The Hippo/STE20 homolog SIK1 interacts with MOB1 to regulate cell proliferation and cell expansion in Arabidopsis

Abstract

Multicellular organisms co-ordinate cell proliferation and cell expansion to maintain organ growth. In animals, the Hippo tumor suppressor pathway is a master regulator of organ size. Central to this pathway is a kinase cascade composed of Hippo and Warts, and their activating partners Salvador and Mob1/Mats. In plants, the Mob1/Mats homolog MOB1A has been characterized as a regulator of cell proliferation and sporogenesis. Nonetheless, no Hippo homologs have been identified. Here we show that the Arabidopsis serine/threonine kinase 1 (SIK1) is a Hippo homolog, and that it interacts with MOB1A to control organ size. SIK1 complements the function of yeast Ste20 in bud site selection and mitotic exit. The sik1 null mutant is dwarf with reduced cell numbers, endoreduplication, and cell expansion. A yeast two-hybrid screen identified Mob1/Mats homologs MOB1A and MOB1B as SIK1-interacting partners. The interaction between SIK1 and MOB1 was found to be mediated by an N-terminal domain of SIK1 and was further confirmed by bimolecular fluorescence complementation. Interestingly, sik1 mob1a is arrested at the seedling stage, and overexpression of neither SIK1 in mob1a nor MOB1A in sik1 can rescue the dwarf phenotypes, suggesting that SIK1 and MOB1 may be components of a larger protein complex. Our results pave the way for constructing a complete Hippo pathway that controls organ growth in higher plants.

The MAP4 Kinase SIK1 Ensures Robust Extracellular ROS Burst and Antibacterial Immunity in Plants

“…Unlike the other Arabidopsis MAP4K members, SIK1 (MAP4K3, At1g69220) exhibits unique domain architecture with a central kinase domain and additional N- and C-terminal regions (Figure 1A). Interestingly, the sik1 mutants displayed a dwarf phenotype, which is consistent with previous findings (Xiong et al, 2016), whereas other mutant lines did not show obvious differences in plant growth compared to wild-type Col-0 (Figure 1B). …”

“…To confirm the role of SIK1 in ROS regulation, we analyzed a second independent homozygous T-DNA mutant line ( sik1–4 ) as well as a UBQ::SIK1-GFP complementation line (Xiong et al, 2016). Flg22-induced ROS production was significantly reduced in both sik1–1 and sik1–4 compared to wild-type Col-0 (Figure 4B and S5A).…”

“…Gene-specific primers (Table S3) were then used for RT-PCR to detect full-length transcripts. Seeds of bik1 (Lu et al, 2010), bik1 pbl1 (Zhang et al, 2010), the bik1 complementation line pBIK1::BIK1-HA (Lin et al, 2013) , fls2 (SALK_062054) (Ranf et al, 2012), sik1–4 (Xiong et al, 2016) and the sik1–4 complementation line pUBQ::SIK1 (Xiong et al, 2016) used in this study were described previously.…”

“…In Arabidopsis , the MAP4K family contains 10 members. The SIK1 MAP4K regulates cell proliferation and cell expansion (Xiong et al, 2016). The Arabidopsis BLUS1 MAP4K is phosphorylated by phototropins and serves as a primary regulator of stomatal opening (Takemiya et al, 2013).…”

Genome-wide identification of miRNAs and their targets during early somatic embryogenesis in Dimocarpus longan Lour.

“…In Arabidopsis thaliana, down-regulation of the mob1 gene may cause cell cycle disorder and affect sporophyte and gametophyte development 69 , and may be involved in the regulation of growth and development via auxin signal transduction 70 . It may also involved in Hippo metabolic pathways, regulating cell division, withdrawal and polarity establishment to control tissue growth and programmed cell death, and determine the size of organs 71 . The most important of these processes must be regulated by binding Pol II.…”