Cytokinins Control Endocycle Onset by Promoting the Expression of an APC/C Activator in Arabidopsis Roots

Takahashi, N.,Kajihara, T.,Okamura, C.,Kim, Y.,Katagiri, Y.,Okushima, Y.,Matsunaga, S.,Hwang, I.,Umeda, M. Current Biology Ltd ; Elsevier Science Ltd 2013 Current biology Vol.23 No.18

Plant roots respond to various internal and external signals and adjust themselves to changes of environmental conditions. In the root meristem, stem cells produce daughter cells that continue to divide several times. When these latter cells reach the transition zone, they stop dividing and enter the endocycle, a modified cell cycle in which DNA replication is repeated without mitosis or cytokinesis. The resultant DNA polyploidization, named endoreduplication, is usually associated with an increase of nuclear and cell volume and with cell differentiation [1-4]. At the transition zone, cytokinin signaling activates two transcription factors, type-B ARABIDOPSIS RESPONSE REGULATOR 1 (ARR1) and ARR12, and induces SHY2/IAA3, a member of the Aux/IAA family of auxin signaling repressors. This inhibits auxin signaling and reduces the expression of auxin efflux carriers, resulting in cell division arrest [5]. Such counteracting actions of two hormones are assumed to determine meristem size. However, it remains unknown whether cytokinins additionally control meristem size through an auxin-independent pathway. Here we show that, in Arabidopsis, the cytokinin-activated ARR2 directly upregulates the expression of CCS52A1, which encodes an activator of an E3 ubiquitin ligase, anaphase-promoting complex/cyclosome (APC/C) [6], thereby promoting the onset of the endocycle and restricting meristem size. Our genetic data revealed that CCS52A1 function is independent of SHY2-mediated control of auxin signaling, indicating that downregulation of auxin signaling and APC/C-mediated degradation of cell-cycle regulators cooperatively promote endocycle onset, and thus fine tune root growth.

TOR Signaling Promotes Accumulation of BZR1 to Balance Growth with Carbon Availability in Arabidopsis

Zhang, Z.,Zhu, J.Y.,Roh, J.,Marchive, C.,Kim, S.K.,Meyer, C.,Sun, Y.,Wang, W.,Wang, Z.Y. Current Biology Ltd ; Elsevier Science Ltd 2016 Current biology Vol.26 No.14

<P>For maintenance of cellular homeostasis, the actions of growth-promoting hormones must be attenuated when nutrient and energy become limiting. The molecular mechanisms that coordinate hormone-dependent growth responses with nutrient availability remain poorly understood in plants [1, 2]. The target of rapamycin (TOR) kinase is an evolutionarily conserved master regulator that integrates nutrient and energy signaling to regulate growth and homeostasis in both animals and plants [3-7]. Here, we show that sugar signaling through TOR controls the accumulation of the brassinosteroid (BR)-signaling transcription factor BZR1, which is essential for growth promotion by multiple hormonal and environmental signals [8-11]. Starvation, caused by shifting of light-grown Arabidopsis seedlings into darkness, as well as inhibition of TOR by inducible RNAi, led to plant growth arrest and reduced expression of BR-responsive genes. The growth arrest caused by TOR inactivation was partially recovered by BR treatment and the gain-of-function mutation bzr1-1D, which causes accumulation of active forms of BZR1 [12]. Exogenous sugar promoted BZR1 accumulation and seedling growth, but such sugar effects were largely abolished by inactivation of TOR, whereas the effect of TOR inactivation on BZR1 degradation is abolished by inhibition of autophagy and by the bzr1-1D mutation. These results indicate that cellular starvation leads sequentially to TOR inactivation, autophagy, and BZR1 degradation. Such regulation of BZR1 accumulation by glucose-TOR signaling allows carbon availability to control the growth promotion hormonal programs, ensuring supply-demand balance in plant growth.</P>

Kinesin-12 Kif15 Targets Kinetochore Fibers through an Intrinsic Two-Step Mechanism

Sturgill, Emma G.,Das, D.,Takizawa, Y.,Shin, Y.,Collier, Scott E.,Ohi, Melanie D.,Hwang, W.,Lang, Matthew J.,Ohi, R. Current Biology Ltd ; Elsevier Science Ltd 2014 Current biology Vol.24 No.19

