ABSTRACT
Selection for flowering time and probably for the timing or rate of the senescence have contributed to the adaptation of maize from a reduced tropical niche to almost any latitude in the globe. The choice of an appropriate flowering time is contributing to mitigate the effect of the climate change. Flowering time is controlled by at least 90 genes of small effects and a few genes of large effect that were consistently found in several independent studies. Among those genes stands out ZCN8, which is homologous of FT, the florigen in Arabidopsis, and functions as an integrator of the floral pathways. This gene forms a module with at least three other genes, ZmCCT9, ZmCCT10, and ZmRap2.7 that repress ZCN8 in long days and delayed the flowering under those conditions. The attenuation of the expression of those genes by transposon insertion probably played an important role in the expansion of maize to temperate areas. Large structural variants seem to have also played a relevant role on the adaptation of maize to those regions. The high density of markers achieved with modern genotyping technologies, the mapping experiments with a large number of genotypes and the statistical models with improved accuracy have allowed the development of equations based on molecular markers that predict pretty well the flowering time of the genotypes. The senescence, although being a complex process involving many different metabolic paths, is not controlled by as many genes as the flowering time. During the senescence the photosynthetic genes are down-regulated and the genes involved in catabolism and transport are up-regulated. The transcription factors, particularly the NACs play an important role coordinating the changes at senescence. The progress in the knowledge of the genetics of senescence and, particularly flowering time has been enormous in the last decade. The rapid advance is expected to continue with efficient molecular technologies as CRISPR/Cas9. All that knowledge will contribute to the development of maize varieties with optimum phenology and adaptation to each of the specific environments where they are grown.
