Abstract
Auxin plays a key role across all land plants in growth and developmental processes. Although auxin signaling function has diverged and expanded, differences in the molecular functions of signaling components have largely been characterized in Arabidopsis (Arabidopsis thaliana). Here, we used the nuclear Auxin Response Circuit recapitulated in yeast (Saccharomyces cerevisiae) system to functionally annotate maize (Zea mays) auxin signaling components, focusing on genes expressed during the development of ear and tassel inflorescences. All 16 maize auxin/indole-3-acetic acid repressor proteins were degraded in response to auxin with rates that depended on both receptor and repressor identities. When fused to the maize TOPLESS homolog RAMOSA1 ENHANCER LOCUS2, maize auxin/indole-3-acetic acids were able to repress AUXIN RESPONSE FACTOR transcriptional activity. A complete auxin response circuit comprising all maize components, including the ZmAFB2/3 b1 maize AUXIN SIGNALING F-BOX (AFB) receptor, was fully functional. The ZmAFB2/3 b1 auxin receptor was more sensitive to hormone than AtAFB2 and allowed for rapid circuit activation upon auxin addition. These results validate the conserved role of predicted auxin response genes in maize as well as provide evidence that a synthetic approach can facilitate broader comparative studies across the wide range of species with sequenced genomes.
Highlights
Auxin plays a key role across all land plants in growth and developmental processes
The maize B73 genome contains 34 auxin/indole-3-acetic acid (Aux/IAA) repressor genes, 16 of which were selected for further analysis here because they are expressed in developing maize inflorescences (Davidson et al, 2011; Bolduc et al, 2012; Eveland et al, 2014) and represent members of six of the eight Aux/IAA clades found in monocots
Synthetic Maize Auxin Response Circuit expression patterns strongly resembled those previously reported for BIF1 and BIF4 (Galli et al, 2015) and suggest a high degree of functional redundancy in this family of transcriptional regulators
Summary
Auxin plays a key role across all land plants in growth and developmental processes. auxin signaling function has diverged and expanded, differences in the molecular functions of signaling components have largely been characterized in Arabidopsis (Arabidopsis thaliana). The ZmAFB2/3 b1 auxin receptor was more sensitive to hormone than AtAFB2 and allowed for rapid circuit activation upon auxin addition These results validate the conserved role of predicted auxin response genes in maize as well as provide evidence that a synthetic approach can facilitate broader comparative studies across the wide range of species with sequenced genomes. The nuclear auxin response is among the best understood signaling pathways in plants and is an excellent model to tackle questions about functional divergence in gene families within a single species and in network functions across multiple species. The wealth of genetic, genomic, and biochemical tools in Arabidopsis have made it possible to rapidly build a strong foundational understanding of auxin response To both explore the extent of shared auxin signaling properties across plants and fully interrogate the connection between natural variation in protein sequences and functional innovations in plant development, this nuclear auxin response must be examined in more species. Maize ARFs show similar preferences for auxin response elements to Arabidopsis ARFs (Galli et al, 2018), and the specificity of activator ARFs within maize is not explained by differences in their promoter preferences (Lanctot et al, 2020)
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