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Roles of mitochondria

In anaerobically grown yeast mitochondria adopt an undeveloped form termed promitochondria. Under non-repressing aerobic conditions these can rapidly adopt a fully functional form (Plattner et al., 1970). Visser et al. (1995) noted that under aerobic de-repressed conditions there were numerous small mitochondria, whereas under glucose repression they became much fewer in number but these were larger and branched. The total mitochondriome was similar in each case. Conversely, working with sake yeast, Kitagaki et al. (2013) developed procedures to monitor mitochondrial morphology throughout fermentation. They noted that initially the organelles had a filamentous tubular structure and as the fermentation progressed these fragmented and became smaller and non-elongated. Rosenfeld et al. (2004) isolated mitochondria from anaerobically grown, repressed yeast cells and reported that they could detect the presence of cyanide-sensitive and non-phosphorylating NADH-dependent oxygen consumption but no antimycin-A-dependent, NADH- or NADPH-dependent oxidase activities and thus they concluded that oxygen consumption by anaerobic mitochondria was negligible. Despite the apparent differences in some reports, the consensus is that the mitochondria are highly dynamic in shape and size and that processes of both fusion and division are of frequent occurrence (Jensen et al., 2000).

The role of mitochondria in brewing yeast under brewing conditions is uncertain; however, it is known that petites produce unsatisfactory fermentation performance, notably slower rates, prolonged vicinal diketone (VDK) stand times, and altered flocculation characteristics (Ernandes et al., 1993). The negative effects on VDK metabolism are perhaps unsurprising, since this organelle is the site of key parts of the ILV pathway in which branched chain amino acids leucine, isoleucine, and valine are synthesized (Kohlhaw, 2003). In modern brewing practice, where large batch sizes and very concentrated worts are commonly used, the increased frequency of occurrence of petites has been observed and ascribed to elevated stress levels associated with these processes ( Jenkins et al., 2009; Lawrence et al., 2012, 2013). The quiescent daughter cells produced during the diauxic shift phase discussed earlier (see section ‘Quiescent yeast cells') inherit only highly functional mitochondria during the asymmetrical division (Peraza-Hayes et al., 2010; McFaline-Figueroa et al., 2011). Under these conditions, where oxygen is present the quiescent cells adopt a respiratory metabolism and maintain their elevated levels of glycogen and trehalose. The non-quiescent cell fraction retains a repressed metabolism and continues to deplete carbohydrate reserves via glycolysis. This helps to explain the relative differences in longevity between the quiescent and non-quiescent phenotypes under non-growth-permitting conditions.

Interestingly, mitochondria seemingly have a role in sterol uptake and transport in anaerobically grown yeast. Reiner et al. (2006) used a range of yeast mutants that were unable to grow under anaerobic conditions. They observed that the largest group of mutants, which were not able to take up exogenous sterols, had disruptions of genes known to have mitochondrial functions. These roles for mitochondria are perhaps unsurprising, since this organelle is the site of several biosynthetic pathways. These include part of those for sterol synthesis, branched-chain amino acids, and the citric acid cycle (Shimizu et al., 1973; Ryan and Kohlhaw, 1974; Jauniaux, 1978). Based on the inhibition of the ADP/ATP transporter by bongkrekic acid, Visser et al. (1994) proposed that the energy to fuel these pathways must be derived from substrate-level phosphorylation. In one of the few studies using a brewing yeast strain, Samp (2012) reported that in lager strains there was a link between mitochondrial function and SO2 production. Respiratory deficient mutants showed reduced conversion rates of sulfate to sulfite compared with the wild type. Conditions that favoured cardiolipin synthesis, an important mitochondrial membrane lipid, also resulted in reduced sulfite.

Mitochondria play a role in apoptosis in yeast. This is the phenomenon of self-programmed suicide. Programmed cell death can occur in response to exposure to external stimuli, such as hydrogen peroxide or acetic acid, and via internal cues, such as cell ageing. In a unicellular organism, such as yeast, it may represent an altruistic cooperative phenomenon whereby unfit cells are eliminated from the population and/or subpopulations are selected that possess enhanced properties, such as improved resistance to external stresses. Several apoptotic pathways have been described and some suicidal processes are mediated by mitochondria. Regulation may involve major signalling pathways, such as Ras and TOR, which serve to link the process to inputs from ageing and nutritional status. A group of proteases termed caspases are involved in the signalling process, and the outward manifestations of mitochondrial involvement are release into the cytosol of cytochrome C and nucleases that migrate to the nucleus and once there disrupt DNA and reactive oxygen species (Peirera et al., 2008). Of course, in the case of yeast subjected to the conditions encountered in brewing, the putative roles of mitochondria in apoptosis are less apparent, since these organelles never become fully developed. For example, Madeo et al. (1999) reported that oxygen radicals played an essential role in apoptosis, since conditions that resulted in their depletion, such as anaerobiosis, prevented apoptosis. Conversely, in another study (Aerts et al., 2008) a mutant yeast strain with a much shorter chronological lifespan compared to the wild type was shown, when growing aerobically, to exhibit increased rates of apoptosis and to have dysfunctional mitochondria.

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