Yujie Fng, Yuqin Zheng, Wei Lu, Jin Li, Yujing Dun, Shui Zhng,Youping Wng,*

aKey Laboratory of Plant Functional Genomics of the Ministry of Education,Jiangsu Key Laboratory of Crop Genomics and Molecular Breeding,Yangzhou University,Yangzhou 225009,Jiangsu,China

bJoint International Research Laboratory of Agriculture and Agri-Product Safety, the Ministry of Education of China, Yangzhou University,Yangzhou 225009,Jiangsu,China

Keywords:TCP miR319 Regulation Growth and development Abiotic stress

ABSTRACT Elaborate regulation of gene expression is required for plants to maintain normal growth,development, and reproduction. MicroRNAs (miRNAs) and transcription factors are key players that control gene expression in plant regulatory networks. The TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) family comprises plantspecific transcription factors that contain a conserved TCP domain of 59 amino acids.Some members of this family are targeted by miR319, one of the most ancient and evolutionarily conserved miRNAs in plants. Accumulating evidence has revealed that miR319-regulated TCP (MRTCP) genes participate extensively in plant development and responses to environmental stress. In this review, the structural characteristics and classifications of TCP transcription factors and the regulatory relationships between TCP transcription factors and miRNAs are introduced. Current knowledge of the regulatory functions of MRTCP genes in multiple biological pathways including leaf development,vascular formation,flowering,hormone signaling,and response to environmental stresses such as cold, salt, and drought is summarized. This review will be beneficial for understanding the roles of the MRTCP-mediated regulatory network and its molecular mechanisms in plant development and stress response,and provides a theoretical basis for plant genetic improvement.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2. Structures and classifications of plant TCP transcription factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3. The regulatory relationship between miRNAs and TCPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4. The regulatory role of MRTCP in plant development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1. The role of MRTCP in leaf development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2. The role of MRTCP in plant secondary cell wall formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.3. The role of MRTCP in flowering time control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.4. The role of MRTCP in plant photomorphogenesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.5. The role of MRTCP in phytohormone metabolism and signaling. . . . . . . . . . . . . . . . . . . . . . . . . . 23

5. The role of MRTCP in plant response to abiotic stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.1. The role of MRTCP in cold stress response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2. The role of MRTCP in drought and salt stress response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.Introduction

Complex and precise regulation of gene expression is required for cell metabolism, tissue formation, organ development, and adaptation to ever-changing environmental conditions over the life of a plant. Interactions between transcription factors and corresponding cis-acting elements as well as microRNAs(miRNAs) and their targets are major determinants of gene expression at the transcriptional and post-transcriptional levels.The TEOSINTE BRANCHED 1, CYCLOIDEA, and PROLIFERATING CELL NUCLEAR ANTIGEN BINDING FACTOR (TCP) family is a family of plant-specific transcription factors that modulate the expression levels of their targets by binding specific promoter sequences [1]. miRNAs, a type of endogenous noncoding small RNA, 20-24 nt in length, are widespread in eukaryotes [2]. Some plant TCP genes have been shown to be targeted by miR319 via base-pair recognition, thereby affecting biological processes including hormone synthesis, cell proliferation and differentiation,and environmental response[3-5]. Here, the roles of miR319-regulated TCPs (MRTCP) in plant growth,development,and stress response are reviewed(Table 1).

2. Structures and classifications of plant TCP transcription factors

Transcription factors,also known as trans-acting factors,are a class of regulatory proteins that can activate or suppress the transcription of target genes by interacting with cis-acting elements in the promoter region of the genes [34]. A total of 32,070 transcription factors in 165 plant species belonging to 58 families had been identified by 2017, 2296 of them in Arabidopsis thaliana[35].TCP transcription factors constitute a small family of plant-specific transcription factors and are represented by their first three characterized members:TEOSINTE BRANCHED 1 (TB1) from maize, CYCLOIDEA (CYC)from snapdragon (Antirrhinum majus), and PROLIFERATING CELL NUCLEAR ANTIGEN BINDING FACTOR (PCF) from rice[34]. The N-termini of TCP proteins contain a noncanonical basic-helix-loop-helix (bHLH) region that is responsible for DNA binding, nuclear localization, and oligomerization [1]. A conserved sequence of 59 amino acid residues was designated as the TCP domain, a name derived from maize TB1,goldfish grass CYC,and rice PCF1/PCF2 [1,34,36,37].

