A lack of mitochondrial alternative oxidase compromises capacity to recover from severe drought stress

Abstract

Alternative oxidase (AOX) constitutes a nonenergy conserving branch of the mitochondrial electron transport chain. AOX activity may be important to avoid reactive oxygen species (ROS) generation by the chain, particularly during abiotic stress. We compared leaf AOX1a transcript and AOX protein amounts in wild-type (WT) Nicotiana tabacum plants experiencing mild to severe drought. Mild to moderate drought resulted in a progressive and modest increase in AOX amount, accompanied by a progressive increased expression of different ROS-scavenging components. Under these conditions, transgenic plants with suppressed AOX amount, due to an RNA interference construct, were not compromised in their ability to manage ROS load and prevent cellular damage. Severe drought stress, particularly when combined with increased irradiance, strongly increased AOX amount in WT tobacco and this coincided with an increase in total respiration and, despite further induction of ROS-scavenging systems, some evidence of cellular damage. Under these severe conditions, plants lacking AOX suffered more cellular damage than WT and, at the most severe stage, were found to downregulate rather than upregulate the transcript level of several important ROS-scavenging components. At this stage, WT plants could still recover rapidly after rewatering, but the recoverability of AOX knockdown plants was strongly compromised.

Mitochondrial Alternative Oxidase Maintains Respiration and Preserves Photosynthetic Capacity during Moderate Drought in Nicotiana tabacum

“…1). This correlates well with our previous studies showing that RI29 is the stronger of the two knockdowns (Wang et al, 2011;Wang and Vanlerberghe, 2013;Cvetkovska et al, 2014). In RI29, the effects of drought on A n , ETR II , excitation pressure, F PSII , and NPQ were all strongly magnified relative to the wild type, whereas RI9 (the slightly weaker AOX knockdown) always showed an intermediate response between RI29 and the wild type (Figs.…”

“…Protein carbonyl content was estimated by immunoblotting using the OxyBlot protein oxidation detection kit, following the manufacturer's instructions (EMD Millipore). Leaf water status was determined by measuring the RWC of leaves, as previously described (Wang and Vanlerberghe, 2013). Separate chloroplast and mitochondrial fractions for immunoblotting were isolated simultaneously from 40 g fresh weight of leaf tissue using the protocol developed by Rödiger et al (2010).…”

“…Transgenic plant lines with suppressed levels of AOX protein (RI9 and RI29) attributable to the presence of an AOX1a RNA interference construct, or elevated levels of AOX protein (B7) due to the presence of an AOX1a transgene driven by a constitutive promoter were previously described (Wang et al, 2011;Wang and Vanlerberghe, 2013;Cvetkovska et al, 2014). In particular, it has been shown that AOX protein levels are strongly suppressed in the knockdowns, even under the strongly AOX-inducing conditions of low temperature (Wang et al, 2011) or severe drought (Wang and Vanlerberghe, 2013). Seeds were germinated in vermiculite for 16 d and then transferred individually to 4-inch plastic pots containing a general purpose growing medium with 4 parts soil (Promix BX; Premier Horticulture) and 1 part vermiculite.…”

The Roles of Reactive Oxygen Metabolism in Drought: Not So Cut and Dried

“…However, drought induced a shift from the cytochrome oxidase to the AOX pathway, although this was not associated with altered AOX abundance (Ribas-Carbo et al, 2005). Current evidence suggests that deficiency in the AOX pathway enhances drought sensitivity (Giraud et al, 2008;Wang and Vanlerberghe, 2013).…”

Variation in relative water content, proline accumulation and stress gene expression in two cowpea landraces under drought

“…For example, mitochondrial alternative oxidase (AOX) isoforms can be induced by drought in plants, including cowpea, to limit mitochondrial ROS production (Vanlerberghe et al, 2016) (Costa et al, 2010) (Wang and Vanlerberghe, 2013). The plastid terminal oxidase PTOX, a chloroplast homologue of AOX, is induced in some plant species to limit excess electron transfer in the chloroplast during stress (Laureau et al, 2013) (Johnson and Stepien, 2016).…”