Live and Let Die: The Core Circadian Oscillator Coordinates Plant Life History and Pilots Leaf Senescence
2018-02-16

For a long time, many scientists were keen on discovering "the master regulator" of plant senescence. However, in the last two decades, it became very clear that this "master regulator" does not exist, but an extremely complex regulatory network controls all aspects and phases of senescence. Temporal transcript profiling of a defined leaf of Arabidopsis with high resolution, which covered 22 time points during onset and progression of leaf senescence, revealed expression changes of more than 6500 genes and disclosed a distinct chronology of events (Breeze et al., 2011Breeze E. Harrison E. McHattie S. Hughes L. Hickman R. Hill C. Kiddle S. Kim Y.S. Penfold C.A. Jenkins D. et al.High-resolution temporal profiling of transcripts during Arabidopsis leaf senescence reveals a distinct chronology of processes and regulation.Plant Cell. 2011; 23: 873-894Crossref PubMed Scopus (623) Google Scholar). Besides autophagy and transport, transcriptional response to reactive oxygen species (ROS), jasmonic acid (JA), and salicylic acid (SA) are activated at early time points. All three signaling molecules are also part of the stress responses to biotic as well as abiotic stresses, indicating a strong cross-talk between the senescence process and stress control. Incoming environmental signals are constantly integrated throughout life history and, if stress conditions produce long-term unfavorable conditions for the plant, premature senescence is initiated as an exit strategy to produce offspring. However, premature senescence often correlates with diminished seed quantity and quality. As almost one fourth of the Arabidopsis genes change their expression during senescence, and transcription factors play a pivotal role in senescence regulation. Especially, the NAC and WRKY transcription factor families, which largely expanded during evolution of land plants, are overrepresented in the senescence transcriptomes, and many members of these families are part of the complex regulatory network, in many cases at hub positions. Nevertheless, members of all transcription factor families participate in the senescence regulatory network. Multi-layer feedback regulatory cues are in place to control senescence and transcriptional, post-transcriptional, post-translational regulatory mechanisms are realized for one and the same gene e.g., WRKY53. Moreover, we are just beginning to understand the dynamic changes in chromatin structure and nuclear architecture during senescence. Here also, multi-layer feedback regulation can be observed, and again the same players are involved. WRKY53 expression, for example, is on the one hand activated by senescence-associated chromatin modifications (Ay et al., 2009Ay N. Irmler K. Fischer A. Uhlemann R. Reuter G. Humbeck K. Epigenetic programming via histone methylation at WRKY53 controls leaf senescence in Arabidopsis thaliana.Plant J. 2009; 58: 333-346Crossref PubMed Scopus (135) Google Scholar), whereas, on the other hand, the WRKY53 protein is part of the powerdress/HDA9 complex and directs chromatin modifications to promoters of senescence-regulating genes (Chen et al., 2016Chen X. Lu L. Mayer K.S. Scalf M. Qian S. Lomax A. Smith L.M. Zhong X. POWERDRESS interacts with HISTONE DEACETYLASE 9 to promote aging in Arabidopsis.Elife. 2016; 5: e17214Crossref PubMed Scopus (101) Google Scholar). Moreover, early developmental processes are closely linked to leaf senescence so that a kind of "leaf developmental memory" appears to exist, meaning if early leaf development is perturbed, leaf senescence is affected. Evidence for this hypothesis is provided by detailed analyses of the transcription factor REVOLUTA (REV), which is involved in early developmental processes such as establishment of leaf polarity, lateral meristem initiation, or vascular development and directly regulates the expression of the senescence-promoting transcription factors WRKY53 in a redox-dependent manner during early senescence (Xie et al., 2014Xie Y. Huhn K. Brandt R. Potschin M. Bieker S. Straub D. Doll J. Drechsler T. Zentgraf U. Wenkel S. REVOLUTA and WRKY53 connect early and late leaf development in Arabidopsis.Development. 2014; 141: 4772-4783Crossref PubMed Scopus (67) Google Scholar). More and more examples of this cross-talk between early and late leaf development have become apparent in recent years. Following the same lines of evidence, cell proliferation activity and leaf senescence are interconnected, as low cell proliferation activity is associated with accelerated leaf senescence and vice versa (for review, see Kim et al., 2018Kim J. Kim J.H. Lyu J.I. Woo H.R. Lim P.O. New insights into the regulation of leaf senescence in Arabidopsis.J. Exp. Bot. 2018; 69: 787-799Crossref PubMed Scopus (98) Google Scholar). This also points to the existence to a "leaf developmental memory". Taken together, our current view on senescence regulation necessitates regulatory factors that co-ordinate not only environmental signals but also developmental events throughout the life history of a plant. Very recently, Zhang et al., 2018Zhang Y. Wang Y. Wei H. Li N. Tian W. Chong K. Wang L. Circadian evening complex represses jasmonate-induced leaf senescence in Arabidopsis.Mol. Plant. 