Abiotic stress has the capacity to influence plant growth, development, quality, and, ultimately, yield. TCP transcription factors are widely involved in the regulation process of plant life and act as key regulators of internal and external signal responses by recruiting other proteins and regulating hormone signal pathways [17,53]. Consequently, it is of crucial interest to examine the latent activities of the TCP gene under varied abiotic stress conditions. When we predicted the cis-acting elements in the EsTCPs promoters, we found that the vast majority of these genes are responsive to stressors like drought and low temperatures and are inducible by plant hormones such as gibberellin, auxin, abscisic acid, and methyl jasmonate. In these results, ABA treatment inhibited 10 EsTCPs in roots at all time points (Figure 7). Mutual inhibition between the activity of Class I TCP and the ABA signaling pathway is expected because ABA inhibits the processes that TCP facilitates, such as plant growth, reproduction, cell division, and elongation. Based on this, the researchers found that [54] MdTCP46 expression was suppressed in apples during ABA and drought conditions, while overexpression of MdTCP46 resulted in lower sensitivity to ABA and resilience to drought stress. The distinction is that transgenic Arabidopsis and transgenic rice plants have decreased ABA sensitivity due to the TCP10 gene found in Moso bamboo (Phyllostachys edulis). Through a thorough examination of phenotypic characteristics and stress-related physiological indicators, it was discovered that the overexpression of PeTCP10 in Arabidopsis facilitated stomatal closure when subjected to ABA treatment. Under drought stress conditions, no notable variation in ABA accumulation was noted between the transgenic Arabidopsis and its wild-type counterpart. Additionally, PeTCP10 was demonstrated to impede lateral root growth via a MeJA-mediated pathway. In this scenario, PeTCP10 may work as a positive regulator of plant drought tolerance via ABA-dependent signaling systems, while acting as well as a negative regulator of lateral root growth via MeJA-mediated pathways [45]. Based on the experimental evidence, we postulate that the ABA signaling pathway may play a regulatory role in the root development of Siberian wildrye, potentially through the suppression of specific TCP gene expression, thereby modulating root growth and development. Furthermore, it has long been shown in Arabidopsis that Class I and Class II TCP proteins regulate leaf growth through the jasmonic acid signaling pathway [53]. Researchers reported that Arabidopsis plants exhibit a root hair shortening phenotype when the GrTCP11 gene in cotton is overexpressed [55]. This discovery indicates that the TCP11 gene limits root hair elongation by controlling the jasmonic acid (JA) signaling pathway. However, our research has demonstrated that the TCP gene family exhibits a complex regulatory response to methyl jasmonate (MeJA). In leaves subjected to methyl jasmonate (MeJA), the EsTCP gene’s expression was also drastically decreased (Figure S3). Interestingly, the EsPCF5 gene was significantly upregulated in the root and displayed broad tissue expression characteristics following MeJA stress. According to promoter cis-acting element analysis, EsPCF5 contains four methyl jasmonate response elements (Table S4), which possibly be the structural basis for its response to MeJA stress. On the other hand, the absence of typical methyl jasmonic acid response elements in the promoter region of the DgTCP16 gene in orchardgrass is closely linked to the significant downregulation of this gene upon MeJA treatment [17]. These results imply that the regulatory network between the methyl jasmonate and EsTCP gene family may be much more intricate than the straightforward negative regulatory relationship. It may involve a variety of regulatory mechanisms, including post-transcriptional regulation, tissue-specific expression, and a variety of cis-acting elements. Similarly, when PEG is used to simulate drought stress, the expression of genes in roots is very little compared with that in leaves, which may also be related to the stress mode of applying drugs to roots in our work, further amplifying the influence of stress treatment on gene expression. The results demonstrated that RNA sequencing under PEG-6000-induced drought stress revealed distinct gene expression profiles between drought-tolerant and drought-sensitive leaves of Siberian wildrye [56]. It was observed that certain differentially expressed genes in tolerant plants exhibited more rapid induction or inhibition. In addition, the EsSnRK2, EsLRK10, and EsCIPK5 genes may regulate stomatal closure via the abscisic acid signaling pathway, providing a novel molecular foundation for elucidating the mechanisms underlying drought stress responses in Siberian wildrye. Then, it is plausible to hypothesize that EsTCP may play a role in regulating stomatal closure via the abscisic acid signaling pathway. More generally, under the condition of low temperature, the expression of each gene also showed an obvious inhibitory effect. But we can see that the EsPCF10 gene is less affected by low temperature stress (Figure 7). Hence, we have effectively predicted the existence of several hormone response elements in the promoter region of this gene, such as the precise binding sites of gibberellin (GA), auxin, abscisic acid (ABA), and other hormones. This discovery implies that this gene’s expression may be cooperatively controlled by several hormone signaling pathways, since contributing to plant development and growth as well as stress response.

