One-third of the global population is affected by micronutrient deficiency, particularly folate. Although folate synthesis has been relatively well characterized, few folate-related genes in maize have been cloned, and the molecular mechanism regulating folate synthesis in maize remains unclear. In this study, transcriptome and proteome analyses of three waxy maize inbred lines with high, medium, and low folate contents were performed to identify key genes controlling folate biosynthesis. Pairwise comparisons revealed 21 differentially expressed genes and 20 differentially expressed proteins potentially associated with folate biosynthesis in the three lines. Six key folate-associated genes, ZmMocos2, ZmGGH, ZmADCL2, ZmCBR1, ZmSHMT, and ZmPurH, were identified. These genes encode enzymes that potentially function in folate biosynthesis. Functional validation of one of these genes, ZmADCL2, using an EMS mutant (Mut9264) showed that a 4-base insertion in an exon increased the folate content of fresh maize kernels 1.37-fold that of the wild type. ZmADCL2 was considered a potential target for generating maize lines with higher folate content. KEGG enrichment analysis of differentially expressed genes and proteins showed that several pathways in addition to folate biosynthesis were likely indirectly involved in folate metabolism and content (e.g., glycine, serine, and threonine metabolism; purine metabolism; cysteine and methionine metabolism; alanine, aspartate and glutamate metabolism; glutathione metabolism; and pyruvate metabolism. The transcriptome and proteomic data generated in this study will help to clarify the mechanisms underlying folate accumulation and aid breeding efforts to biofortify maize with folate.

Maize is the most widely cultivated, used and highest yield crop in the world and China. Southern corn rust (SCR) is an air borne disease caused by Puccinia polysora Underw., which mainly occurs in tropical and subtropical maize growing areas. In recent years, SCR has become one of the major diseases in the Huang-Huai-hai maize production region due to the climate change, which directly leads to compromised grain quality and poor yields in maize and significantly jeopardizes maize production in China. At present, SCR usually spreads in a large area within a short period of time once occurred because most maize varieties promoted in China are susceptible, and conventional chemical measures is usually in vain. Therefore, cultivating resistant cultivars by exploiting resistance genes in maize germplasm resources is the most effective and economical strategy for controlling SCR. The highly resistant germplasm is scarce in maize resources, mainly from tropical and subtropical regions, and barely no temperate germplasm can be directly used in breeding practice. Compared with foreign maize germplasm, the highly resistant maize germplasms of China were much less, mainly from local landraces or P group materials containing tropical origins with relatively limited genetic variation. The identification and cloning of SCR resistance genes in maize is essential for promoting molecular marker-assisted breeding, as well as accelerating the breeding process of new varieties with desired resistance. At present, several SCR resistance genes have been identified and cloned, laying a foundation for molecular marker-assisted selection. Over the years, Chinese breeders have developed a number of elite maize inbred lines resistant to SCR with limited resistance germplasm resources, and successfully created disease-resistant hybrids. Recent studies on the genome of SCR pathogens revealed that pathogens have differentiated into highly toxic lineages in China, thus escaping the recognition of resistance genes. Therefore, the exploration and utilization of extensive genetic resources in resistant germplasm still need to be further strengthened. In this paper, we outlined the biological characteristics and hazards of SCR, systematically summarized the research progresses in the identification and utilization of maize germplasm resources resistant to SCR, the mapping and cloning of SCR resistant genes and the breeding of resistant varieties, and prospect the future research direction of SCR. This review will provide references for the prevention and control of SCR, as well as the breeding of resistant maize varieties.