Regulatory Roles of MicroRNAs in Maize Growth, Development and Abiotic Stress Responses

MicroRNAs (miRNAs) are a class of endogenous non-coding small RNAs with a length of 20 24 nucleotides that present widely in eukaryotes. miRNAs regulate the expression of their target genes posttranscriptionally through transcript cleavage or translation inhibition. Recent studies have shown that miRNAs are involved in a wide variety of biological processes of plant growth, development and stress responses, and play essential roles in regulating agronomic traits of crops. Maize is an important staple food, feed and industrial raw material, and thus it is crucial to improve maize yield and quality to ensure world food security. Compared to model plants Arabidopsis and rice, the studies on maize miRNAs are still relatively limited. The understanding of functions and regulatory mechanisms of miRNAs in maize is essential for engineering important agronomic traits genetically through molecular breeding. In this article, we review the discovery and identification of maize miRNAs, most of which are tissue-specific and spatiotemporally expressed. Up to now, a total of 325 mature miRNAs from 174 precursors were identified in maize genome, belonging to 29 miRNA families. We also systematically summarize the functions of key components in maize miRNA biogenesis pathways, including DCL, AGO and HEN1. Mutations in these miRNA processing proteins result in pleiotropic developmental phenotypes, suggesting the important regulatory roles for miRNAs in maize development. MiRNAs whose functions have been characterized in maize growth and development are discussed, including those involved in root formation, leaf morphogenesis, grain maturation and reproductive development. Furthermore, function of miRNAs in responses to abiotic stresses, such as salt stress, drought stress, temperature stress and nutrition stress are elaborated, with the highlight on miR169-NF-YA, miR399-PHO2, and miR528-LAC3 regulatory modules. We also discuss the current existed issues and future perspectives in maize miRNA study. Despite the identification of a large number of maize miRNAs, research on the functions and regulatory mechanisms of miRNAs in maize is still very limited, and it is still required to generate essential genetic materials and take advantage of multiple experimental strategies to perform in-depth and systematic studies on miRNA and their target genes. It is believed that miRNAs are valuable gene resources and a better understanding of miRNA-mediated regulatory network would be beneficial for engineering maize varieties with improved agronomic traits.