Molecular Basis of Kernel Development and Kernel Number in Maize (Zea mays L.)

Grain yield per ear of maize (Zea mays L.) is composed of both kernel number and grain weight. The number of kernels on an ear is determined by not only the number of kernel rows which is closely related to the inflorescence development, but also the number of fertile florets generated by the flower meristem. Therefore, those genes for inflorescence architecture and flower development are potentially involved in the genetic control of kernel number. Maize kernel is a single-seeded fruit comprised of the maternally derived pericarp, and embryo and endosperm derived from double fertilization. Both embryo and endosperm account for the vast majority of the mature kernel mass, and directly determine the kernel size and weight. In this paper, we outlined the genetic controls of kernel number with the emphasis on the inflorescence and floret related genes that are involved in the CLAVATA- WUSCHEL (CLV-WUS) feedback loop, hormone biosynthesis and signaling, floral organ development and sex determination. In particular, we described the regulatory network models for interplays among phytohormones including auxin, gibberellin, cytokinin and strigolactone in the inflorescence architecture and floral organ development. We also summarized those embryo and endosperm developmental genes involving in processing and editing of mitochondrial transcripts, transcription and translation of some chloroplast DNAs as well as nuclear RNAs. Most of these genes encode PPR proteins targeted to mitochondria or plastids. Recently, several studies have identified a new pathway to control kernel development by regulating the transcription and processing of pre-mRNA within the nucleus. Here, we also discussed the association between these genes and kernel number or kernel weight, and the potential areas of research for deciphering molecular mechanisms of grain yield in maize.