Signaling Role of ROS
During evolution ROS were steadily forming a constraint for the growth of both, autotrophic and heterotrophic life forms as living plants that always needed to adapt to the ROS containing environment. Therefore in billions of years of evolutionary acclimation not only the damaging effects of ROS were defeated but ROS developed a strong signaling role as the trigger mechanism for the processes that help to acclimatize to high ROS levels or even need ROS as cofactors. ROS have not only damaging but also a signaling role (Hung et al., 2005; Mubarakshina et al., 2010; Zorina et al., 2011; Kreslavski et al., 2012b, 2013a; Schmitt et al., 2014a). Figure 67 gives a general overview on the molecular generation and signaling of ROS and the acclimation of the photosynthetic apparatus (PA) (Zorina et al., 2011). The response of cells starts with the perception of stress by sensors or the response of sensors to ROS that are formed under stress conditions (Kanesaki et al., 2010).
Typically, cascades of mitogen-activated protein kinases (MAPK), other transcription factors (TFs), Ca2+, phytohormones and other compounds function as sensors and/or transducers (Kaur and Gupta, 2005; Jung et al., 2009). In the following, characteristic examples will be described for the signaling action of :AgO2, H2O2 and O in cells, separately discussed for cyanobacteria (chap. 4.1) and plants (chap. 4.2).
Figure 67. Scheme for perception and transduction of stress signals and formation of ROS as signal molecules for genetic signaling supporting the acclimation of cells to stress conditions (adapted from Zorina et al., 2011). Image reproduced with permission.
In plant cells, ROS are produced in different organelles, predominantly in chloroplasts and peroxisomes, while the contribution from mitochondria is smaller (Foyer and Noctor, 2005). Imaging of oxidative stress in leaves of Arabidopsis thaliana revealed that :AgO2 and O are primarily located in mesophyll tissues, while H2O2 was predominantly detected in vascular tissues (Fryer et al., 2002), see Figure 63 in chap. 3.2.1.
Another important approach to study effects of different ROS next to the selection of special assay conditions is given by the use of mutant strains that differ in the generation of individual ROS and/or the content of protection systems/enzymes. This point is of high relevance for studies on specific signaling pathways (see chap. 4 “ROS signaling in coupled nonlinear systems”).
Detailed studies of such systems have been conducted in cyanobacteria. The important aspects of superoxide and hydrogen peroxide signa?ling in cyanobacteria have been treated separately (chap. 4.1). As mentioned above, cyanobacteria serve as efficient models for studying the molecular mechanisms of stress responses. The genes of these cells can be easily knocked out or overexpressed which permit straightforward approaches to investigate the genetic aspects of signaling. This enabled the intensive studies of the potential stress sensors and signal transducers in cyanobacteria (Los et al. 2010; Kanesaki et al., 2010; Zorina et al., 2011, Kreslavski et al., 2013a).
Resulting from a long evolutionary adaption process, systems of perception and transduction of stress signals, as well as the hormonal regulation system (see Figure 67) work in close coordination. Their interaction was fine-tuned during billions of years of evolution.
In the cytoplasm of plant cells, low temperature, drought and salinity cause an increased concentration of Ca2+. In this case, calcium channels may serve as multifunctional sensors that perceive stress-induced changes in the physical properties of cell membranes (see Figure 67). The discovery of such multifunctional sensory systems is important to understand perception and transmission of stress signals. Apparently, changes in membrane fluidity, regardless of the nature of the stress effect are a signal that is perceived by sensory histidine kinases or ion channels localized in the membranes (Kanesaki et al., 2007; Zorina et al., 2011; Los et al., 2013; Kreslavski et al., 2013a).
