Advances in research and utilization of maize wild relatives

Abstract

<p indent="0mm">Maize (<italic>Zea mays</italic> ssp. <italic>mays</italic>, 2<italic>n</italic>=2<italic>x</italic>=20), which belongs to the <italic>Zea</italic> genera of the Maydeae tribe of the Gramineae family, is an important crop in guaranteeing national food security due to its high grain yield, forage yield and nutritive value for human food and animal feed as well as its use as an industrial material. The development and application of hybrid breeding, generous use of pesticides and fertilizers, and continuous improvement of cultivation techniques have greatly improved the yield of maize in the past several decades. However, this growth in yield has been declining. Meanwhile, the occurrence of recent extreme weather events threatens maize production. In addition, due to prevailing crop breeding practices, yield-related traits have been focused on by plant breeders, which led to narrowing genetic diversity of maize germplasm, severe cultivar homogeneity, and poor resistance to biotic and abiotic stress. Therefore, it is of the utmost importance to cultivate newer maize varieties that are highly adaptable to unfavorable environments, along with high yielding and excellent quality. Crop wild relatives (CWRs) appear promising in enhancing the genetic diversity of cultivated crops. The wild relatives of maize, such as teosinte and <italic>Tripsacum</italic> L., contain abundant genetic variation and excellent resistance to salt, cold, disease, and insect. They are a treasure for maize breeding and mining excellent exogenous gene resources. In this review, we will first discuss the phylogeny and cross-compatibility of maize and its wild relatives. Although previous studies have investigated the phylogeny and cross-compatibility of maize, teosinte and <italic>Tripsacum</italic> L. in detail, the phylogeny and cross-compatibility of maize and its wild relatives have recently been further clarified based on phenotypic, molecular geographic, and genomic evidence. Secondly, we will describe the advances in genomics of maize and its wild relatives. Over the past decade, reference genome or transcriptome sequences have been obtained for numerous crops. However, genomic studies of CWRs have trailed behind those of domesticated and other model plants. The opportunities afforded by CWRs reference genomes are only now beginning to be realized thanks to diminishing cost of next-generation sequencing technologies making population-scale data feasible. Thirdly, we will systematically cover the recent successful utilization of maize wild relatives in maize breeding for improvements in yield and quality, biotic and abiotic stress resistance, perennial maize, and forage maize. These success examples demonstrate that maize wild relatives are effective and valuable to feed back into the improvement of maize. Lastly, we will discuss the future development and research direction of maize wild relatives. It is urgent to strengthen the collection, protection and genome sequencing of maize wild relatives, and establish maize-teosinte mapping populations, including classic F₂, backcross, chromosome segment substitution line, and recombinant inbred line (RIL). Molecular markers based on the genome and transcriptome of maize and its wild relatives will be utilized to construct linkage maps and to map QTLs or genes. The cross between maize and <italic>Tripsacum</italic> is extremely difficult under the natural state, but the polyploid as bridge material (such as MTP, a maize-<italic>T</italic>.<italic> dactyloides</italic>-<italic>Z</italic>.<italic> perennis</italic> allohexaploid) can transfer <italic>T</italic>.<italic> dactyloides</italic> genes introgressed into maize. <italic>De novo</italic> domestication of maize wild relatives via gene editing technology, functional genomics and bioinformatics will accelerate maize improvement. This review aims to provide important insights into how maize wild relatives can be utilized as a powerful gene pool for improving maize varieties and guaranteeing national food security.