TCP transcription factors are found in many plant species and involved in diverse aspects of plant growth and development, including branching, flower development, female and male gamete development, leaf development, and senescence [3,8,10,38-40]. TCP factors have been found to take part in the regulation of circadian rhythms and the biosynthesis and signaling pathways of jasmonic acid (JA),auxin, gibberellin, and other hormones [38,41]. The TCP family in Arabidopsis contains 24 members, which can be divided into two classes (I and II) according to the sequence similarity of the TCP domain [42]. A yeast two-hybrid assay and an electrophoretic mobility shift assay(EMSA)indicated that TCP factors in the same class have a tendency to form homo- or hetero-dimers [42]. Compared with the class I members, the class II members have four more amino acid residues in the TCP domain,and some class II proteins(such as members of TB1/CYC group)contain a conserved R domain composed of polar amino acids that mediates protein-protein interactions by forming hydrophilic helices[38].DNA binding sites of the class I and class II TCP proteins were identified by random binding site selection and EMSA,showing that class I TCP proteins bind GGNCCCAC whereas class II TCP proteins bind GTGGNCCC [42]. The DNA binding specificities and transcriptional regulation of the two classes of TCP factors were suggested to vary in plants[42].

?

In general, class I TCP proteins are involved mainly in promoting the cell cycle, while class II TCP proteins are involved mainly in inhibiting cell cycle progression [42,43].PCF1 and PCF2, two of the first experimentally characterized TCP factors in rice,form homo-or hetero-dimers and bind the PCNA promoter to mediate its specific expression in the meristem [34]. Arabidopsis TCP14 and TCP15 were reported to be involved in the activation and inhibition of cell proliferation during leaf and flower development, respectively[44,45].TCP20 was found to regulate cell elongation,cell division,and nuclear cell differentiation in Arabidopsis [39]. TCP22 negatively controls leaf senescence and regulates cell proliferation with TCP7, TCP8,and TCP23[46]. TCP1 governs the transcription of DWF4 by interacting with the DWF4 promoter to affect BR synthesis, and further regulates plant growth and development [47]. TCP16 plays an important role in early pollen development, and repression of TCP16 in Arabidopsis using RNA interference (RNAi) technology caused abnormal pollen in half of the transgenic plants [48]. TCP5 was shown to participate in the regulation of ethylene-mediated petal development by binding the promoter of the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene ACS2 [49]. Arabidopsis TCP23-overexpressing lines showed a late-flowering phenotype together with morphological changes including altered leaf shape, leaf margin whitening,and altered root size, suggesting that AtTCP23 participates in the regulation of flowering time and development[50].

The class II proteins include TB1, CYC, and CINCINNATA(CIN) can be divided into two categories: CYC/TB1 and CIN[51]. Eight CIN genes in Arabidopsis (TCP2, TCP3, TCP4, TCP5,TCP10,TCP13,TCP17,and TCP24)from the CIN branch showed partial functional redundancy in regulating leaf development[51]. The CIN gene in snapdragon was implicated in the development of lateral organs and the regulation of cytokinin and auxin signaling pathway genes [52,53], while the CYC/TB1 genes were reported to regulate flowering [54]. The CYC/TB1 genes are further classified into three categories(CYC1, CYC2, and CYC3) based on phylogenetic analysis [54].The CYC1/TB1 gene plays an important role in controlling axillary bud differentiation and branching [38]. The CYC2 gene plays a major role in controlling adaxial and abaxial symmetry in flowers [55,56]. CYC3 is expressed in both flower primordia and lateral branches but exerts little control over flower branching, and the function of CYC3 in regulating flower development is unknown [57,58]. Thus far, many plant TCP genes have been characterized (Tables 1 and 2).

