No abstract

The anaphase-promoting complex/cyclosome (APC/C) represents a large multisubunit E3-ubiquitin ligase complex that controls the unidirectional progression through the cell cycle by the ubiquitination of specific target proteins, marking them for proteasomal destruction. Although the APC/C's role is largely conserved among eukaryotes, its subunit composition and target spectrum appear to be species specific. In this review, we focus on the plant APC/C complex, whose activity correlates with different developmental processes, including polyploidization and gametogenesis. After an introduction into proteolytic control by ubiquitination, we discuss the composition of the plant APC/C and the essential nature of its core subunits for plant development. Subsequently, we describe the APC/C activator subunits and interactors, most being plant specific. Finally, we provide a comprehensive list of confirmed and suspected plant APC/C target proteins. Identification of growth-related targets might offer opportunities to increase crop yield and resilience of plants to climate change by manipulating APC/C activity. Expected final online publication date for the Annual Review of Cell and Developmental Biology Volume 38 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Anaphase Promoting Complex/Cyclosome (APC/C) controls unidirectional progression through the cell cycle by marking key cell cycle proteins for proteasomal turnover. Its activity is temporally regulated by the docking of different activating subunits, known in plants as CELL DIVISION PROTEIN 20 (CDC20) and CELL CYCLE SWITCH 52 (CCS52). Despite the importance of the APC/C during cell proliferation, the number of identified targets in the plant cell cycle is limited. Here, we used the growth and meristem phenotypes of Arabidopsis thaliana CCS52A2-deficient plants in a suppressor mutagenesis screen to identify APC/C CCS52A2 substrates or regulators, resulting in the identification of a mutant cyclin CYCA3;4 allele. CYCA3;4 deficiency partially rescues the ccs52a2-1 phenotypes, whereas increased CYCA3;4 levels enhance the ccs52a2-1 phenotypes. Furthermore, whereas CYCA3;4 proteins are promptly broken down after prophase in wild-type plants, they remain present in later stages of mitosis in ccs52a2-1 mutant plants, marking them as APC/C CCS52A2 substrates. Strikingly, increased CYCA3;4 levels result in aberrant root meristem and stomatal divisions, mimicking phenotypes of plants with reduced RETINOBLASTOMA-RELATED PROTEIN 1 (RBR1) activity. Correspondingly, RBR1 hyperphosphorylation was observed in CYCA3;4 gain-of-function plants. Our data thus demonstrate that an inability to timely destroy CYCA3;4 contributes to disorganized formative divisions, possibly in part caused by the inactivation of RBR1.

32The Anaphase Promoting Complex/Cyclosome (APC/C) controls unidirectional progression 33 through the cell cycle by marking key cell cycle proteins for proteasomal turnover. Its 34 activity is temporally regulated by the docking of different activating subunits, known in 35 plants as CDC20 and CCS52. Despite the importance of the APC/C during cell 36 proliferation, the number of identified targets in the plant cell cycle is limited. Here, we 37 used the growth and meristem phenotypes of Arabidopsis CCS52A2-deficient plants in a 38 suppressor mutagenesis screen to identify APC/C CCS52A2 substrates or regulators, resulting 39 in the identification of a mutant cyclin CYCA3;4 allele. CYCA3;4 deficiency partially 40 rescues the early ccs52a2-1 phenotypes, whereas increased CYCA3;4 levels enhances them. 41 Furthermore, whereas CYCA3;4 proteins are promptly broken down after prophase in 42 wild-type plants, they remain present in later stages of mitosis in ccs52a2-1 mutant plants, 43 marking them as APC/C CCS52A2 substrates. Strikingly, CYCA3;4 overexpression results in 44 aberrant root meristem and stomatal divisions, mimicking phenotypes of plants with 45 reduced RBR1 activity. Correspondingly, RBR1 hyperphosphorylation was observed in 46 CYCA3;4-overproducing plants. Our data thus demonstrate that an inability to timely 47 destroy CYCA3;4 attributes to disorganized formative divisions, likely in part caused by 48 the inactivation of RBR1.49 4 109 ENHANCER (TE) / TILLERING AND DWARF (TAD1) was shown to mediate the 110 ubiquitination and subsequent degradation of MONOCULM 1 (MOC1), called LATERAL 111 6SUPPRESSOR (LAS) in Arabidopsis, a GRAS-family transcription factor that promotes shoot 112 branching and tillering (Lin et al., 2012; Xu et al., 2012). 113 Here, we have utilized an ethyl methanesulfonate (EMS) suppressor screen to identify 114 novel APC/C CCS52A2 targets, based on the growth inhibitory phenotype of ccs52a2-1 knockout 115 plants. We show that one of the identified revertants encodes a mutant allele of the CYCA3;4 116 gene and demonstrate this cyclin to be a specific target of APC/C CCS52A2 to ensure correct stem 117 cell organization. 118 119 RESULTS 120 Identification of pkn2 as a ccs52a2-1 Suppressor Mutant 121 Compared to wild type (WT, Col-0) plants, ccs52a2-1 mutant seedlings display a short root 122 phenotype (Figures 1A and 1B; Supplemental Figures 1A and 1B) (Vanstraelen et al., 2009; 123 Heyman et al., 2013). This phenotype was used to screen for putative targets or regulators of the 124 APC/C CCS52A2 ubiquitin ligase complex through a mutagenesis revertant screen. Therefore, ethyl 125 methanesulfonate (EMS)-mutagenized ccs52a2-1 plants were screened in the M 2 generation for a 126 recovered root growth. Out of a total of 260 initially identified revertants, 33 were confirmed in 127 the next generation. Among these, one revertant mutation, named pikmin 2 (pkn2), yielded a root 128 length in between that of WT and ccs52a2-1 mutant plants (Figures 1A to 1C; Supplemental 129 Figures 1A to 1C). 1...

