RNA polymerase III (RNAPIII) is specialized for transcription of short, abundant nonprotein-coding RNAs, such as tRNAs and 5S rRNA, which have fundamental roles in the protein synthesis. Highly abundant ribosomal proteins, tRNAs and rRNAs, are required to achieve high-efficiency functioning of the protein translation machinery in the maize endosperm, which accounts for 90% of the dry seed weight and serves as the main organ of nutrient storage. Transcriptional regulation of protein-coding genes by RNAPII has been extensively studied in almost all model organisms, but regulation of tRNAs and 5S rRNA expression have been considerably less understood especially in plants.

Previous studies have demonstrated that PLATZ (plant AT-rich sequence- and zinc-binding) family proteins are a novel class of plant-specific zinc-dependent DNA-binding proteins and are classified as transcription factors (TFs) in plant TFs databases. However, none of them has been genetically characterized to regulate specific genes in plants.

Recently, through genetic cloning and functional characterization, Dr. WU Yongrui and his colleagues at CAS Center for Excellence in Molecular Plant Sciences, Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, reported a maize classic endosperm-specific mutant floury3 (fl3).
In this research, they discovered that the fl3 mutation causes severe defects in endosperm development and a dramatic reduction in seed weight. The mutant phenotype only occurs when fl3 is transmitted through the female. fl3 is caused by a dominant mutation and regulated by genomic imprinting, leading to a semi-dominant behavior.
Yeast two-hybrid screening and bimolecular luciferase complementation (BiLC) experiments further showed that Fl3 interacts with two critical factors, RPC53 and TFC1, in the RNAPIII transcription complex. The fl3 mutation causes tremendous cutting-down in levels of many tRNAs and 5S rRNA, as well as dramatic alterations in transcriptome. Consequently, those morphs are responsible for the defects in endosperm development and filling in fl3 seeds. Taken together, these findings provide new insight into our understanding of the endosperm development and storage reserve filling, as well as RNAPIII transcription modulation.
This work entitled “The Maize Imprinted Gene Floury3 Encodes a PLATZ Protein Required for tRNA and 5S rRNA Transcription Through Interaction with RNA Polymerase III” has been published in The Plant Cell (Li, et al., 2017).

This work was supported by the National Natural Science Foundation of China, Chinese Academy of Sciences, the Ministry of Science and Technology of China and a Chinese Thousand Talents Program Grant.

Author contact:
WU Yongrui, principal investigator
Institute of Plant Physiology & Ecology
Chinese Academy of Sciences
300 Fenglin Road
Shanghai 200032
Phone: (86) 21- 54924341
Email: yrwu@sibs.ac.cn

                                                                                                 A proposed model for FL3/fl3 controlling the storage filling in the maize endosperm.