Gene duplication at the Fascicled ear1 locus controls the fate of inflorescence meristem cells in maize

Abstract

Plant meristems are self-renewing groups of pluripotent stem cells that produce lateral organs in a stereotypical pattern. Of interest is how the radially symmetrical meristem produces laminar lateral organs. Both the male and female inflorescence meristems of the dominant Fascicled ear (Fas1) mutant fail to grow as a single point and instead show deep branching. Positional cloning of two independent Fas1 alleles identified an ∼160 kb region containing two floral genes, the MADS-box gene, zmm8, and the YABBY gene, drooping leaf2 (drl2). Both genes are duplicated within the Fas1 locus and spatiotemporally misexpressed in the mutant inflorescence meristems. Increased zmm8 expression alone does not affect inflorescence development; however, combined misexpression of zmm8, drl2, and their syntenic paralogs zmm14 and drl1, perturbs meristem organization. We hypothesize that misexpression of the floral genes in the inflorescence and their potential interaction cause ectopic activation of a laminar program, thereby disrupting signaling necessary for maintenance of radially symmetrical inflorescence meristems. Consistent with this hypothesis, RNA sequencing and in situ analysis reveal altered expression patterns of genes that define distinct zones of the meristem and developing leaf. Our findings highlight the importance of strict spatiotemporal patterns of expression for both zmm8 and drl2 and provide an example of phenotypes arising from tandem gene duplications.

The Impact of Fasciation on Maize Inflorescence Architecture

“…However, DRL1 and DRL2 seem to be involved in positive feedback signaling. Consistent with this speculation, duplicate copies of two transcription factor genes, MADS-box gene Zmm8 and YABBY gene DRL2, at the Fascicled ear1 (Fas1) locus, are ectopically overexpressed in the CZ of the IM (Du et al 2021), leading to an enlarged IM. In addition to these positive stem cell regulators, the bZIP transcription factor FEA4 is a negative stem cell regulator and a PERIANTHIA ortholog in maize (Pautler et al 2015).…”

Tissue-specific transcriptomics reveal functional differences in maize floral development

“…Importantly, our data included three UFM-enriched genes with known RNA expression patterns (Supplemental Figure 2 and Supplemental File 2). zmm8 /GRMZM2G102161 and zmm14 /GRMZM2G099522 encode MADS-box transcription factors that are broadly expressed in the meristem and floral organs of the upper floret, but not detected in the lower floret (Cacharron et al, 1999; Du et al, 2021). barren stalk1 (ba1) /GRMZM2G397518 encodes a bHLH protein required for axillary meristem initiation and is expressed in a diffuse pattern in UFM and in a group of cells at the UFM/LFM boundary, but not detected in LFM (Gallavotti et al, 2004).…”

“…DEG in the RNA biosynthesis group contains several classes of transcription factors (TFs) with well-known roles in plant growth and development, including APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF), myeloblastosis (MYB), homeobox, basic helix-loop-helix (bHLH), TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL NUCLEAR ANTIGEN FACTOR (TCP), and WRKY TFs ( Supplemental Figure 2 ). DEG also included TFs with known functions in maize floral development, including ba1 /GRMZM2G397518, (Gallavotti et al, 2004), branched silkless1 (bd1) /GRMZM2G307119 (Chuck et al, 2002), gnarley1 (gn1) /GRMZM2G452178 (Foster et al, 1999a; Foster et al, 1999b), teosinte branched1 (tb1) /AC233950.1_FG002 (Hubbard et al, 2002), Wavy auricle in blade1 (Wab1) / branched angle defective1 (bad1) /GRMZM2G110242 (Hay and Hake, 2004; Bai et al, 2012; Lewis et al, 2014), GRF-interacting factor1 (gif1) /GRMZM2G180246 (Zhang et al, 2018), zfl2 /GRMZM2G180190 (Bomblies et al, 2003), zmm8 /GRMZM2G102161 and zmm14 /GRMZM2G099522 (Du et al, 2021). Thus, transcriptional regulation significantly differs between UFM and LFM.…”

“…Microarray analysis indicates that approximately 9% of genes are differentially expressed in equivalently staged anthers from the upper and lower florets (Skibbe et al, 2008); thus, floret-specific gene expression persists even in differentiated floral organs. Finally, two MADS-box genes, zmm8 and zmm14 , are only detectable in the upper florets by RNA in situ hybridization (Cacharron et al, 1999; Du et al, 2021). These data support the model that UFM and LFM use distinct gene regulatory networks and have divergent developmental fates from the very earliest stages of floral development.…”

“…Surprisingly, zmm8 and zmm14 were the only two DE MADS-box genes identified in our samples (Supplemental Figure 2 and Supplemental File 2), both of which were previously shown to be strongly UFM-enriched and have been hypothesized to act as upper floret selector genes (Cacharron et al, 1999). Recent analysis of zmm8; zmm14 double mutants, however, indicate that zmm8/zmm14 promote FM meristem determinacy in both florets and do not have floret-specific functions (Du et al, 2021). Although zmm8/zmm14 are highly UFM-enriched in our data, they are also expressed in LFM (Supplemental File 1).…”