Characterizing the Expression Pattern of miR167-Regulated ZmArf3 and ZmArf30 in Maize Inflorescences

Abstract

Plant growth and development depends on the activity of meristems, pools of stem cells that generate leaves and other organs. Maize (corn) produces two inflorescences, the tassel and ear, which are patterned by a series of branching events that depend on the initiation of axillary meristems. The maize fuzzy tassel (fzt) mutant contains a mutation in dicer-like1, a key enzyme require for microRNA (miRNA) biogenesis in plants. MiRNAs small non-coding RNAs repress target mRNA expression and function in diverse developmental and physiological processes in plants. fzt plants have severe vegetative and reproductive defects, including highly branched inflorescences. Most miRNAs are moderately reduced in fzt plants compared to normal controls, but some miRNAs are more dramatically affected. One of the most significantly reduced miRNA in fzt plants is miR167d-3p, which is reduced >25-fold in fzt mutant tassel primordia compared to normal controls. miR167 targets mRNAs that encode the auxin response factors (ARF), ARF3 and ARF30 in maize. ARFs function as part of the auxin-signaling network to regulate transcription of auxin sensitive genes. The plant hormone auxin is necessary for virtually all aspects of plant growth, including axillary meristem initiation. The highly branched inflorescences in fzt initiate many more axillary meristem primordia than normal, consistent with increased auxin signaling. ZmArf3 and ZmArf30 are related to the functionally redundant Arabidopsis ARF6 and ARF8, which are also targeted by miRNA167. ZmArf3 and ZmArf30 have similar expression patterns in the maize tassel, localizing in all meristem regions, including the inflorescence meristem, branch meristem, spikelet pair meristem, spikelet meristem, the upper and lower floral meristems, and are also strongly expressed in initiating organ primordia and tassel vasculature. In situ hybridization experiments with ZmArf3 and ZmArf30 in normal and fzt tassel tissue show little differences in expression pattern domains, and we are investigating whether the fzt phenotype is due to increased auxin transport and signaling using ZmArf3 and ZmArf30 mutants. However, the expression patterns documented suggest a unique role for ZmArf3 and ZmArf30 in the initiation of axillary meristems and lateral organ primordia in the maize inflorescences.