Molecular Basis of Escape and Anticipation of Stress

The environment on our planet is characterised by regularly occurring changes in abiotic factors. First, the rotation of the Earth causes a day/night cycle with obvious consequences for factors such as light availability, temperature and moisture. Second, as a result of the Earth’s orbit around the sun and the tilt in the axis of the Earth’s rotational plane relative to the orbital plane, different seasons exist—periods of the year marked by typical changes in light intensity, temperature or precipitation.

Day/night cycles and seasonal changes also influence biotic factors. The threat of fungal pathogens, for example, is dependent on temperature and moisture, as spore germination is favoured by high humidity.

Given the amplitude of such regular changes— for example, day/night temperature fluctuations in deserts, warm summers and winters with subzero temperatures in Central Europe or mild winters and dry summers in the Mediterranean— they can potentially represent severe stress. The anticipation of recurring stressful environmental conditions is therefore an extremely useful strategy employed by all kinds of organisms to limit damage and the cost of investment in acclimative processes. The needles of many conifers, for example, begin their cold acclimation, measurable as an increase in cold hardiness, even before the temperatures actually drop in autumn (Fig. 2.26).

Fig. 2.26. Changes in the frost hardiness of pine needles over the course of a year. The upper panel shows temperature data (upper/lower line: 2 m/5 cm, respectively above ground) and day lengths in Bayreuth (330 m above sea level). The frost hardiness of Pinus sylvestris needles was determined as the temperature at which tissue damage occurs (lower panel).

An increase in fros tolerance is already detectable when the day length falls below a threshold (1) and before frost actually occurs for the first time (2). Green indicates the phase of relative frost sensitivity, magenta indicates the frost-hardening phases, blue indicates the phases of extreme frost tolerance, and yellow the phases of de-hardening (Modified from Hansen (2000))

More generally, the restriction of physiological activity to seasons with more favourable conditions (the escape strategy) can ensure survival and reproductive success. Across the eukaryotic domain, biological clocks generate intrinsic circadian rhythms that help organisms coordinate physiological activities with fluctuating environmental conditions in day/night cycles.

Furthermore, combined with the perception of light, the circadian clock enables the perception of changes in the day length (photoperiodism) and thereby the anticipation of seasons. The latter is also afforded by reliable perception of abiotic conditions typical of a cold winter. This is exemplified by vernalisation. Many plants require prolonged exposure to cold temperatures before they develop an inflorescence. This ensures a switch to reproduction only after a typical winter in the subpolar oceanic climate has been endured.