NOVEL ALLIES OF ENDOCYCLE DIRECTED CELL AND ORGAN GROWTH MODULATION PATHWAYS IN PLANTS

How plants modulate growth and definite size of their organs is a fundamental question in plant biology. In agronomic context, organ size provides ultimate forecast of yield and total biomass which are key determinants of food and biofuel crop production. Typically, organ growth is controlled by coordinated maneuver of cell proliferation, expansion and differentiation into particular tissues. Many studies have highlighted the role of endoreduplication in development and physiology, however, the underlying mechanisms linking endopolyploidy with cell growth and final organ size are still nature’s secrets. Previously, UBIQUITIN-SPECIFIC PROTEASE 14 (UBP14) encoded by DA3 (“DA” means big in Chinese) was described to function with anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase through interacting its inhibitor ULTRAVIOLET-B INSENSITIVE 4 (UVI4) to modulate endoreduplication and organ growth in Arabidopsis thaliana. The Arabidopsis da3-1 mutant possesses larger cotyledons, leaves and flowers with higher ploidy levels. Using Next Generation Mapping (NGM) approach, I genetically identify several modifier genes of da3-1 from EMS mutagenized Arabidopsis populations that influence the ploidy and growth phenotypes of da3-1. I selected two of them for functional analysis and designated as mod1-1 and mod2-1. The cellular and genetic studies suggested that they strongly represses the cellular ploidy and synergistically regulate the organ growth phenotypes of da3-1. The DNA and protein sequence analysis revealed that they are key players of splicing complex in higher eukaryotes, however, their role in endoreduplication or any organ growth related mechanisms has never been described. The expression pattern analysis of MOD1 and MOD2 revealed that they are highly expressed in transcriptionally active and differentiating cells. Bimolecular fluorescence complementation (BiFC), co-immunoprecipitation and pull down experiments demonstrate that MOD1 and MOD2 physically interacts with UBP14 and CDKB1;1 in vivo and in vitro. Further studies proves that UBP14 influences the stability of MOD1 and MOD2. We already knew that cdkb1;1-1 enhanced the ploidy phenotype of ubp14-1 and uvi4-1, however, in contrast mod1-1 and mod2-1 suppresses the cellular ploidy of uvi4-1 and cdkb1;1-1 suggesting MOD1 and MOD2 are downstream components of UBP14 in endocycle pathway. Overall, these findings define a genetic and molecular mechanism of MOD1 and MOD2 function in endoreduplication and plant organ growth regulation.