Proteins that recognize and act on specific subsets of microtubules (MTs) enable the varied functions of the MT cytoskeleton. We recently discovered that Kif15 localizes exclusively to kinetochore fibers (K-fibers) [1, 2] or bundles of kinetochore-MTs within the mitotic spindle. It is currently speculated that the MT-associated protein TPX2 loads Kif15 onto spindle MTs [3-5], but this model has not been rigorously tested. Here, we show that Kif15 accumulates on MT bundles as a consequence of two inherent biochemical properties. First, Kif15 is self-repressed by its C terminus. Second, Kif15 harbors a nonmotor MT-binding site, enabling dimeric Kif15 to crosslink and slide MTs. Two-MT binding activates Kif15, resulting in its accumulation on and motility within MT bundles but not on individual MTs. We propose that Kif15 targets K-fibers via an intrinsic two-step mechanism involving molecular unfolding and two-MT binding. This work challenges the current model of Kif15 regulation and provides the first account of a kinesin that specifically recognizes a higher-order MT array.

Phytochrome-interacting factor from Arabidopsis to liverwort

Lee, N.,Choi, G. Current Biology, Ltd ; Elsevier Science Ltd 2017 Current opinion in plant biology Vol.35 No.-

<P>Phytochromes are red and far-red light photoreceptors that regulate the responses of plants to light throughout their life cycles. Phytochromes do this in part by inhibiting the function of a group of basic helix-loop-helix transcription factors called phytochrome-interacting factors (PIFs). Arabidopsis has eight PIFs that function sometimes redundantly and sometimes distinctively depending on their expression patterns and protein stability, as well as on variations in the promoters they target in vivo. PIF-like proteins exist in other seed plants and non-vascular plants where they also regulate light responses. The mechanism by which phytochrome regulates light responses by promoting the degradation of the PIFs is conserved in liverwort, suggesting it must have evolved some time before the last common ancestor shared by seed plants and non-vascular plants.</P>

Identification of a Peptidergic Pathway Critical to Satiety Responses in Drosophila

Min, S.,Chae, H.S.,Jang, Y.H.,Choi, S.,Lee, S.,Jeong, Y.,Jones, Walton D.,Moon, S.,Kim, Y.J.,Chung, J. Current Biology Ltd ; Elsevier Science Ltd 2016 Current biology Vol.26 No.6

<P>Although several neural pathways have been implicated in feeding behaviors in mammals [1-7], it remains unclear how the brain coordinates feeding-motivations to maintain a constant body weight (BW). Here, we identified a neuropeptide pathway important for the satiety and BW control in Drosophila. Silencing of myoinhibitory peptide (MIP) neurons significantly increased BW through augmented food intake and fat storage. Likewise, the loss-offunction mutation of mip also increased feeding and BW. Suppressing the MIP pathway induced satiated flies to behave like starved ones, with elevated sensitivity toward food. Conversely, activating MIP neurons greatly decreased food intake and BW and markedly blunted the sensitivity of starved flies toward food. Upon terminating the activation protocol of MIP neurons, the decreased BW reverts rapidly to the normal level through a strong feeding rebound, indicating the switch-like role of MIP pathway in feeding. Surprisingly, the MIP-mediated BW decrease occurred independently of sex peptide receptor (SPR), the only known receptor for MIP, suggesting the presence of a yet-unknown MIP receptor. Together, our results reveal a novel anorexigenic pathway that controls satiety in Drosophila and provide a new avenue to study how the brain actively maintains a constant BW.</P>

Journal of Cell Science

Current Biology Ltd ; Elsevier Science Ltd 1996 Current biology Vol.6 No.10

Inhibition of Respiration Extends C. elegans Life Span via Reactive Oxygen Species that Increase HIF-1 Activity

Lee, S.J.,Hwang, A.B.,Kenyon, C. Current Biology Ltd ; Elsevier Science Ltd 2010 Current biology Vol.20 No.23

A mild inhibition of mitochondrial respiration extends the life span of many organisms, including yeast, worms, flies, and mice [1-10], but the underlying mechanism is unknown. One environmental condition that reduces rates of respiration is hypoxia (low oxygen). Thus, it is possible that mechanisms that sense oxygen play a role in the longevity response to reduced respiration. The hypoxia-inducible factor HIF-1 is a highly conserved transcription factor that activates genes that promote survival during hypoxia [11, 12]. In this study, we show that inhibition of respiration in C. elegans can promote longevity by activating HIF-1. Through genome-wide screening, we found that RNA interference (RNAi) knockdown of many genes encoding respiratory-chain components induced hif-1-dependent transcription. Moreover, HIF-1 was required for the extended life spans of clk-1 and isp-1 mutants, which have reduced rates of respiration [1, 4, 13]. Inhibiting respiration appears to activate HIF-1 by elevating the level of reactive oxygen species (ROS). We found that ROS are increased in respiration mutants and that mild increases in ROS can stimulate HIF-1 to activate gene expression and promote longevity. In this way, HIF-1 appears to link respiratory stress in the mitochondria to a nuclear transcriptional response that promotes longevity.

