Unraveling cold tolerance in maize

Li et al. discover a cold-tolerance allele that was not selected during maize domestication.

Background: Cold stress is an important environmental factor that limits plant growth, development, and geographical distribution. Maize originated in Mexico and is sensitive to low temperatures. Cold stress not only inhibits the germination and seedling growth of maize, but it also affects ear formation and grain filling, resulting in decreased crop production. However, the molecular and genetic basis of the cold response in maize remains unclear.

Question: We aimed to identify genes that regulate the cold response in maize and dissect the underlying mechanism.

Findings: We identified the transcription factor bZIP68, a negative regulator of the cold-stress response, from a population of transgenic maize plants overexpressing more than 700 maize genes. bZIP68 interacts with MPK8 (MITOGEN-ACTIVATED PROTEIN KINASE8), which is also a negative regulator of the cold-stress response. MPK8-mediated phosphorylation of the Ser250 of bZIP68 not only increases the protein stability of bZIP68 at 4°C but also improves its binding ability to the promoter of the transcription factor gene DREB1.7. Re-sequencing analysis of the promoter region of bZIP68 revealed that the nucleotide diversity (π) was strongly reduced in maize compared with its ancestor teosinte, supporting the notion that bZIP68 was selected during maize domestication. A 358-bp Indel was selected in the bZIP68 promoter region during maize domestication, which leads to increased bZIP68 expression and decreased cold tolerance in maize.

Next steps: We determined that the bZIP68 locus was a target of selection during early domestication; however, the cold-tolerance allele of bZIP68 was not selected. Therefore, it will be interesting to explore why bZIP68 was selected during maize domestication. Meanwhile, we plan to generate cold-resistant maize varieties by editing the bZIP68 promoter.