It is known that ROS are produced in all cell compartments and their formation is necessary for the functioning of photosynthetic organisms (Suzuki and Mittler, 2006). Certain ROS are considered as signaling molecules and regulators of expression of some chloroplast and nuclear genes (Schmitt et al., 2014a; Kreslavski et al., 2013a; Minibayeva et al., 1998; Minibayeva and Gordon, 2003; Desikan et al., 2001, 2003; Hung et al., 2005; Galvez-Valdivieso and Mullineaux, 2010; Mubarakshina et al., 2010; Dickinson and Chang, 2011). A new view on the effects of ROS as signaling molecules first appeared in the study of hormone signaling and the regulation of expression of genes involved in plant protection from pathogen infections (Chen et al., 1993), conditions under which interactions of ROS with salicylic acid and nitric oxide play a crucial role in regulation of the response to infection (Wilson I.D. et al., 2008; Kreslavski et al., 2013a; Vallad and Goodman, 2004).
One of the key points in understanding of the effect of ROS on photosynthesis was the discovery of the formation of the superoxide anion and hydrogen peroxide in the pseudo-cyclic electron transport (See Figure 53 and Figure 54), which does not lead to the reduction of NADP+, but to the absorption of O2 (Asada, 1999). In addition, it was shown that the activation of plasma membrane redox-systems and the increased formation of ROS in the apoplast is one of the universal reactions of plant cells to stress (Kreslavski et al., 2013a; Minibayeva et al., 1998, 2009; Minibayeva and Gordon, 2003; Dickinson and Chang, 2011).
It was found that the main generators of ROS in the apoplast of root cells are the cell wall peroxidases (Minibayeva et al., 2009; Minibayeva and Gordon, 2003). Apparently, the release of ROS from cells followed by a switch of peroxidase/oxidase modes of extracellular peroxidases form the basis for the fast response of plant cells to stress.
In addition to ROS, the stress signaling functions may be attributed to some metabolites, whose formation is initiated by ROS, for example, the products of lipid peroxidation (LP). The primary subjects for peroxidation in living cells are unsaturated fatty acids that constitute major components of phospho- and glycolipids of biological membranes.
ROS regulate the processes of polar growth, the activity of stomata, light-induced movement of chloroplasts and plant responses to the action of biotic and abiotic environmental factors (Pitzschke and Hirt, 2006; Miller et al., 2007; Swanson and Gilroy, 2010).
Signaling by ROS may be realized through changes in potential of the redox-sensitive cell systems and through phosphorylation/dephos- phorylation cycles of signaling proteins (transcription factors, etc). The accumulation of redox-active compounds such as ROS within the chlo- roplast is associated with the rate of photosynthetic electron transport. Redox-sensitive thioredoxin or PQ may act as sensors of changes in redox properties under stress conditions (Figure 54). Signals generated from modulation in the activity of ETC may also lead to changes in gene expression (Vallad and Goodman, 2004).
Although many things have been ruled out from the mechanisms of action of ROS as signal molecules, there are still many gaps in understanding the complete network of these regulatory events. The sensor(s) of H2O2 in higher plants remain largely unknown (Galvez-Valdivieso and Mullineaux, 2010; Mubarakshina et al., 2010; Kreslavski et al., 2012b). There is no information about specific proteins that convert a signal about an increase in the intracellular ROS levels to a biochemical response in the cells. It is not known exactly which particular ROS play a signaling role in the chloroplast and other cellular compartments and how different signaling pathways respond to an increase in the level of different types of ROS. Knowledge of the mechanisms of regulation of these signaling pathways may help to construct biochemical pathways and to produce genetically engineered plants with enhanced stress resistance.
High light, especially high doses of UV-A or UV-B lead to the damage of the PA. Plastoquinones (the primary and secondary plastoquinone, Qa and Qb, respectively) as well as the D1 and D2 proteins are amongst the primary targets of UV radiation (Strid et al., 1994; Babu et al., 1999; Asada, 2006; Carvalho et al., 2011). The MmCaOs cluster of PSII is also vulnerable to damage by UV irradiation (Ohnishi et al., 2005, Najafpour et al., 2013, 2015, 2016). However, UV light-induced damage can also depend on the additional interaction with light in the visible region. Or - more generally spoken - the light induced signaling after interaction of visible light with certain sensors leads to the activation of protection mechanisms against UV-A and UV-B.
