ANAC017 Coordinates Organellar Functions and Stress Responses by Reprogramming Retrograde Signaling 1[OPEN] Xiangxiang Meng, a Lu Li, a Inge De Clercq, b Reena Narsai, a Yue Xu, a Andreas Hartmann, a Diego Lozano Claros, a Eddie Custovic, c Mathew G. Lewsey, a James Whelan, a and Oliver Berkowitz a,2,3 a Department of Animal, Plant, and Soil Science, Australian Research Council Centre of Excellence in Plant Energy Biology, La Trobe University, Bundoora, Victoria 3086, Australia b Ghent University, Department of Plant Biotechnology and Bioinformatics, and VIB Center for Plant Systems Biology, 9052 Ghent, Belgium c School of Engineering and Mathematical Sciences, La Trobe University, Bundoora, Victoria 3086, Australia ORCID IDs: 0000-0002-5956-2011 (X.M.); 0000-0002-7410-8706 (L.L.); 0000-0002-4615-5441 (Y.X.); 0000-0003-2987-293X (A.H.); 0000-0003-1329-1946 (E.C.); 0000-0002-2631-4337 (M.G.L.); 0000-0001-5754-025X (J.W.); 0000-0002-7671-6983 (O.B.). Mitochondria adjust their activities in response to external and internal stimuli to optimize growth via the mitochondrial retrograde response signaling pathway. The Arabidopsis (Arabidopsis thaliana) NAC domain transcription factor ANAC017 has previously been identified as a regulator of the mitochondrial retrograde response. We show here that overexpression of ANAC017 in Arabidopsis leads to growth retardation, altered leaf development with decreased cell size and viability, and early leaf senescence. RNA sequencing analyses revealed that increased ANAC017 expression leads to higher expression of genes related to mitochondrial stress, cell death/autophagy, and leaf senescence under nonlimiting growth conditions as well as extensive repression of chloroplast function. Gene regulatory network analysis indicated that a complex hierarchy of transcription factors exists downstream of ANAC017. These involve a set of up-regulated ANAC and WRKY transcription factors associated with organellar signaling and senescence. The network also includes a number of ethylene- and gibberellic acid-related transcription factors with established functions in stress responses and growth regulation, which down-regulate their target genes. A number of BASIC LEUCINE-ZIPPER MOTIF transcription factors involved in the endoplasmic reticulum unfolded protein response or balancing of energy homeostasis via the SNF1-RELATED PROTEIN KINASE1 were also down-regulated by ANAC017 overexpression. Our results show that the endoplasmic reticulum membrane tethering of the constitutively expressed ANAC017, and its controlled release, are crucial to fine-tune a fast reactive but potentially harmful signaling cascade. Thus, ANAC017 is a master regulator of cellular responses with mitochondria acting as central sensors. Signaling pathways integrate developmental and environmental processes to control germination, growth, and the transition to flowering that produces the next generation of seeds. Mitochondria and chlo- roplasts provide the energy and are also the location of biosynthetic pathways that are essential for these processes (Van Dingenen et al., 2016). Anterograde signaling pathways directly affect the expression of nucleus-located genes encoding organellar proteins with concomitant downstream impact on organelle function. Mitochondria and chloroplasts actively feed back their functional status to the nucleus, referred to as retrograde signaling. Retrograde and anterograde signaling pathways are interconnected to optimize organellar functions for whole-plant growth (Chan et al., 2016; de Souza et al., 2017), with plant hormones acting as additional mediators (Berkowitz et al., 2016). Chloroplast retrograde signaling can be divided into biogenic and operational control mechanisms that regulate organelle biogenesis and acclimation to environmental conditions, respectively (Pogson et al., 2008). Chloroplast retro- grade regulation also interacts with the phytochrome system to regulate light-induced gene expression (Martín et al., 2016). Signaling components include tetrapyrrole pathway intermediates, metabolites such 1 This work was supported by the facilities of the Australian Re- search Council Centre of Excellence in Plant Energy Biology (CE140100008). I.D.C. is supported by the Research Foundation Flan- ders (postdoctoral fellowship 12N2415N, travel grant 450215N). R.N. is supported by an Australian Research Council DECRA fellowship (DE160101536). X.M. and D.L.C. are supported by a La Trobe Uni- versity postgraduate scholarship. 2 Author for contact: o.berkowitz@latrobe.edu.au. 3 Senior author. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy de- scribed in the Instructions for Authors (www.plantphysiol.org) is: James Whelan (j.whelan@latrobe.edu.au). J.W. and O.B. conceived the project; X.M., L.L., Y.X., A.H., D.L.C., and O.B. performed the experiments; X.M., R.N., I.D.C., D.L.C., E.C., M.G.L., and O.B. analyzed the data and contributed to the article preparation; X.M., J.W., R.N., and O.B. wrote the article with contributions by all authors. [OPEN] Articles can be viewed without a subscription. www.plantphysiol.org/cgi/doi/10.1104/pp.18.01603 634 Plant Physiology Ò , May 2019, Vol. 180, pp. 634–653, www.plantphysiol.org Ó 2019 American Society of Plant Biologists. All Rights Reserved. www.plantphysiol.org on May 24, 2019 - Published by Downloaded from Copyright © 2019 American Society of Plant Biologists. All rights reserved.