the strength of the B73 and CML52 promoters in transient maize protoplast assays. We concluded that the upstream region of is important in conditioning the response to heat stress and was under selection in maize when adapted to different environments. Heat stress has large negative effects on maize grain yield. Heat stress creates ER stress in maize and sets off the UPR. We searched for factors conditioning heat induction of the UPR in maize seedlings by conducting an association analysis based on the upregulation of unspliced and spliced forms of mRNA (ZmbZIP60u and ZmbZIP60s, respectively). ZmbZIP60u was upregulated more robustly by heat stress in the tropical maize line, CML52, than in B73, and a major QTL derived from the analysis of RILs from a cross of these two lines mapped in the vicinity of . We conducted a test to determine whether the QTL was acting as a regulatory element or in , as might be expected for a transcription factor. We found that the QTL was acting , likely involving the promoter. promoters in other temperate and tropical lines similar to CML52 showed enhanced expression of ZmbZIP60u by heat. The contribution of the CML52 promoter to heat induction of was confirmed by analyzing the CML52 and B73 promoters linked to a luciferase reporter and assayed in heat-treated maize protoplasts.