Understanding Callus Types in Maize by Genetic Mapping and Transcriptional Profiling

Plant transformation efficiency is highly dependent on species, individual genotypes, and tissue types. In maize, immature embryos are regularly used for transformation. The process relies heavily on callus development, as it is intricately associated with somatic embryogenesis and subsequent plant regeneration, both of which directly affect transformation efficiency. Immature embryos of the segregation progeny derived from the two inbred parents, a transformation-amenable line A188 and a recalcitrant line B73, can be cultured to form two primary callus types: Type I and Type II. The Type II callus grows faster and is a favorable type for regeneration. Here, Type I and II calli from the B73xA188 F2 population were genotyped by Genotyping-By-Sequencing (GBS). Quantitative trait locus (QTL) analysis of the callus type identified QTLs at chromosomes 2, 5, 6, 8, and 9. The result was largely supported by the bulk segregant RNA-seq (BSR-seq) genetic analysis using RNA from separately pooled Type I and II calli. Both analyses revealed that an allele of A188 on chromosome 6 and B73 alleles on chromosomes 2, 5, 8, and 9 promoted the formation of the Type II callus. Differentially expressed genes (DEGs) between the Type II and I F2 calli were also identified. In addition, the A188 calli developed from the same immature embryos often exhibit heterogeneous morphology, including the fast- and slow-growing callus sectors. The transcriptional comparison between the two sectors was performed to identify DEGs. Both sets of DEGs were enriched in genes involved in cell-wall organization and wax biosynthesis pathways.