Push-pull strategy in the regulation of postembryonic root development

Choe, G.,Lee, J.Y. Current Biology, Ltd ; Elsevier Science Ltd 2017 Current opinion in plant biology Vol.35 No.-

<P>Unlike animals, plants continue to grow throughout their lives. The stem cell niche, protected in meristems of shoots and roots, enables this process. In the root, stem cells produce precursors for highly organized cell types via asymmetric cell divisions. These precursors, which are 'transit-amplifying cells,' actively divide for several rounds before entering into differentiation programs. In this review, we highlight positive feedback regulation between shoot- and root-ward signals during the postembryonic root growth, which is reminiscent of a 'push-pull strategy' in business parlance. This property of molecular networks underlies the regulation of stem cells and their organizer, the 'quiescent center,' as well as of the signaling between stem cell niche, transit-amplifying cells, and beyond.</P>

Regulatory network of NAC transcription factors in leaf senescence

Kim, H.J.,Nam, H.G.,Lim, P.O. Current Biology, Ltd ; Elsevier Science Ltd 2016 Current opinion in plant biology Vol.33 No.-

<P>Leaf senescence is finely tuned by many regulatory factors such as NAC (NAM/ATAF/CUC)) transcription factors (TFs). NACs comprise one of the largest families of TFs in plants, many of which are differentially regulated during leaf senescence and play a major role in leaf senescence. Recent studies advanced our understanding on the structural and functional features of NAC TFs including target binding specificities of the N-terminal DNA binding domain and dynamic interaction of the C-terminal intrinsically disordered domain with other proteins. NAC TFs control other NACs and also interact with NACs or other TFs to fine-tune the expression of target genes. These studies clearly demonstrated the highly complex characteristics of NAC regulatory networks, which are dynamically regulated temporally and spatially and effectively integrate multiple developmental and environmental signals.</P>

Adaptation through horizontal gene transfer in the cryptoendolithic red alga Galdieria phlegrea

Qiu, H.,Price, D.C.,Weber, A.P.M.,Reeb, V.,Chan Yang, E.,Lee, J.M.,Kim, S.Y.,Yoon, H.S.,Bhattacharya, D. Current Biology Ltd ; Elsevier Science Ltd 2013 Current biology Vol.23 No.19

Thriving in the hot, acidic, and metal-rich environments associated with geothermal areas is possible for only a few eukaryotes, with the Cyanidiophytina red algae (Cyanidium, Galdieria, and Cyanidioschyzon) being a famous example. These unicellular taxa can live in pH 0-4 and temperatures reaching up to 56<SUP>o</SUP>C [1,2]. Because Cyanidiophytina is sister to a vast array of mesophilic red algae (the Rhodophytina), such as the unicellular Porphyridium and the seaweed Chondrus[3], the genetic basis of their adaptation to extreme environments is of great interest from both the perspective of biotechnology and of evolution. The recently completed 13.7 Mbp genome sequence from the hot-spring dwelling Galdieria sulphuraria demonstrated that horizontal gene transfer (HGT) from prokaryotic sources provided this taxon with remarkable metabolic versatility (e.g., glycerol metabolism) and the ability to survive in its hostile environment (e.g., genes to detoxify mercury and arsenic) [4]. To explore the role of HGT in other members of this genus, we generated an 11.4 Mbp draft genome assembly from the sister taxon G. phlegrea DBV 009 [5]. In contrast to G. sulphuraria, this species is adapted to dry habitats near fumaroles such as fissures between rocks or cryptoendolithic environments [5,6]. Here, we provide evidence for extensive gene loss in the common ancestor of Cyanidiophytina that includes the eukaryote-derived loci required for urea utilization. Surprisingly, we find that G. phlegrea has regained the complete set of genes required for urea hydrolysis through HGT from eubacteria. The unlinked nature of these genes is likely explained by multiple gene transfers that resulted in assembly of the pathway in G. phlegrea. Our study demonstrates that genome reduction, a common outcome in eukaryotes for adaptation to a specialized niche, can be ameliorated by the gain of once lost, or novel functions through HGT.