RT Journal Article SR Electronic T1 Genome and time-of-day transcriptome of Wolffia australiana link morphological extreme minimization with un-gated plant growth JF bioRxiv FD Cold Spring Harbor Laboratory SP 2020.03.31.018291 DO 10.1101/2020.03.31.018291 A1 Todd P. Michael A1 Evan Ernst A1 Nolan Hartwick A1 Philomena Chu A1 Douglas Bryant A1 Sarah Gilbert A1 Stefan Ortleb A1 Erin L. Baggs A1 K. Sowjanya Sree A1 Klaus J. Appenroth A1 Joerg Fuchs A1 Florian Jupe A1 Justin P. Sandoval A1 Ksenia V. Krasileva A1 Ljudmylla Borisjuk A1 Todd C. Mockler A1 Joseph R. Ecker A1 Robert A. Martienssen A1 Eric Lam YR 2020 UL http://biorxiv.org/content/early/2020/04/01/2020.03.31.018291.abstract AB Wolffia is the fastest growing plant genus on Earth with a recorded doubling time of less than a day. Wolffia has a dramatically reduced body plan, primarily growing through a continuous, budding-type asexual reproduction with no obvious phase transition. Most plants are bound by the 24-hour light-dark cycle with the majority of processes such as gene expression partitioned or phased to a specific time-of-day (TOD). However, the role that TOD information and the circadian clock plays in facilitating the growth of a fast-growing plant is unknown. Here we generated draft reference genomes for Wolffia australiana (Benth.) Hartog & Plas to monitor gene expression over a two-day time course under light-dark cycles. Wolffia australiana has the smallest genome size in the genus at 357 Mb and has a dramatically reduced gene set at 15,312 with a specific loss of root (WOX5), vascular (CASP), circadian (TOC1), and light-signaling (NPH3) genes. Remarkably, it has also lost all but one of the NLR genes that are known to be involved in innate immunity. In addition, only 13% of its genes cycle, which is far less than in other plants, with an overrepresentation of genes associated with carbon processing and chloroplast-related functions. Despite having a focused set of cycling genes, TOD cis-elements are conserved in W. australiana, consistent with the overall conservation of transcriptional networks. In contrast to the model plants Arabidopsis thaliana and Oryza sativa, the reduction in cycling genes correlates with fewer pathways under TOD control in Wolffia, which could reflect a release of functional gating. Since TOD networks and the circadian clock work to gate activities to specific times of day, this minimization of regulation may enable Wolffia to grow continuously with optimal economy. Wolffia is an ideal model to study the transcriptional control of growth and the findings presented here could serve as a template for plant improvement.