3. The regulatory relationship between miRNAs and TCPs

miRNAs are well known for their negative regulation of gene expression at the post-transcriptional level and have been extensively studied in plants[106].These 20-24 nt small noncoding RNAs are generated from precursors with special stem-loop structures and cleave or regulate the translation of target gene mRNA by binding the target mRNA according to sequence complementarity [107]. Previous research [108]has established that plant miRNAs play critical roles in a wide range of developmental processes, including auxin signaling;metabolism; and root, stem, leaf and flower organ development.The regulatory function of miRNAs depends principally on controlling the expression of target genes. Accordingly,clarifying the functions of target genes would be useful for identifying the roles of miRNAs and dissecting the regulatory network mediated by miRNA-target modules. Many of the target genes of miRNAs encode regulatory proteins such as transcription factors and kinases [109]. Previous studies suggest that TCP genes are essential components of miRNA related regulatory networks.Schommer et al.[110]confirmed that several TCP genes in the CIN branch in Arabidopsis were regulated by miR319. These TCP genes were shown to be inhibited by miR319 at the post-transcriptional level,whereas other TCP genes lacking miR319 target sequence were not affected [110]. TCP2, TCP3, TCP4, TCP10, and TCP24 in Arabidopsis and their class II TCP homologs in other plant species were suggested to be regulated by miR319[110],which is evolutionarily conserved. In rice, five (OsPCF5, OsPCF6,OsPCF7, OsPCF8, and OsTCP21) of 26 TCP genes were verified to be targets of miR319[17].

Intriguingly, Koyama et al. [111]and Rodriguez et al. [112]reported that some miR319-targeted TCP genes also control the expression of miRNAs. For instance, chromatin immunoprecipitation(ChIP)analysis showed that Arabidopsis TCP3 can directly activate the expression of miR164 by binding the GnCCC motif in the promoter of miR164A[111].Another study[112]suggested that TCP4 regulates the expression of miR396,as overexpression of TCP4 in Arabidopsis led to increased expression of miR396 and reduced growth hormone-releasing(GRF)activity.A previous study[4]showed that TCP4 can bind GGACCA(C) or its reverse complementary sequence, (G)TGGTCC,which is present upstream of the miR396b precursor in Arabidopsis. A series of experiments confirmed that TCP4 directly regulates the expression of miR396b by binding ciselements in the promoter region of miR396b, thereby inhibiting cell proliferation in plants[4].

4. The regulatory role of MRTCP in plant development

4.1. The role of MRTCP in leaf development

Considerable progress has been achieved in characterizing the role of MRTCP genes in regulating plant development(Fig.1). Accumulated evidence suggests that MRTCP genes play a crucial role in plant leaf morphogenesis[15].Silent mutations in target genes that impair their interaction with miRNA are typically generated to investigate target gene regulation by miRNAs. Transgenic Arabidopsis plants overexpressing mutant forms of TCP2 and TCP4 with synonymous changes that prevented sequence identification by mature miR319 and miR319-directed inhibition had smaller leaves and longer hypocotyls than wild type(WT)plants,whereas tcp2,tcp4,and tcp10 single-knockout mutants had slightly larger leaves than WT plants [3,4]. The endogenous promoter was used to drive the mutant form of TCP4 fused to the GFP protein(rTCP4:GFP),and the transgenic plants showed green rosette leaves that were smaller,rounder and darker than those in WT plants[4].Leaf size was significantly increased in loss-of-function tcp2 tcp4 double mutant compared to WT, and the leaves of tcp2 tcp4 tcp10 triple mutant and tcp2 tcp3 tcp4 tcp10 quadruple mutant were crinkled [15]. In tomato, LANCEOLATE (LA)encodes a CIN-class TCP factor that is regulated by miR319,and the normally large compound leaves were converted into small simple leaves in partially dominant La mutants [113].Expression of LAmdriven by the LA promoter caused the production of reduced, simple leaves and partially fused primary leaflets in transgenic tomato plants (LApro ?LAm),suggesting that LA positively regulates differentiation during leaf development[20].Mutation of the CIN gene in Antirrhinum caused the transformation of flat leaves to crinkled leaves[114].

?

?

MRTCP genes not only affect leaf size, shape, and curvature, but also control the pace of leaf senescence in plants. Senescence was accelerated in rTCP4:GFP transgenic plants[4].Transgenic plants expressing a hyperactivated form of TCP4 (TCP4:VP16-C) showed faster leaf initiation and early senescence, revealing the role of TCP4 in plant maturation programs[8].Pro-35S:mTCP4 Arabidopsis leaves showed earlier yellowing than leaves of WT Arabidopsis, while tcp3/4/10 mutant plants showed delayed leaf yellowing and maintained a higher chlorophyll content than WT plants[9].