Molecular biology studies have demonstrated that the promoter region of the AsTCP gene family also contains a rich diversity of cis-acting elements. Through systematic analysis of promoter sequences, researchers identified 19 distinct types of hormone response elements, with ABRE being the most prominent, comprising 21% of all hormone response elements [57]. Although this study did not further explore the effect of exogenous ABA treatment on AsTCP expression patterns, the distribution characteristics of promoter elements strongly suggest that the AsTCP transcription factor family may be deeply involved in abiotic stress response networks in oats and play an important regulatory role in plant hormone signal transduction pathways. Plant hormone response elements have been proven to be highly conserved across a wide range of species. Using abscisic acid (ABA), auxin (IAA), and methyl jasmonate (MeJA) as examples, their respective response elements—ABRE (ABA response element), AuxRE (Auxin response element), and JARE (jasmonic acid response element)—have been further identified in Arabidopsis, a model plant, and rice, an important food crop [58,59,60]. The researchers performed a comprehensive analysis of the AtTCP8 gene targets in Arabidopsis, identifying TCP8-bound gene promoters and differentially expressed genes in tcp8 mutants. These datasets exhibited significant enrichment in components related to various plant hormone signaling pathways, including brassinosteroids (BRs), auxin, and jasmonic acid [61]. And in Platycodon grandiflorus, MeJA response elements were found to be engaged in MEJA-mediated gene expression regulation networks. Further research found that when subjected to low temperatures and MeJA therapy, 10 members of the PgbZIP gene family demonstrated substantial expression responses [62]. This occurrence is directly related to the existence of cis-regulatory elements, which emphasizes their importance. Furthermore, these highly conserved cis-acting elements are present in many phylogenetically distinct plant species. This occurrence not only verifies the functional conservation of the TCP gene family in the plant hormone signal transduction pathway from an evolutionary standpoint, but it also illustrates a potential critical regulatory role in the plant’s response to abiotic stress. On top of that, the EsPCF10 gene showed significant expression changes under both low-temperature stress and exogenous growth hormone (IAA) treatment, highlighting its critical role in integrating hormone signaling and abiotic stress response and implying that it may be involved in plant adaptive regulation to various environmental stresses with complex regulatory networks. Significantly, Class I EsTCP (PCF) genes have dual functional properties: they exhibit broad tissue-specific expression profiles while sustaining high transcriptional activity under a variety of stress settings. Indeed, these transcription factors serve critical roles in regulating plant morphogenesis and development, especially via altering hormone biosynthesis pathways and reprogramming stress-responsive signaling networks [63]. Also, they manage the processes of cell elongation and the cell cycle [10]. These observations indicate that Class I EsTCPs may play different regulatory roles in different developmental stages and may have certain advantages in coping with abiotic stress (Figure 7, Figure 8, and Figure S3). Our findings indicate that several EsTCP genes may have powerful adaptive and multifunctional regulation capacities in response to abiotic stress. EsPCF10 and EsPCF16 genes (Figure 7 and Figure S3), in particular, demonstrated significant expression characteristics under various stress conditions and were closely related to hormone signaling pathways, making them prime candidates for improving crop stress resistance and agricultural productivity in plateau areas. The work we performed was the first to thoroughly examine the expression patterns of the TCP gene family in Siberian wildrye under varied abiotic stress conditions. Compared to earlier studies, which mostly focused on the limitations of single environmental conditions such as salt stress and drought stress, our work made an important advance in experimental design: The research was expanded to include plant hormone control, with a focus on the regulatory mechanisms of major plant hormones such as abscisic acid (ABA), growth hormone (IAA), and methyl jasmonate (MeJA), as well as novel models of low temperature and drought stress. Employing this comprehensive multi-dimensional experimental framework, we systematically elucidated the expression regulation patterns of the TCP gene family in Siberian wildrye under diverse abiotic stress conditions. Our work not only addresses a critical knowledge gap in understanding the TCP gene expression profiles of Siberian wildrye under combined stress conditions but also establishes a robust experimental foundation and presents novel conceptual insights for deciphering the intricate molecular regulatory networks mediated by TCP transcription factors during plant stress responses.