Heterosis or hybrid vigor refers to the superior phenotypic traits of hybrids relative to their parental inbred lines. An imbalance between the expression levels of two parental alleles in the F1 hybrid has been suggested as a mechanism of heterosis. Here, based on genome-wide allele-specific expression analysis using RNA sequencing technology, 1689 genes exhibiting genotype-dependent allele-specific expression (genotype-dependent ASEGs) were identified in the embryos, and 1390 genotype-dependent ASEGs in the endosperm, of three maize F1 hybrids. Of these ASEGs, most were consistent in different tissues from one hybrid cross, but nearly 50% showed allele-specific expression from some genotypes but not others. These genotype-dependent ASEGs were mostly enriched in metabolic pathways of substances and energy, including the tricarboxylic acid cycle, aerobic respiration, and energy derivation by oxidation of organic compounds and ADP binding. Mutation and overexpression of one ASEG affected kernel size, which indicates that these genotype-dependent ASEGs may make important contributions to kernel development. Finally, the allele-specific methylation pattern on genotype-dependent ASEGs indicated that DNA methylation plays a potential role in the regulation of allelic expression for some ASEGs. In this study, a detailed analysis of genotype-dependent ASEGs in the embryo and endosperm of three different maize F1 hybrids will provide an index of genes for future research on the genetic and molecular mechanism of heterosis.

Genomic imprinting is a classic epigenetic phenomenon related to the uniparental expression of genes. Imprinting variability exists in seeds and can contribute to observed parent-of-origin effects on seed development. Here, we conducted allelic expression of the embryo and endosperm from four crosses at 11 days after pollination (DAP). First, the F1 progeny of B73(♀) × Mo17(♂) and the inducer line CAU5 were used as parents to obtain reciprocal crosses of BM-C/C-BM. Additionally, the F1 progeny of Mo17(♀) × B73(♂) and CAU5 were used as parents to obtain reciprocal crosses of MB-C/C-MB. In total, 192 and 181 imprinted genes were identified in the BM-C/C-BM and MB-C/C-MB crosses, respectively. Then, by comparing the allelic expression of these imprinted genes in the reciprocal crosses of B73 and CAU5 (BC/CB), fifty-one Mo17-added non-conserved genes were identified as exhibiting imprinting variability. Fifty-one B73-added non-conserved genes were also identified by comparing the allelic expression of imprinted genes identified in BM-C/C-BM, MB-C/C-MB and MC/CM crosses. Specific Gene Ontology (GO) terms were not enriched in B73-added/Mo17-added non-conserved genes. Interestingly, the imprinting status of these genes was less conserved across other species. The cis-element distribution, tissue expression and subcellular location were similar between the B73-added/Mo17-added conserved and B73-added/Mo17-added non-conserved imprinted genes. Finally, genotypic and phenotypic analysis of one non-conserved gene showed that the mutation and overexpression of this gene may affect embryo and kernel size, which indicates that these non-conserved genes may also play an important role in kernel development. The findings of this study will be helpful for elucidating the imprinting mechanism of genes involved in maize kernel development.

Maize (Zea mays L.) is a globally important crop, but its sensitivity to cold stress leads to substantial yield losses in temperate zones, where low temperatures, particularly below 10 °C, critically constrain cultivation in high-altitude and high-latitude regions. Recent progress has identified key cold-tolerance genes, dissected natural variations, elucidated signaling pathways, and enabled multi-omics network construction. Some studies have provided evidence for the proposed coupling of ZmICE1 with amino acid metabolism and COR gene expression, the association of COOL1 promoter variation with HY5 binding and high-latitude adaptation, and the involvement of HSF21 in lipid homeostasis without an apparent yield penalty. Several novel genes (ZmWAK3, ZmMAPK4, TIP4;3, ZmDUF966-10) have been identified as promising candidates, expanding the genetic reservoir for further validation. Mechanistically, an unconventional ZmBSK4-ZmMAPK4 cascade, calcium-MAPK crosstalk, histone modifications, and small RNA-mediated regulation have been implicated in cold responses, although the extent to which each mechanism has been experimentally validated varies. Integrative approaches (chromatin accessibility, WGCNA, multi-omics) have revealed hierarchical synergies among transcription factors, epigenetics, hormones, and metabolism. Favorable alleles (COOL1HapA, HSF21Hap1, and the TIP4;3 beneficial haplotype) represent potential targets for molecular design, while CRISPR/Cas9 offers opportunities to accelerate gene editing and pyramiding. This review synthesizes these advances, and offers a comprehensive framework for cold-tolerance breeding in maize.