Shuwan Chen

The SCARECROW (SCR) transcription factor governs cell-type patterning in plant roots and Kranz anatomy of leaves, serving as a master regulator of root and shoot morphogenesis. Foxtail millet (Setaria italica), characterized by a compact genome, self-pollination, and a short growth cycle, has emerged as a C₄ model plant. Here, we revealed two SCR paralogs in foxtail millet—SiSCR1 and SiSCR2—which exhibit high sequence conservation with ZmSCR1/1h (Zea mays), OsSCR1/2 (Oryza sativa), and AtSCR (Arabidopsis thaliana), particularly within the C-terminal GRAS domain. Both SiSCR genes exhibited nearly identical secondary structures and physicochemical profiles, with promoter analyses revealing five conserved cis-regulatory elements. Robust phylogenetic reconstruction resolved SCR orthologs into monocot- and dicot-specific clades, with SiSCR genes forming a sister branch to SvSCR from its progenitor species Setaria viridis. Spatiotemporal expression profiling demonstrated ubiquitous SiSCR gene transcription across developmental stages, with notable enrichment in germinated seeds, plants at the one-tip-two-leaf stage, leaf 1 (two days after heading), and roots during the seedling stage. Co-expression network analysis revealed that there is a correlation between SiSCR genes and other functional genes. Abscisic acid (ABA) treatment led to a significant downregulation of the expression level of SiSCR genes in Yugu1 roots, and the expression of the SiSCR genes in the roots of An04 is more sensitive to PEG6000 treatment. Drought treatment significantly upregulated SiSCR2 expression in leaves, demonstrating its pivotal role in plant adaptation to abiotic stress. Analysis of heterologous expression under the control of the 35S promoter revealed that SiSCR genes were expressed in root cortical/endodermal initial cells, endodermal cells, cortical cells, and leaf stomatal complexes. Strikingly, ectopic expression of SiSCR genes in Arabidopsis led to hypersensitivity to ABA, and ABA treatment resulted in a significant reduction in the length of the meristematic zone. These data delineate the functional divergence and evolutionary conservation of SiSCR genes, providing critical insights into their roles in root/shoot development and abiotic stress signaling in foxtail millet.

The CLAVATA3/EMBRYO-SURROUNDING REGION (CLE) genes encode signaling peptides that play important roles in various developmental and physiological processes. However, the systematic identification and characterization of CLE genes in foxtail millet (Setaria italica L.) remain limited. In this study, we identified and characterized 41 SiCLE genes in the foxtail millet genome. These genes were distributed across nine chromosomes and classified into four groups, with five pairs resulting from gene duplication events. SiCLE genes within the same phylogenetic group shared similar gene structure and motif patterns, while 34 genes were found to be single-exon genes. All SiCLE peptides harbored the conserved C-terminal CLE domain, with highly conserved positions in the CLE core sequences shared among foxtail millet, Arabidopsis, rice, and maize. The SiCLE genes contained various cis-elements, including five plant hormone-responsive elements. Notably, 34 SiCLE genes possessed more than three types of phytohormone-responsive elements on their promoters. Comparative analysis revealed higher collinearity between CLE genes in maize and foxtail millet, which may be because they are both C₄ plants. Tissue-specific expression patterns were observed, with genes within the same group exhibiting similar and specific expression profiles. SiCLE32 and SiCLE41, classified in Group D, displayed relatively high expression levels in all tissues except panicles. Most SiCLE genes exhibited low expression levels in young panicles, while SiCLE6, SiCLE24, SiCLE25, and SiCLE34 showed higher expression in young panicles, with SiCLE24 down-regulated during later panicle development. Greater numbers of SiCLE genes exhibited higher expression in roots, with SiCLE7, SiCLE22, and SiCLE36 showing the highest levels and SiCLE36 significantly down-regulated after abscisic acid (ABA) treatment. Following treatments with ABA, 6-benzylaminopurine (6-BA), and gibberellic acid 3 (GA3), most SiCLE genes displayed down-regulation followed by subsequent recovery, while jasmonic acid (JA) and indole-3-acetic acid (IAA) treatments led to upregulation at 30 min in leaves. Moreover, identical hormone treatments elicited different expression patterns of the same genes in leaves and stems. This comprehensive study enhances our understanding of the SiCLE gene family and provides a foundation for further investigations into the functions and evolution of SiCLE genes in foxtail millet.