Why do plants need so many cyclin-dependent kinase inhibitors?

Abstract

Cell cycle regulation is fundamental to growth and development, and Cyclin-Dependent Kinase Inhibitors (CKIs) are major negative regulators of the cell cycle. Plant genomes encode substantially more CKIs than metazoan or fungal genomes. Plant CKIs fall into 2 distinct families, KIP-RELATED PROTEINS (KRPs) and SIAMESE-RELATED proteins (SMRs). SMRs can inhibit both S-phase and M-phase CDK complexes in vitro and are transcribed throughout the cell cycle, yet SMRs do not inhibit DNA replication in vivo. This suggests that SMRs must be activated post transcriptionally after the start of S-phase, but the mechanism of this hypothesized activation is unknown. Recent work indicates that even distantly related SMRs have the same biochemical function, and that differential transcriptional regulation likely maintains their distinct roles in integrating various environmental and developmental signals with the cell cycle.

Deceleration of cell cycle underpins a switch from proliferative- to terminal division in plant stomatal lineage

“…To identify a factor that plays a role in the decelerating cell cycle during the ACD-to-SCD transition, we surveyed publicly-available transcriptome data (Han et al, 2018;Lau et al, 2014) to search for CKIs that are induced by SPCH and MUTE. Among the 7 KRP and 17 SIM /SMR genes (Kumar and Larkin, 2017;Peres et al, 2007), only SMR4 exhibits marked increase by the induced MUTE overexpression (iMUTE) (Figure 2A). On the other hand, a majority of SMRs and KRPs is either downregulated or unchanged upon iMUTE or iSPCH (Figure 2A).…”

“…Our study revealed that SMR4 is a direct MUTE target and expresses during the transition from proliferating meristemoid state to differentiating GMC state. SMR proteins are known to promote endoreduplication in trichomes, pavement cells and sepal giant cells (Hamdoun et al, 2016;Kumar and Larkin, 2017;Roeder et al, 2010;Walker et al, 2000). However, unlike trichomes and pavement cells, stomatal lineage cells do not undergo endoreduplication (Melaragno et al, 1993).…”

Multiple mechanisms explain how reduced KRP expression increases leaf size of Arabidopsis thaliana

“…Strong overexpression of KRPs inhibits cell cycle progression by blocking both G 1 /S and G 2 /M transition, resulting in plants with large cells and reduced endoreplication (Kumar & Larkin, ) as well as induction of cell death (Schnittger et al ., ). Conversely, weaker ectopic expression of KRPs inhibits only mitotic entry and promotes endoreplication (Kumar & Larkin, ). When KRP4 was overexpressed in the central domain of the shoot apex using the CLV3 promoter, cell size was strongly increased, but cell‐size homeostasis was restored once cells exited the expression domain (Serrano‐Mislata et al ., ).…”

“…Ectopic expression of KRP genes in Arabidopsis results in small, serrated leaves with reduced cell number, but increased cell size (De Veylder et al ., ; Bemis & Torii, ; Jun et al ., ). Strong overexpression of KRPs inhibits cell cycle progression by blocking both G 1 /S and G 2 /M transition, resulting in plants with large cells and reduced endoreplication (Kumar & Larkin, ) as well as induction of cell death (Schnittger et al ., ). Conversely, weaker ectopic expression of KRPs inhibits only mitotic entry and promotes endoreplication (Kumar & Larkin, ).…”

Suppressor of Gamma Response 1 Modulates the DNA Damage Response and Oxidative Stress Response in Leaves of Cadmium-Exposed Arabidopsis thaliana

“…To date, 7 KRPs and 17 SMR proteins have been identified in A. thaliana. Some SMR proteins have been suggested to play a role in the DNA damage response (DDR) (Yi et al, 2014;Kumar and Larkin, 2017). Yi et al (2014) demonstrated that exposure to hydroxyurea induced DNA double strand breaks (DSBs), likely through induction of ROS, and increased SMR5 and SMR7 transcript levels in A. thaliana roots.…”