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Transition from quiescence to growth

The passage from quiescence to growth occurs in the lag phase of fermentation. It is in the interest of brewers to have as short a lag time as possible, and therefore identification of the factors that regulate its duration is of commercial importance. Just as entry into quiescence is via a programmed sequence of cellular events, so is exit. The process is distinct from growth, as measured in terms of increase in cellular biomass, and proliferation, as manifested by budding. Exit from quiescence must be as tightly regulated as entry and must only occur when conditions are favourable for growth to occur. By inference, premature exit will cause cells to lose their resistant phenotypes and almost certainly result in cell death. Nevertheless, as described in the previous section, the evidence suggests that in the quiescent phase cells have adopted a phenotypic state in which they are poised to pass into the growth phase as soon as conditions allow. Daignan-Fornier and Sagot (2011) summarized the quiescent stage as a convergence between cellular adaption to cope with adversity and preparation for efficient resumption of proliferation.

Laporte et al. (2011) used re-ordering of the actin skeleton and disassembly of the proteasome storage granule as markers of passage into growth, and noted that addition of glucose alone was sufficient to trigger these changes. They concluded that the presence of the sugar was sufficient to initiate the early steps of passage from quiescence, and this occurred even when de novo protein synthesis was inhibited, and thus independent of growth and proliferation. Activation by glucose required metabolism at least as far as pyruvate but did not require the formation of ATP. This adds weight to the suggestion that there is a two-way interplay between cellular signalling and regulatory systems and the intracellular concentrations of relatively small molecular weight metabolic intermediates. These metabolic activities are not simply required for energy generation (McKnight, 2010).

 
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