The Dual Significance of Light

Sunlight is by far the most dominant energy source for all life on Earth. For plants, as sessile photoautotrophic organisms, it is one of the most important environmental factors. Light varies in intensity and spectral composition from place to place, from season to season, and in the course of the daily photoperiod. In order to optimally harvest the energy of light and to minimise stress arising from insufficient or supraoptimal absorption of photons, plants have evolved multiple ways to modulate light exposure and photosynthesis.

Furthermore, light quality (colour), intensity (quantum flux) and duration (day length) have a second function as environmental cues, which guide the plant through its entire life cycle. Developmental plasticity of plants enables effective adaptation and stress avoidance strategies and is to a large extent based on the ability to perceive and transduce light signals. Key transitions from seed to germination, through subsequent phases of vegetative growth, to flowering and finally senescence of the entire plant or its organs are regulated in response to a fluctuating light environment.

Plants have evolved photosynthetic pigments such as chlorophylls to harvest the energy of light, and a diverse set of photoreceptors to monitor the intensity, spectral composition and direction of light. Photoreceptors differ from the photosynthetic pigments with respect to absorption spectra, intracellular localisation and the effects of excitation by photons. While the lightharvesting systems are found in the chloroplasts/ thylakoids, the sensors regulating developmental processes are localised in the cytosol and the nucleus. Excitation of chlorophyll initiates an electron transfer chain coupled to the production of adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH).

Excitation of photoreceptors triggers signal transduction processes and concomitant changes in gene expression. In spite of the fundamental differences in the molecular, biochemical and biophysical processes involved, the two functions of light for plants—as an energy source and as an environmental cue—are tightly interlaced. Many of the responses of a plant to changing light conditions result in an optimisation of utilisation of light for photosynthesis.

Terrestrial plants, as well as the majority of algae, use blue and red light for photosynthesis. The respective accessory pigments are carotenoids and chlorophyll b (in some algae also other chlorophylls), while the reactive photopigments are dimers of chlorophyll a, termed P680 and P700, respectively. The bulk of chlorophyll a serves as a light-harvesting pigment. Cyanobacteria and red algae have no chlorophyll b but have additional accessory pigments, which allow them to inhabit aquatic biotopes with an altered composition of the visible spectrum.

Phycobiliproteins, assembled in the phycobilisomes, allow these organisms to effectively use blue-green light for photosynthesis and thus to inhabit deeper water layers or muddy waters where green algae cannot harvest sufficient light for photosynthesis. Red light is absorbed in the top layers of a water body and blue light is scattered, resulting in the dominance of blue green at greater depth. Red algae and cyanobacteria are able to dynamically modify the proportions of the red-absorbing and blue-green-absorbing pigments in the phycobilisomes to acclimate to special light climates (MacIntyre et al. 2002) (Fig. 3.1).

Fig. 3.1. Cyanobacteria are photosynthesising prokaryotes in saline as well as fresh waters. a Whips of cyanobacteria accumulating close to the surface in the southern Baltic Sea. The brownish colour of these unicellular organisms does not support their common name, blue-green algae, as the typical phycobilins are often overlaid by carotenoids (Rastogi et al. 2010) (Nordic Microalgae c/o SMHI Oceanographic Unit Sven Källfelts gata 15, SE-426 71 Västra Frölunda Sweden; Copyright by Bengt Karlson). b Colourful zonation of cyanobacteria in the Morning Glory Pool at the Yellowstone National Park, Wyoming (from Encyclopaedia Britannica Online (2016); license no. SSTK-041E2-CF2B by Shutterstock)

A minor yet highly relevant component of the solar radiation reaching the Earth’s surface is ultraviolet (UV) light. Because of its high energy it can be damaging to the macromolecules of biological systems, including DNA. Therefore, plants—like other organisms colonising the land—have evolved very effective mechanisms for protection against UV radiation and to repair UV-induced damage. Activation of repair is in part dependent on the perception of UV light as a signal.

Avoidance of damage by excessive irradiation or of low light, as well as acclimation to different and changing light climates, occur at different levels: morphological, anatomical, cellular, subcellular and molecular. Several of the responses, particularly the morphological and structural ones, are principally irreversible—for instance, the formation of shade and sun leaves. Others are dynamic and reversible, such as hinge movements of entire leaves or the displacement of chloroplasts within the cells. In a few cases (e.g. the shade avoidance response and state transitions), mechanisms have been elucidated in molecular detail.

This chapter will focus on avoidance and acclimation responses of plants to fluctuating light climate and to UV exposure, as well as the perception systems underlying light-controlled plasticity of plant development. For the mechanisms of photosynthesis and comprehensive accounts of light signal transduction, the reader is referred to plant physiology and plant biochemistry textbooks.