Selenite activates the alternative oxidase pathway and alters primary metabolism in Brassica napus roots: evidence of a mitochondrial stress response

Abstract

BackgroundHuman requirements for dietary selenium are met mainly by crops. However, excessive uptake of selenium in plants can restrict growth, and its toxicity has been postulated to target roots. Selenite toxicity can be attributed to its assimilation into selenocysteine, which can replace cysteine to yield malformed selenoproteins. Additionally, selenite has pro-oxidant properties. In this study, the effects of selenite on root tissue in Brassica napus (canola) were investigated to better understand its mode of toxicity and the metabolic adjustments needed to mediate a selenite-response.ResultsSelenite induced the rapid formation of mitochondrial superoxide, which led to decreased aconitase activity and involvement of the alternative oxidase pathway. Although selenite altered primary metabolism, as observed by the increased amino acids and decreased TCA cycle metabolites, increased glucose presumably supported higher respiratory rates and ATP levels reported in this study. Additionally, evidence is presented indicating that selenite suppressed the ubiquitin-proteasome pathway, and induced the pentose phosphate pathway needed to maintain antioxidant metabolism. Selenite treatment also elevated glutathione concentration and coincided with increased levels of γ-glutamyl cyclotransferase, which may possibly degrade selenium metabolites conjugated to glutathione.ConclusionCollectively, the data indicate that selenite necessitates the reconfiguration of metabolic pathways to overcome the consequences of mitochondrial oxidative stress in root tissue. Efforts to mitigate the detrimental effects of selenite-induced oxidative stress may ultimately improve selenium tolerance and accumulation in crops.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0259-6) contains supplementary material, which is available to authorized users.

Stuck between a ROS and a hard place: Analysis of the ubiquitin proteasome pathway in selenocysteine treated Brassica napus reveals different toxicities during selenium assimilation

“…The fluorescent probe MitoSox Red was recently used to demonstrate that selenite induces the accumulation of mitochondrial superoxide in B. napus (Dimkovikj and Van Hoewyk, 2014). However, in this study, wide-field microscopy did not reveal a difference in red fluorescence between plants treated with or without Sec ( Fig.…”

“…Plants likely respond to selenite and Sec differently in order to mount a successful stress response. Selenite treatment in B. napus increased antioxidant metabolism after 24 h, as judged by increased levels of NADPH and activity of glucose-6-phosphate dehydrogenase (Dimkovikj and Van Hoewyk, 2014), which diverts glucose into the oxidative-pentose phosphate pathway. In that same study, selenite intriguingly decreased proteasome activity and levels of ubiquitinated proteins.…”

“…Briefly, 10-15 root tips (0.8-1.2 cm) were allowed to incubate in 1 mL of Hoagland's media containing 5 M Mitosox Red (Ex 510 /Em 580 ) for 15 min on a rotating platform, as described (Dimkovikj and Van Hoewyk, 2014). Roots were washed 3x in Hoagland's media before fluorescence was visualized using a TRITC filter on an Olympus BX51 fluorescent microscope.…”

“…It is well established that inorganic selenium, such as selenate and selenite, deplete glutathione and cause oxidative stress (Hugouvieux et al, 2009;Łabanowska et al, 2012). Recently, Brassica napus (canola) plants treated with selenite rapidly accumulated mitochondrial superoxide; in response to selenite, roots reconfigured primarily metabolism -including antioxidant metabolism -to counter the effects of oxidative stress (Dimkovikj and Van Hoewyk, 2014). Additionally, both selenate and selenite can be metabolized by sulfur assimilatory enzymes and assimilated into Sec, as extensively reviewed elsewhere (Zhu et al, 2009).…”

Growth inhibition by selenium is associated with changes in primary metabolism and nutrient levels in Arabidopsis thaliana

“…Se increased the amino acids levels in Brassica napus , while TCA intermediates were decreased in roots exposed to Se (Dimkovikj and Van Hoewyk ). Minor changes in amino acids levels in plants treated with Se were observed here (Fig.…”