The photoreceptor interaction network

Abstract

Plants are exposed to stressful conditions including the aggressive transition between belowground and aboveground environments, the presence of competitors, and seasons with extreme temperatures or shortage of water. Specific combinations of irradiance, spectral composition, duration of the daily photoperiod and angle of incidence of light are tightly linked to these threats and very often anticipate their impending occurrence (Figure 1). When an organ emerges from the soil, it experiences a transition between full darkness and daily light cycles. The ratio between red light and far-red light (R:FR) is inversely related to the presence, size and position of neighbour plants that can compete for resources, because green leaves reflect and transmit much more far-red than red light (see Chapter 22). The photoperiod correlates with time of the year. The angle of incidence depends on the position of neighbours and soil aggregates or stones. The perception and transduction of these signals by the network formed by phytochromes, cryptochromes, phototropins and their downstream signalling elements leads to changes in plant growth and development that favour adjustment to the associated stressful conditions. In addition to the light signals, plants are also exposed to noise from the light environment. This noise is caused by fluctuations that do not carry ecologically relevant information. Some of these fluctuations are in the range of magnitude of the light signals. This is the case, for instance, of the reductions in R:FR that take place at the beginning and the end of the photoperiod due to atmospheric reasons and not to the presence neighbours. Furthermore, some fluctuations can be signals at a given stage and noise at another. The photoperiod is indicative of season but a seedling that emerges from the soil has to respond to light no matter whether the day is short or long. The evolutionary significance of the occurrence of different photoreceptors is partially accounted for by presence of the divergent signals described above. Phytochrome A (phyA), for instance, is the only photoreceptor able to perceive the difference between belowground darkness and the low R:FR experienced beneath a deep canopy (Yanovsky et al., 1995). However, as a result of its photochemical properties and signalling cascade, phyA is not a good receptor of changes in R:FR or light direction. Conversely, phytochrome B (phyB) is a good sensor of changes in R:FR but it is not involved in the response to very low R:FR compared to darkness (Quail et al., 1995). In this chapter, we will describe an additional consequence of the occurrence of multiple photoreceptors: the formation of a network that provides a refined response to the signals against the background noise.