Influences on the photosynthetic process by hormones and sugars

1.3.3.1. Salicylic acid

Salicylic acid (SA) is a compound that can be found in all parts of a plant where it has a diversity of functions (Lee et al. 1995). It plays an important role together with ROS in the response to biotic stresses (Draper 1997; Durner et al 1997; Lamb and Dixon 1997; van Camp et al. 1998) and is also required for a process called systemic acquired resistance (SAR). SAR is defined as a resistance to subsequent pathogen attack that develops in the uninfected, pathogen-free parts of the plant after the initial inoculation (Ross 1961). Additionally, it has been shown that SA signalling is affected by EEE and that SA effects both photosynthesis and stomatal conductance (Genoud et al. 2002; Karpinski et al. 2003; Chaerle et al. 2004; Zeier et al. 2004). SA is also involved in long-term acclimatory processes (Karpinski et al. 2003). These observations suggest that SA is in a central position of crosstalk between the signalling pathways of acclimatory and defence responses.

1.3.3.2. Ethylene

Ethylene (ET) plays a vital role in several aspects of plant growth and development (Johnson and Ecker 1998) and is particularly important regulator of stress responses (Wang et al. 2002). Its synthesis follows via a well defined and tightly regulated pathway that responds to several developmental and environmental stimuli. Furthermore, it plays an important role in cell death, such as senescence (Hadfield and Benett 1997), aerenchyma formation (Drew et al. 2000) and in

ROS-Figure 7: Schematic overview of the five retrograde signalling pathways D,H,I refer to three subunits from the Mg²⁺-Chelatase (from Beck et al. 2005)

induced cell death (Kangasjarvi et al. 2005). Additionally, ET interacts with SA signalling (Devadas 2002) indicating its role in biotic stress responses. Studies have shown that ET can inhibit photosynthesis (Kays and Pallas 1980) and that nutrient status and leaf age can affect ET signalling (Legé et al. 1997). It is discussed that ET plays a role in the crosstalk between EEE and the programmed cell death signalling pathway (Mühlenbock 2006).

1.3.3.3. Nitric oxide

Another group of signalling molecules involved in biotic and abiotic stress responses are reactive nitrogen species (RNS). The most conspicuous of them is the gaseous secondary messenger NO, also highly reactive with different biomolecules, in particular proteins (Neill et al. 2002). The role of NO as an antioxidant agent has been shown in a broad range of abiotic stress responses. This molecule increases the plant tolerance to drought (García-Mata and Lamattina 2002), is involved in the induction of salt resistance (Zhao et al. 2004) and protects from the oxidative stress derived from treatment with methylviologen (Beligni and Lamattina 1999). On the other hand, biotic stress responses are also associated with NO production. Accumulation of NO occurs after infection with avirulent strains of plant pathogens (Delledonne et al. 1998; Wojtaszek 2000). As NO reacts with O2−, peroxynitrite is formed (Stamler et al. 1992). Apparently, this NO derivative is not involved in plant cell death, as it was initially believed by analogy with the mammalian system.

In fact, the high degree of reactivity between NO and O2−is rather considered to accomplish ROS-scavenging or regulatory functions in planta (Beligni and Lamattina 1999; Delledonne et al. 2001;

Delledonne et al. 2002; Romero-Puertas et al. 2004). NO itself also modifies thiol-containing residues in a process denominated S-nitrosylation. This capacity to modify proteins confers potential to trigger redox signalling. Moreover, NO is able to react with transition metal centres. Therefore, it affects the activity of heme- and iron-sulphur containing proteins such as mitochondrial cytochrome c, guanylyl cyclase and aconitase (Stamler et al. 1992; Yamasaki and Sakihama 2000).

1.3.3.4. Sugars

In plants, sugars not only function as metabolic resources and structural constituents of cells, but they also act as important regulators of various processes associated with plant growth and development. A variety of genes, whose products are involved in diverse metabolic pathways and cellular functions, are either induced or repressed depending upon the availability of soluble sugars. In general, sugars favour the expression of enzymes in connection with biosynthesis, use and storage of reserves (including starch, lipid, and proteins), while repressing the expression of enzymes involved in photosynthesis and reserve mobilization (Koch 1996).

In oxygenic photosynthetic organisms, especially higher plants, sucrose and all the array of enzymes and proteins related to its processing developed into a central role between photosynthesis, transport, and heterotrophic utilization (Salerno and Curatti 2003). Soluble sugars seem to assume a dual role with respect to ROS. Soluble sugars can be involved in, or related to, ROS-producing metabolic pathways. In reverse, soluble sugars can also feed NADPH-producing metabolic pathways, such as the oxidative pentose-phosphate pathway, which can contribute to ROS scavenging.

However, an important ROS-producing situation, such as high photosynthetic activity is associated with accumulation of soluble sugars. Moreover, in reverse, accumulation of soluble sugars negatively regulates photosynthesis gene expression (Koch 1996; Pego et al. 2000; Rolland et al. 2002),

including expression of Calvin cycle genes. This may cause, at least transiently, poor recycling of NADP⁺ and excessive electron transfer that may lead to ROS production, even under ambient illumination.

These relationships between excess light and sugar accumulation could be the basis for the selection of parallel induction of gene expression by light and sugar in plant cells. Moreover, these relationships between light and sugar are strongly compounded in situations of abiotic stress, such as chilling. Chilling promotes both photo-oxidative damage (Harvaux and Kloppstech 2001) and the accumulation of sugars, which are supposed to act as cold-stress protectants (Ciereszko et al. 2001).

Indeed, this parallel induction of genes by excess light and excess sugar is verified for genes encoding proteins involved in excess photon removal, such as chalcone synthase, the pivotal step of flavonoid synthesis, or in ROS defence, such as SOD (Feinbaum et al. 1991; Koch 1996; Rossel et al. 2002).