Functional Conservation in the SIAMESE-RELATED Family of Cyclin-Dependent Kinase Inhibitors in Land Plants

Abstract

The best-characterized members of the plant-specific SIAMESE-RELATED (SMR) family of cyclin-dependent kinase inhibitors regulate the transition from the mitotic cell cycle to endoreplication, also known as endoreduplication, an altered version of the cell cycle in which DNA is replicated without cell division. Some other family members are implicated in cell cycle responses to biotic and abiotic stresses. However, the functions of most SMRs remain unknown, and the specific cyclindependent kinase complexes inhibited by SMRs are unclear. Here, we demonstrate that a diverse group of SMRs, including an SMR from the bryophyte Physcomitrella patens, can complement an Arabidopsis thaliana siamese (sim) mutant and that both Arabidopsis SIM and P. patens SMR can inhibit CDK activity in vitro. Furthermore, we show that Arabidopsis SIM can bind to and inhibit both CDKA;1 and CDKB1;1. Finally, we show that SMR2 acts to restrict cell proliferation during leaf growth in Arabidopsis and that SIM, SMR1/LGO, and SMR2 play overlapping roles in controlling the transition from cell division to endoreplication during leaf development. These results indicate that differences in SMR function in plant growth and development are primarily due to differences in transcriptional and posttranscriptional regulation, rather than to differences in fundamental biochemical function.

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

“…18,20,21 Of particular note, Kumar and coworkers found that 10 different SMRs, including an SMR from the bryophyte Physcomitrella patens, can complement the trichome cell division phenotype of Arabidopsis sim mutants. 13 Additionally, individual KRP and the SMR genes show distinct patterns of tissue-specific and stress-regulated expression, consistent with the individual genes playing roles in integrating the cell cycle with a wide variety of signals. 12,22,23 This level of transcriptional control is presumably secondary to the post-transcriptional control of CKI protein levels acting within the cell cycle itself that was described in the previous section.…”

“…[15][16][17] In contrast, while SIM overexpression can induce endoreplication, resulting in DNA contents as high as 128C, overexpression of SIM or other SMRs has never been observed to inhibit DNA replication or cause cell death. 8,13 Thus, available evidence indicates that SMRs only inhibits M-phase, while KRPs can block entry into both M-and S-phases.…”

“…Recent work firmly establishes that SMRs interact with and inhibit both CDKA:1 and CDKB1;1 both in vitro and in vivo. 13 Because CDKA;1 is the main Arabidopsis G1/S CDK, yet SMR overexpression does not inhibit S-phase entry in vivo, this recent work suggests that SMRs are inactive throughout G1 and the G1/S transition, allowing S phase to proceed, but that in mitotically arrested or endoreplicating cells, SMRs are activated to block entry into mitosis. Transcription of SIM or other SMRs does not appear to be regulated relative to cell cycle phase, and thus it seems likely that SMRs are activated post-transcriptionally after the initiation of S-phase.…”

“…29 SMR2 also plays a role in restricting cell proliferation early in leaf development, negatively regulating leaf size. 13 Expression of several SMRs is regulated by biotic or abiotic stress. 12,22 SMR5 and SMR7 are direct targets of the DNA damage-responsive transcription factor SOG1, and inhibit cell proliferation and promote endoreplication in response to DNA damage.…”

“…Most land plant genomes contain multiple genes in both CKI families; for example the Arabidopsis genome contains 7 KRP genes and 17 SMR genes, numbers that are typical of angiosperm genomes. 10,13 The large number of ICK/KRPs and SMRs encoded by plant genomes raises the question of why plants need so many CKI genes.…”