2018; 11: 326-337Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar added a new piece to the puzzle. One good candidate for this coordinator function is the circadian clock. First, the circadian rhythm is affected by leaf age. The circadian periods of clock-regulated genes as well as the periods of the core clock genes are shorter in older leaves compared with young leaves and appear to be regulated through the clock oscillator TOC1 (Kim et al., 2016Kim H. Kim Y. Yeom M. Lim J. Nam H.G. Age-associated circadian period changes in Arabidopsis leaves.J. Exp. Bot. 2016; 67: 2665-2673Crossref PubMed Scopus (36) Google Scholar). By the way, disruption of the circadian clock in animal systems can also affect senescence and age-related disorders. Second, Ezer et al., 2017Ezer D. Jung J.H. Lan H. Biswas S. Gregoire L. Box M.S. Charoensawan V. Cortijo S. Lai X. Stöckle D. et al.The evening complex coordinates environmental and endogenous signals in Arabidopsis.Nat. Plants. 2017; 3: 17087Crossref PubMed Scopus (151) Google Scholar analyzed in detail that the circadian evening complex (EC) coordinates environmental and endogenous signals in Arabidopsis. Zhang et al., 2018Zhang Y. Wang Y. Wei H. Li N. Tian W. Chong K. Wang L. Circadian evening complex represses jasmonate-induced leaf senescence in Arabidopsis.Mol. Plant. 2018; 11: 326-337Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar have disclosed a molecular mechanism how EC interferes with JA-induced leaf senescence in Arabidopsis. EC is a critical component of the core oscillator regulating circadian outputs and consists of three proteins: EARLY FLOWERING3 (ELF3), EARLY FLOWERING4 (ELF4), and LUX ARRHYTHMO (LUX). LUX is a GARP transcription factor mediating DNA-binding activity, while ELF3 functions as an adaptor between LUX and ELF4 to form a ternary transcriptional repression complex. Plants with mutations in the EC components all showed accelerated natural as well as more pronounced JA-induced leaf senescence. This indicates that the core component of the circadian clock, EC, negatively regulates senescence induction. In agreement, EC component overexpression led to stay-green phenotypes after JA treatment. Moreover, transcriptome analyses revealed that EC is involved in JA signaling and response reactions but also controls senescence regulators, such as WRKY53 or WRKY70 or the NAC factors ORE1 or NAP. The EC component LUX directly interacts with the promoter of the MYC2 transcription factor gene, which encodes a key activator of JA-induced leaf senescence, but not with the promoter of other JA signaling components such as COI1 or JAZ1. LUX interaction with the MYC2 promoter represses MYC2 expression. When plants with mutations in the components of EC were crossed with the myc2 mutant or the triple myc2 myc3 myc4 mutant plants, the homozygous offspring abrogated the accelerated JA-induced leaf senescence observed in EC mutants. This confirms that a core component of the circadian clock controls JA signaling via MYC transcription factors to regulate leaf senescence. Moreover, all plants with mutations in the components of EC contained lower JA levels, suggesting further involvement of EC in JA production (Zhang et al., 2018Zhang Y. Wang Y. Wei H. Li N. Tian W. Chong K. Wang L. Circadian evening complex represses jasmonate-induced leaf senescence in Arabidopsis.Mol. Plant. 2018; 11: 326-337Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). Remarkably, ELF3 can affect dark-induced leaf senescence also in an EC-independent manner. ELF3 and red light-activated phytochrome B can repress the senescence-promoting activity of PIF4/PIF5 at the transcriptional and the post-translational levels, respectively. These two PIFs activate the expression of ORE1 through multiple feed-forward loops, thus connecting a clock component and the classical light signaling component to leaf senescence (Sakuraba et al., 2014Sakuraba Y. Jeong J. Kang M.Y. Kim J. Paek N.C. Choi G. Phytochrome-interacting transcription factors PIF4 and PIF5 induce leaf senescence in Arabidopsis.Nat. Commun. 2014; 5: 4636Crossref PubMed Scopus (271) Google Scholar). Sanchez and Kay, 2016Sanchez S.E. Kay S.A. The plant circadian clock: from a simple timekeeper to a complex developmental manager.Cold Spring Harb. Perspect. Biol. 2016; 8: a027748Crossref PubMed Scopus (125) Google Scholar already entitled the circadian clock as the "mastermind" of plant life modulating a myriad of plant responses. Carbohydrate and nitrogen metabolism, calcium, iron, and copper as well as hormone homeostasis are modulated by the central clock but in turn, all these components feed back to the core oscillator except JA, gibberellic acid, and ethylene (for review see Sanchez and Kay, 2016Sanchez S.E. Kay S.A. The plant circadian clock: from a simple timekeeper to a complex developmental manager.Cold Spring Harb. Perspect. Biol. 2016; 8: a027748Crossref PubMed Scopus (125) Google Scholar). CCA1 was already disclosed as a master regulator of ROS homeostasis through association with the EC in promoters of ROS-responsive genes in vivo; on the other hand, ROS also functions as an input signal that affects the transcriptional output of the clock (Lai et al., 2012Lai A.G. Doherty C.J. Mueller-Roeber B. Kay S.A. Schippers J.H. Dijkwel P.P. CIRCADIAN CLOCK-ASSOCIATED 1 regulates ROS homeostasis and oxidative stress responses.Proc. Natl. Acad. Sci. USA. 2012; 109: 17129-17134Crossref PubMed Scopus (251) Google Scholar). As carbohydrate and nitrogen metabolism, ROS as well as almost all hormone signaling pathways are also integral parts of the regulatory network of senescence, senescence and circadian regulatory circuits appear to have huge overlaps. Thus, plant senescence can definitely be added to the developmental steps governed by this pacemaker (Figure 1). L.R. and J.D. were supported by the Deutsche Forschungsgemeinschaft (SFB1101, B06).