Fig.1- Regulatory network of MRTCP genes during plant growth and development.Red arrows represent promotion.Green line segments represent repression.Blue and yellow line segments represent regulation.Light yellow ellipses represent MRTCPs involved in plant growth and development.Orange ellipses represent targets or downstream components of MRTCPs.Purple ellipses represent phytohormone-associated pathways.Light blue rectangles represent miRNAs.Light green rectangles represent biological processes in plants.At,Arabidopsis thaliana;Os,Oryza sativa;Sl, Solanum lycopersicum;Pt,Populus tomentosa;Pv, Panicum virgatum;Phv,Phaseolus vulgaris;Brp,Brassica rapa;Gh,Gossypium hirsutum;Mt.,Medicago truncatula.

4.2. The role of MRTCP in plant secondary cell wall formation

The secondary cell wall in plants is composed mainly of lignin, cellulose, and hemicellulose. TCP4 was considered to activate secondary cell wall biosynthesis in Arabidopsis[12,115]. The vessel wall thickness of transgenic plants constitutively expressing miR319-resistant TCP4 (rTCP4) was markedly thicker than that of WT plants, and increased abundance of lignin and cellulose was detected in rTCP4-OX lines [12]. VND7 functions as a vital regulator of xylem vessel differentiation in Arabidopsis [116]. Further study [12]suggested that TCP4 activates VND7 transcription by directly interacting with its promoter, thereby activating the expression of genes involved in secondary cell wall biosynthesis and programmed death to accelerate vessel differentiation.

Another target gene of miR319, TCP24, was believed to reduce the thickening of the secondary cell wall in roots and floral organs[16].Optimal shear force produced by secondary cell wall thickening is necessary for anther dehiscence,which is essential for plant fertilization[117].Overexpression of mTCP24,the miR319-resistant version of TCP24,prevented the thickening of secondary walls in the anther endothecium, leading to anther dehiscence failure and sterility [16].Suppression of TCP24 by introduction of p35S:TCP24SRDX into Arabidopsis prompted secondary cell wall thickening in the anther endothecium as well as in roots [16]. Several genes responsible for lignin and cellulose biosynthesis were downregulated in p35S:mTCP24 plants, testifying to the negative regulatory role of TCP24 in secondary cell wall thickening[16].

4.3. The role of MRTCP in flowering time control

MRTCP factors in plants can also control flowering time. A loss-of-function TCP4 mutant (tcp4-1) showed delayed flowering time compared with that of WT, whereas plants expressing a hyperactivated form of TCP4 (TCP4:VP16-C)showed an early-flowering phenotype [8]. Additional investigation revealed that TCP4 acts as a transcription activator of CO, which regulates photoperiod-dependent flowering [118].Early flowering was observed in 35S:TCP4 transgenic Arabidopsis plants; however, late flowering was observed in co-9 mutants expressing 35S:TCP4 constructs, showing that TCP4-mediated early flowering is dependent on CO [118].TCP4 could directly bind the CO promoter, facilitate CO transcription, and physically interact with PHYTOCHROME AND FLOWERING TIME 1 (PFT1), which is required for the function of TCP4 in promoting flowering [118].

4.4. The role of MRTCP in plant photomorphogenesis

Cryptochromes (CRYs) and phytochromes (PHYs) are photoreceptors that play essential roles in light-associated plant growth and development processes including seed germination, seedling elongation, leaf expansion, photoperiodic flowering, and the circadian clock [119]. As a blue-light receptor in Arabidopsis, CRY1 not only modulates deetiolation responses, stomata movements, and root growth in a blue-light-dependent manner,but also mediates complex signaling networks to control plant growth and development[120]. TCP2 specifically interacted with CRY1 in a blue lightdependent manner in a yeast two-hybrid system, and CRY1-TCP2 interaction in plant cells was further verified by bimolecular fluorescence complementation (BiFC) assay [6].Overexpression of TCP2 led to repressed hypocotyl elongation under blue light and advanced cotyledon development under blue or red light [6]. ChIP-qPCR results suggested that TCP2 might control hypocotyl elongation by binding the chromatin regions of HYH and HY5 in a blue light-induced manner and promoting their mRNA accumulation [6]. This study suggested the role of TCP2 as a downstream component of the CRY1 photosensory signaling pathway.