The anaphase-promoting complex/cyclosome (APC/C) marks key cell cycle proteins for proteasomal breakdown, thereby ensuring unidirectional progression through the cell cycle. Its target recognition is temporally regulated by activating subunits, one of which is called CELL CYCLE SWITCH 52 A2 (CCS52A2). We sought to expand the knowledge of identified APC/C targets by using the severe growth phenotypes of CCS52A2-deficient Arabidopsis thaliana plants as a readout in a suppressor mutagenesis screen, resulting in the identification of the previously undescribed gene called PIKMIN1 (PKN1). PKN1 deficiency rescues the disorganized root stem cell phenotype of the ccs52a2-1 mutant, whereas an excess of PKN1 inhibits growth of ccs52a2-1 plants, indicating the importance of PKN1 abundance for proper development. Accordingly, the lack of PKN1 in a wild-type background negatively impacts cell division, while its ectopic expression promotes proliferation. PKN1 shows a cell cycle phase-dependent accumulation pattern, localizing to microtubular structures, including the preprophase band, the mitotic spindle, and phragmoplast. PKN1 is conserved throughout the plant kingdom, with its function in cell division being evolutionary conserved in the liverwort Marchantia polymorpha. Our data thus demonstrate that PKN1 represents a novel, plant-specific gene with a rate-limiting role in cell division, which is proteolytically controlled by the CCS52A2-activated APC/C.

The anaphase-promoting complex/cyclosome (APC/C) marks key cell cycle proteins for proteasomal breakdown, thereby ensuring unidirectional progression through the cell cycle. Its target recognition is temporally regulated by activating subunits, one of which is called CELL CYCLE SWITCH 52 A2 (CCS52A2). We sought to expand the knowledge on the APC/C by using the severe growth phenotypes of CCS52A2-deficient Arabidopsis (Arabidopsis thaliana) plants as a readout in a suppressor mutagenesis screen, resulting in the identification of the previously undescribed gene called PIKMIN1 (PKN1). PKN1 deficiency rescues the disorganized root stem cell phenotype of the ccs52a2-1 mutant, whereas an excess of PKN1 inhibits growth of ccs52a2-1 plants, indicating the need for control of PKN1 abundance for proper development. Accordingly, the lack of PKN1 in a wild-type background negatively impacts cell division, while its systemic overexpression promotes proliferation. PKN1 shows a cell cycle phase-dependent accumulation pattern, localizing to microtubular structures, including the preprophase band, the mitotic spindle, and phragmoplast. PKN1 is conserved throughout the plant kingdom, with its function in cell division being evolutionary conserved in the liverwort Marchantia polymorpha. Our data thus demonstrate that PKN1 represents a novel, plant-specific gene with a role in cell division that is likely proteolytically controlled by the CCS52A2-activated APC/C.