Mitochondrial and Chloroplast Stress Responses Are Modulated in Distinct Touch and Chemical Inhibition Phases

Abstract

Previous studies have identified a range of transcription factors that modulate retrograde regulation of mitochondrial and chloroplast functions in Arabidopsis (Arabidopsis thaliana). However, the relative importance of these regulators and whether they act downstream of separate or overlapping signaling cascades is still unclear. Here, we demonstrate that multiple stressrelated signaling pathways, with distinct kinetic signatures, converge on overlapping gene sets involved in energy organelle function. The transcription factor ANAC017 is almost solely responsible for transcript induction of marker genes around 3 to 6 h after chemical inhibition of organelle function and is a key regulator of mitochondrial and specific types of chloroplast retrograde signaling. However, an independent and highly transient gene expression phase, initiated within 10 to 30 min after treatment, also targets energy organelle functions, and is related to touch and wounding responses. Metabolite analysis demonstrates that this early response is concurrent with rapid changes in tricarboxylic acid cycle intermediates and large changes in transcript abundance of genes encoding mitochondrial dicarboxylate carrier proteins. It was further demonstrated that transcription factors AtWRKY15 and AtWRKY40 have repressive regulatory roles in this touch-responsive gene expression. Together, our results show that several regulatory systems can independently affect energy organelle function in response to stress, providing different means to exert operational control.

Mitochondrial Energy Signaling and Its Role in the Low-Oxygen Stress Response of Plants

“…A similar pattern was observed when mitochondrial dysfunction was induced by AA. The AOX1a transcript was unresponsive in the anac017 null mutant but regulated similar to wild type in the absence of other NACs, ABI4, and WRKY40/63 (Van Aken et al, 2016a). A central role for ANAC017 is further supported by a pronounced reduction of primary root growth induced by AA, rotenone, or MV in anac017.…”

“…AA treatment has become an intensively used model of respiratory inhibition and induction of a mitochondrial retrograde response (MRR) also in plants (Yu et al, 2001;Schwarzländer et al, 2012a;Umbach et al, 2012;De Clercq et al, 2013;Ng et al, 2013a;Van Aken et al, 2016a). AA inhibits the mitochondrial electron transport chain (mtETC) at the N site of complex III and leads to an increased rate of superoxide generation (Cadenas et al, 1977;Murphy, 2009).…”

“…Notably, ANAC017, WRKY40 and ABI4 are also involved in retrograde signaling of the chloroplast (Koussevitzky et al, 2007;Shang et al, 2010;Van Aken et al, 2013;2016a). Converging retrograde signaling cascades may allow for synchronization of the downstream response in both endosymbiotic energy organelles, but the level at which the integration occurs remains elusive (Finkemeier and Schwarzländer, 2017).…”

“…ABI4 in the retrograde response of the chloroplast has recently been shown to be activated through phosphorylation by a MAP kinase cascade in a Ca 2+ -dependent manner, suggesting potential molecular entry points of signal integration (Guo et al, 2016). ANAC017 appears also crucial for the transcriptional response to methyl viologen (MV; paraquat) that produces large amounts of superoxide using electrons from PSI under illuminated conditions and eventually leads to cell death (Van Aken and Van Breusegem, 2015;Van Aken et al, 2016a). Importantly, several MRR target genes (including AOX1a) are not induced by MV.…”

“…Particularly high expression as compared to the other transcription factors of the NAC family is characteristic for ANAC017. This may provide an explanation for the phenotypes associated with its loss, implying that functional backup may be available for less abundant family members (Van Aken et al, 2016a).…”