4.5. The role of MRTCP in phytohormone metabolism and signaling

Earlier observations indicated that MRTCP genes participate in the regulation of multiple aspects of plant growth and development by regulating hormone metabolism. JA is a lipid-derived phytohormone whose content is regulated by a positive feedback loop in plants [121]. LOX2, a key enzyme in the JA and oxylipin synthesis pathways, catalyzes the first critical step in the oxidation of α-linolenic acid to 13(S)-HPOT during JA synthesis[122].Previous research[4,10]established that TCP4 regulates leaf senescence by directly binding the LOX2 promoter and controlling the transcript abundance of LOX2. The TCP binding motif (GGACCA) was detected in the promoter regions of eight oxylipin biosynthetic genes,and the expression levels of JA biosynthetic genes were approximately fourfold higher in rTCP4:GFP transgenic plants than in WT plants, revealing the essential role of TCP4 in regulating JA-mediated senescence in plants[4].

IAA3/SHY2 is one of the direct targets of TCP3 in Arabidopsis,as revealed by ChIP analysis,and TCP3 negatively regulates IAA3/SHY2 transcriptional accumulation [111].Arabidopsis plants overexpressing an miR319-resistant form of mTCP3 (p35S::mTCP3) showed auxin-associated developmental defects including a decussate phyllotaxy, reduced primary and lateral roots,and impaired apical dominance[7].Comparative expression analysis of the auxin response reporter pDR5::GUS and auxin efflux carrier pPIN1::PIN1-GFP in p35S::mTCP3 transgenic and WT plants suggested that the overexpression of mTCP3 weakens auxin transport ability and/or sensitivity in Arabidopsis, providing additional evidence of the negative effects of TCP3 on the auxin response[7].

5. The role of MRTCP in plant response to abiotic stresses

5.1. The role of MRTCP in cold stress response

Plants are constantly exposed to unfavorable environmental conditions. Several researchers have found an association between MRTCP and plant response to abiotic stresses.MiR319 positively regulates cold tolerance in rice [123].Repression of the expression of OsPCF5 and OsPCF8, two MRTCP genes in rice, led to increased cold tolerance of rice seedlings after chilling acclimation[17].Another two potential targets of miR319,OsPCF6 and OsTCP21,showed obvious coldinduced expression,and this induction pattern was markedly inhibited in miR319b-overexpressing plants [18]. OsPCF6 and OsTCP21 knockdown lines produced by RNAi showed greater cold tolerance than WT plants, partly owing to improved reactive oxygen species(ROS)scavenging[18].

Stem-loop RT-PCR showed that miR319 was up-regulated by cold(4°C)stress for 24 h in sugarcane plants,and PCF5 and PCF6 were predicted to be targeted by miR319 in sugarcane[27].PCF6 was further confirmed to be the target of miR319 by cleavage site mapping using 5′-rapid amplification of cDNA ends(RACE),and its expression was reduced by 50%in plants under cold treatment for 24 h [27]. The TCP family in the cassava genome contains 36 members [24]. MeTCP2a,MeTCP2b, MeTCP3a, MeTCP3b, and MeTCP4 (which are homologous to the miR319-targeted genes-AtTCP2/3/4/10/24) were shown to contain miR319 binding sites [24,25]. RNA sequencing (RNA-Seq) analysis of the leaves of cassava seedlings treated with cold (4 °C) stress showed that the expression of MeTCP3a and MeTCP4 declined after cold stress [26].

