**1. Introduction**

Severe drought causes a grievous decline in crop yield by negatively affecting plant growth and reproduction [1,2]. Maize (*Zea mays* L.)is considered a major crop for food, feed, and fuel, but its production is frequently hampered by water scarcity [3,4]. While the molecular mechanisms of drought stress response in plants have not been fully elucidated, many transcription factors (TFs)such as DREB1/CBF, MYB, and AREB/ABF, which regulate drought-responsive genes have been well-studied [5–7]. These reports support the idea that identification of key TFs will help us to better understand the molecular and cellular responses to drought stress.

*Teosinte branched 1*/*Cycloidea*/*Proliferating (TCP)* genes encode plant-specific TFs and are named after the first three functionally characterized members of this TF family—*Teosinte Branched 1* (*TB1*)in maize (*Zea mays* L.), *Cycloidea* (*CYC*)in snapdragon (*Antirrhinum majus*), and *Proliferating Cell nuclear antigen Factor* (*PCF*)in rice (*Oryza sativa*) [8]. This class of TFs share a highly conserved TCP domain, which contains a 59-amino acid, non-canonical basic-Helix-Loop-Helix (bHLH)structural motif that

allows DNA binding, protein–protein interaction, and protein nuclear localization [8–10]. More than 20 TCP TF members were identified in various plant species, such as *Arabidopsis*, rice, tomato, cotton, sorghum, and wheat [11–17]. Based on the TCP domain, these genes were divided into two classes. Class I is the PCF class; class II is subdivided into two clades—CIN (*CINCINNATA* of *Antirrhinum*) and CYC/TB1 [10,15,18]. Many members of the TCP family have been shown to be involved in the regulation of many biological processes during plant growth and development, such as leaf development, branching, floral organ morphogenesis, and senescence [8,19–22]. Possible mechanisms for the regulation were studied. These mechanisms were found to involve either a direct transcriptional control of the cell cycle genes by TCP TFs, or an indirect adjustment of the hormone activity [23]. For example, in *Arabidopsis* seeds, the DELLA proteins GAI (GA-Insensitive)and RGA (Repressor of GA)formed an unproductive complex with the class II TCP protein AtTCP14 or AtTCP15, which prevented the binding of TCPs to promoters of the core cell cycle genes [24]. In turn, GA might induce ubiquitination and degradation of DELLA proteins to relieve the constitutive inhibition of TCP transcriptional activity [24]. Interestingly, the orthologous TCP protein LANCEOLATE was found to participate in GA biosynthesis by upregulating the *SIGA-oxidase 1* gene [25]. It was also found that a subset of class I TCP proteins, such as AtTCP1, AtTCP2, AtTCP3, and AtTCP14, behaved in a similar fashion by acting as an inducer or repressor to influence the biosynthesis or signaling of several hormones during different developmental processes [26–28].

Abscisic acid (ABA)is a known stress response hormone that can mitigate physiological and environmental stresses, including drought stress, by inducing the closure of stomata, thus reducing water loss [29–32]. Although the *TCP* gene family is primarily involved in the regulation of plant growth and leaf development, other functions, such as involvement in the ABA signaling pathway, have been noted for specific *TCP* genes. For example, overexpression of *OsTCP19* in *Arabidopsis* significantly conferred both drought and heat tolerance during seedling establishment and in mature plants [33]. Furthermore, the interaction of OsTCP19 with OsABI4, which encodes a TF involved in the ABA signal transduction, suggests its function in fine-tuning drought-induced ABA signaling [33]. Additionally, creeping bentgrass plants (*Agrostis stolonifera*) that overexpresses *Osa-miR319*, in which four putative target genes, *AsPCF5*, *AsPCF6*, *AsPCF8*, and *AsTCP14* are down-regulated, significantly enhance plant tolerance to salt and drought stress associated with an increased leaf wax content and water retention [34]. It is noteworthy that AtTCP14 antagonizes ABA signaling by interacting with the DOF6 (DNA binding with one finger)TF, preventing the activation of the downstream ABA biosynthetic gene *ABA1* (*ABA deficient 1*) and other ABA-responsive genes in *Arabidopsis* seeds [35]. In contrast, *AtTCP18*, also known as *BRC1* (*Branched 1*), induces *ABF3* (*ABA responsive elements-binding factor 3*), and *ABI5* (*ABA insensitive 5*)—two key regulators of the ABA response—to maintain ABA signaling when the axillary buds enter dormancy [36,37]. These reports indicate a strong association of TCPs with ABA-mediated abiotic stress signaling. However, how the TCP TF genes function in maize, especially in response to drought stress, still remains to be elucidated. Due to the rapid linkage disequilibrium (LD)decay in the maize genome, association study is able to provide a gene-level resolution, which facilitates the genetic detection of several complex traits, such as drought tolerance [38–40]. However, limited allelic variations underlying drought tolerance have been identified [41], and rarely favorable alleles could be used for the genetic improvement of drought tolerance in maize.

Here, we comprehensively analyzed the *ZmTCP* gene family in the maize genome. Previous research has identified 29 *TCP* genes in the maize genome and subsequently, phylogeny, gene structure, chromosomal location, gene duplication, and expression levels of the 29 *ZmTCP* genes were investigated [42]. In this study, we searched against the updated maize genome B73\_RefGen\_v3 and identified 46 *ZmTCP* genes, and further systematically analyzed to determine their phylogenetic relationships and synteny with rice, sorghum, and *Arabidopsis TCPs*. In addition, we studied the expression profiles of these *ZmTCP*s upon drought stress. Importantly, a family-based, genome-wide association study revealed a significant association between the natural variations of *ZmTCP42* and maize drought tolerance. Ectopic expression of *ZmTCP42* in *Arabidopsis* led to enhanced drought tolerance, validating its function in drought tolerance.
