A critical evaluation of the role of alternative oxidase in the performance of strobilurin and related fungicides acting at the Q_o site of Complex III

Abstract

Mitochondrial respiration conserves energy by linking NADH oxidation and electron-coupled proton translocation with ATP synthesis, through a core pathway involving three large protein complexes. Strobilurin fungicides block electron flow through one of these complexes (III), and disrupt energy supply. Despite an essential need for ATP throughout fungal disease development, strobilurins are largely preventative; indeed some diseases are not controlled at all, and several pathogens have quickly developed resistance. Target-site variation is not the only cause of these performance difficulties. Alternative oxidase (AOX) is a strobilurin-insensitive terminal oxidase that allows electrons from ubiquinol to bypass Complex III. Its synthesis is constitutive in some fungi but in many others is induced by inhibition of the main pathway. AOX provides a strobilurin-insensitive pathway for oxidation of NADH. Protons are pumped as electrons flow through Complex I, but energy conservation is less efficient than for the full respiratory chain. Salicylhydroxamic acid (SHAM) is a characteristic inhibitor of AOX, and several studies have explored the potentiation of strobilurin activity by SHAM. We present a kinetic-based model which relates changes in the extent of potentiation during different phases of disease development to a changing importance of energy efficiency. The model provides a framework for understanding the varying efficacy of strobilurin fungicides. In many cases, AOX can limit strobilurin effectiveness once an infection is established, but is unable to interfere significantly with strobilurin action during germination. A less stringent demand for energy efficiency during early disease development could lead to insensitivity towards this class of fungicides. This is discussed in relation to Botrytis cinerea, which is often poorly controlled by strobilurins. Mutations with a similar effect may explain evidence implicating AOX in resistance development in normally well-controlled plant pathogens, such as Venturia inaequalis.

Effects of KCN, SHAM and Oxygen Concentrations on Respiratory Properties of Purified Mitochondria Isolated fromAnanas comosus(Pineapple) andKalanchoë daigremontiana

“…NADH is oxidized by two different dehydrogenases. An external NADH dehydrogenase (NDE) enables cytoplasmic NADH to be used directly, while an internal NADH dehydrogenase (NDI) catalyses a rotenone-insensitive oxidation of mitochondrial NADH generated by TCA cycle (Wood and Hollomon, 2003). NADH is unable to pass through the inner mitochondrial membrane so it is oxidized outside of the mitochondria by NDE and donate their electron directly to ubiquinone (UQ) pool (Siedow and Day, 2000;Møller, 2002).…”

“…1A, B), so the third reason is unsuitable. Furthermore, Wood and Hollomon (2003) reported that NDE lack inhibitors, so that the second and third reasons can also be eliminated. The effect of SHAM on NADH oxidation (Fig.…”

“…1D) supported reason 1 in which the activity of NDE was closely related to COX in NADH oxidation of A. comosus mitochondria. Wood and Hollomon (2003) indicated that in plant mitochondria, the electron fl ow through NDE is divided into the Cyt and Alt pathways at the UQ pool depending on AOX and COX activities. Day et al (1988) observed that the ETC through the NDI was not rapid enough to engage with the AOX.…”

Industry Views of Monitoring and Mitigation of Herbicide Resistance

“…It is noticeable that QoI resistant oomycete pathogens are sensitive to cyazofamid, a QiI fungicide that blocks electron flow through the second quinone binding site of cytochrome bc-1 which faces the inside of the mitochondrial matrix (Mitani et al, 2005). Cyazofamid may be used as a partner to QoIs in resistance management programmes, although it should be recognised that both QoI and QiI fungicides activate alternative oxidase, which causes low levels of resistance to both fungicide groups (Wood and Hollomon, 2003;Hollomon et al, 2005;Gisi et al, 2005). General FRAC guidelines for use of QoI fungicides include the following key instructions: -Apply QoI fungicides at effective rates and intervals, according to manufacturers' recommendations.…”

First report of G143A strobilurin resistance in Cercospora beticola in sugar beet (Beta vulgaris) in Poland

“…The phenomenon was attributed to the additional supply of energy due to increased alternative oxidase (AOX) activity or due to other reasons (Pieczul and Perek 2015). Previous studies indicated that AOX can be induced by fungicides causing ubiquinone inhibition (QoI), what increased survival of fungi under QoI treatment (Wood and Hollomon 2003).…”