5.2. The role of MRTCP in drought and salt stress response

The transcript abundance of mature miR319 in creeping bentgrass (Agrostis stolonifera) was higher in plants under drought or high-salinity stress than in plants under normal growth conditions [28]. Constitutive overexpression of OsamiR319a not only increased the width and thickness of the leaf blade and the stem diameter, but also improved drought and high-salinity tolerance in the transgenic plants [28]. Under drought and high-salinity stress, transgenic plants overexpressing Osa-miR319a displayed superior cell membrane integrity and water retention [28]. Osa-miR319a transgenic plants had higher photosynthesis rates and stomatal conductance than control plants under drought stress, and accumulated less Na+than control plants under salt stress [28]. The up-regulation expression of four putative miR319 target genes(AsPCF5,AsPCF6,AsPCF8,and AsTCP14)was observed after salt or desiccation stress, suggesting the involvement of these target genes in response to salt and drought stress [28].Comparative transcriptome analysis of the salt-tolerant Yakutiye and salt-sensitive Zulbiye genotypes of common bean revealed that Pvul-TCP-1,-11,-13,-22,and-27,which are targeted by miR319, showed salt-responsive expression patterns [22]. Pvul-TCP-1 was expressed at a lower level in roots under salt stress than in untreated roots [22]. The expression of Pvul-TCP-13, -20, and -26 was more than twofold higher in Zulbiye leaves under salt stress than in those under normal conditions, and salt stress induced a more than twofold increase in Pvul-TCP-20 expression in Yakutiye leaves compared to leaves under normal conditions [22]. Our investigation found that miR319 and the TCP target genes are responsive to drought and salt stress in rapeseed (Brassica napus),and that miR319 might negatively regulate the drought and salt tolerance of seedlings by controlling the expression of the TCP targets.These results suggested that miR319-targeted TCP genes might perform divergent functions in monocotyledonous and dicotyledonous plant species, although the miR319/TCP module in plants is evolutionarily conserved.

6. Conclusions and perspectives

The study of miRNAs and their targets has become a plant research focus over the last few decades,and a large number of miRNAs and their target genes in various plant species have been characterized. MiR319 is an evolutionarily ancient miRNA that was first identified via activation-tagging genetic screening. The miR319/TCP-mediated regulatory module plays pivotal roles in a complex gene expression regulatory network and is implicated in multiple key biological processes during the life of a plant. The miR319/TCP module not only affects cell proliferation, leaf and flower shape, stem branching, and development of the pistil and stamen, but also participates in the biosynthesis and transport of phytohormones including JA and auxin, as well as plant responses to abiotic stresses [124]. Furthermore, recent findings in Populus tomentosa and Panicum virgatum suggested that MRTCPs might be potential targets for improving plant tolerance to biotic stress and yield.

Some knowledge of the structures and biological roles of MRTCP transcription factors has been gained over the last few years. However, our understanding of the molecular mechanism by which MRTCPs regulate plant growth and development as well as their effect on environmental adaptation remains deficient. Which genes are directly targeted by MRTCPs? How do these target genes work in plants? Moreover, functional redundancy is often found among members of the TCP family, making it difficult to study thoroughly the functions of TCP factors and requiring the creation of geneediting genetic materials for multiple TCP genes in plants using CRISPR technology.

Several TCP genes targeted by miR319 exhibit distinct spatiotemporal expression patterns, suggesting that they function at different plant developmental stages. How miR319 precisely regulates different TCP factors remains an open issue. The evolutionary modes and characteristics of miR319 and TCP target genes in plants also await elucidation.Studies have revealed the diverse functions of MRTCPs in plants. Identification of MRTCPs that play vital roles in plant growth and development and stress response, as well as the MRTCP-mediated regulatory network and its molecular mechanism, will provide resources for crop genetic improvement and offer directions for practical crop breeding.

Declaration of competing interest

Authors declare that there are no conflicts of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(31501335,31872874),the Natural Science Foundation of Jiangsu Province (BE2018356), the Undergraduate Training Program for Innovation and Entrepreneurship(XKYCX18_120, XKYCX19_151), the Top Talent Support Program and the Qinglan Project of Yangzhou University for Yujie Fang, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Project of Special Funding for Crop Science Discipline Development.We are grateful to Prof. James C Nelson in Kansas State University for a critical revision of the manuscript.

CRediT authorship contribution statement

Yujie Fang contributed to the conception of the review and drafted the manuscript. Yuqian Zheng and Wei Lu contributed to manuscript preparation. Jian Li, Yujing Duan, and Shuai Zhang contributed to collection and sorting of the references.Youping Wang revised the manuscript.All authors read and approved the final manuscript.