Red light (RL) of low intensity can alleviate the negative effect of UV radiation on plants and their PA (Lingakumar and Kulandaivelu, 1993; Qi et al., 2000, 2002; Biswal et al., 2003; Sicora et al., 2003; Kreslavski et al., 2012a,b, 2013a,b,c, 2014a,b). Recent studies have shown that low intensity RL pulses activate the phytochrome system, which triggers protective mechanisms against UV-radiation (Kreslavski et al., 2013a, b). However, many details of this protective action of RL acting via the phytochrome system on PA have not been clarified so far.
The phytochrome system plays an important role in plant growth and PA development. This concept is in agreement with recent studies on mutant Arabidopsis strains with deficiencies in different types of phytochromes, which demonstrated that deletion of phytochromes is critical for plant development (Strasser et al., 2010; Zhao et al., 2013). Even if light capable of driving photosynthesis is available, normal seedling greening and plant development is impossible if phytochromes are absent (Strasser et al., 2010; Zhao et al., 2013). The effects of phytochrome deficiency on photosynthetic parameters have been investigated in previous studies, including the impact on PSII activity (Kreslavski et al., 2013b) and Chl (a+b) content (Strasser et al., 2010; Zhao et al., 2013).
Protective effects against UV are caused by the RL-induced formation of the far-red-absorbing active form of phytochrome and/or enhancement of phytochrome biosynthesis as a result of RL illumination (Kreslavski et al., 2012a, 2013b,c). It was suggested that this protective effect is due to decreased Chl degradation and higher stability of the PSII, as well as higher photochemical activity and a reduced damage of thylakoid membranes (Lingakumar and Kulandaivelu, 1993; Biswal et al., 2003; Kreslavski et al., 2004). On the other hand, a decreased phytochrome level can reduce the resistance of the PA. For example, hy2 mutants of Arabidopsis show a decreased level of PhyB and other pytochromes due to reduced biosynthesis of the phytochrome chromophore, phytochromobilin (Parks and Quail, 1991). This hy2 mutant also showed decreased UV-A resistance of PSII, as determined from delayed luminescence emission (Kreslavski et al., 2013b). It was also shown that the resistance of PA in Arabidopsis WT increased after preillumination with RL, whereas in the hy2 mutant the PSII resistance to UV-A did not change upon the same treatment. It was suggested that the PA resistance to UV radiation depends on the ratio of pro- and antioxidant compounds, which can be affected by PhyB and other phytochromes (Kreslavski et al., 2013b). The role of different phytochromes for the UV resistance of PSII has not been studied so far.
PhyB, one of the key phytochromes in green plants, is involved in the synthesis of photosynthetic pigments, chloroplast development (Zhao et al., 2013), as well as in the synthesis of some photosynthetic proteins and stomatal activity (Boccalandro et al., 2009). It is also known that an increased PhyB content can enhance the resistance of the photosynthetic machinery to environmental stress (Thiele et al., 1999; Boccalandro et al., 2009; Carvalho et al., 2011; Kreslavski et al., 2004, 2012a,c, 2013b,c). In particular, transgenic cotton plants, in which the phytochrome B (PhyB) gene of Arabidopsis thaliana was introduced, showed more than a two-fold increase in the photosynthetic rate and more than a four-fold increase in transpiration rate and stomatal conductance (Rao et al. 2011). In addition, the increase of PhyB content in transgenic potato plants (Dara-5 and Dara-12), which are superproducers of PhyB, led to enhanced resistance of the PA to high irradiance (Thiele et al., 1999). It can be suggested that the increased resistance results from higher Chl content or enhanced stomatal conductance.