In Plant Reaction to Stress

Salicylic acid (SA) (ortho-hydroxybenzoic acid) is one of the phenolic compounds commonly present in higher plants. The highest contents of salicylic acid in the free form as well as in the form of its glucoside have been found and reported in plants of Salicaceae, Betulaceae and Ericaceae families. Volatile methyl salicylate (MeSA) is one of the main ingredients of essential oils of Apiaceae, Violaceae and Rosaceae families, attracting insects and aiding plant reproduction this way (Shlaev et al. 1997) . Salicin, an active extract from Salix alba L. bark, had been used for centuries as a pain-relieving and fever-reducing remedy of which the active ingredient was first isolated as a crystalline compound in 1828 and further converted into a sugar and salicylic acid (Pierpoint 1997) . According to some authors, salicylic acid belongs to a group of plant hormones according to the function it exhibits in plant growth and development, but its content in plants is substantially higher than other phyto-hormones, e.g. jasmonic acid or ethylene. Salicylic acid level in plants is species- and tissue-specific in a concentration range from a few nanograms up to 75 mg g-1 of fresh weight. The highest contents have been documented for ther-mogenic plants of Araceae and Nymphaeaceae families, the compound's function and role lying in an alternative form of respiration to generate heat, spreading their scent, and plants challenged

Fig. 23.1 Salicylic acid biosynthesis in plant cells [according to Czerpak and Bajguz (1998)]

Fig. 23.1 Salicylic acid biosynthesis in plant cells [according to Czerpak and Bajguz (1998)]

with incompatible pathogen infection (Pierpoint 1997; O'Donnell et al. 2001). Salicylic acid influences several physiological and metabolic processes, regulates seed germination, growth of the root system and leaves, chlorophyll biosynthesis, as well as flowering and thermogenesis (Pancheva et al. 1996; Raskin 1992). In plant cytosol, amino acid phenylalanine (2-amino-3-phenylpropionic acid) is a precursor of salicylic acid biosynthesis in the phenylpropanoid pathway via benzoic or coumaric acids as alternative substrates (Metroux 2002) (Fig. 23.1). Salicylic acid synthesis is often accompanied by the generation of other compounds playing a role in resistance mechanisms

Fig. 23.2 Conjugates of salicylic acid in plant cells [according to Czerpak and Bajguz (1998)]

Fig. 23.2 Conjugates of salicylic acid in plant cells [according to Czerpak and Bajguz (1998)]

of plant cells, including phenolic acids (cinnamic, benzoic, coumaric, caffeic, ferulic, etc.) and flavonoids of antioxidant and chelating properties, phytoalexins and lignin (Chong et al. 2001; Metroux 2002). Salicylic acid can be further conjugated into methyl salicylate, a volatile intra-and interplant signal transducer, alerting plant organs as well as neighbouring plants of adverse conditions in the environment (Fig. 23.2). The biosynthesis of methyl salicylate was found for Nicotiana tabacum L. infected with tobacco mosaic virus (TMV). Furthermore, exogenous MeSA induced the biosynthesis of salicylic acid and pathogenesis-related (PR) proteins in tobacco leaves (Lee et al. 1995; Shlaev et al. 1997). Among the conjugated forms, salicylic acid ortho-b-D - glucoside was observed at the highest concentration level in plant cells. Salicylic acid self-regulates the biosynthesis of the glu-coside by activating a specific cytoplasmic UDPglucose:SA glucosyltransferase (GTase) catalyzing its conjugation with a glucose molecule (Fig. 23.2) (Lee et al. 1995), which confirms an active storage mechanism of a locally and sys-temically active free salicylic acid, probably for the purpose of further exposure to stress and

Electromagnetic Spectrum Black And White

Fig. 23.3 The influence of salicylic acid on hydrogen peroxide metabolism during oxidative stress superoxide dismutase ascorbate peroxidase *02" + 2 H+ H202 H202 RO + H20

Fig. 23.3 The influence of salicylic acid on hydrogen peroxide metabolism during oxidative stress cross-tolerance. Moreover, the conjugation is probably an effective detoxification of salicylic acid, when its local concentration may exceed the phytotoxicity threshold (Ribnicky et al. 1998).

The induction of salicylic acid biosynthesis and its function are crucial for plant defence mechanisms and have been well documented for plant-pathogen/herbivore interactions (Klessig et al. 2000; Meuwly et al. 1995). Information on the compound's biosynthesis in response to abiotic factors causing oxidative stress is not as well documented, but its function seems to be unspe-cific and probably mimics a hypersensitive response (HR) triggering systemic acquired resistance (SAR) to pathogens. The HR develops directly at the infection site in the early hours after primary infection and is accompanied by an oxidative burst, i.e. overproduction of reactive oxygen species (ROS), enhanced biosynthesis of phenolic compounds (including salicylic acid), cell wall lignification and induction of PR proteins at the infection site and in surrounding cells (Klessig et al. 2000; Wojtaszek 1997). Salicylic acid is also a non-specific regulator of plant resistance to pathogens and probably to other environmental factors responsible for oxidative stress, i.e. tropospheric ozone, xenobiotics, heavy metals, toxic secondary metabolites (mycotox-ins), UV radiation, salt and drought stress, etc. (Koch et al. 2000; Pal et al. 2002; Pasqualini et al. 2002; Zhu 2002). During the hypersensitive response, salicylic acid alters hydrogen peroxide metabolism within plant cells by the induction of superoxide dismutase (SOD), NADPH oxidase activity and suppression of catalase (CAT) and ascorbate peroxidase (APx), responsible for hydrogen peroxide accumulation, which oxidizes cell constituents, reduces photosynthesis efficiency and destroys cell membranes, leading to programmed cell death (PCD) (Fig. 23.3) (Rao et al. 1997).

The function of the apoptosis is to form a lesion in the form of a ring of dead cells around the infection site to prevent the spread of the pathogen. Later developed SAR requires the existence of a mobile molecule for signal transduction via phloem from the infection site to uninfected plant organs. Salicylic acid in the free form is postulated to serve as a signal transduction compound, a messenger inducing the biosynthesis of PR proteins in healthy plant parts to enhance plant resistance and prevent future infections.

In our earlier studies ((Drzewiecka et al. 2012), we investigated the impact of ambient ozone on the biosynthesis of salicylic acid in leaves of two tobacco cultivars showing diverse sensitivity to ozone (Bel-W3 - sensitive, and Bel-B -resistant). The main aim of the study was to generate information on the possibility of the compound's application as a biomarker of ozone-caused oxidative stress informing of the possible negative influence of tropospheric ozone on trees, crops and plants used in phytoremediation. Tropospheric (ground level) ozone has been known to be a highly oxidizing molecule for plant tissue since the first observations of ozone-caused injury of crops in the 1950s in the United States. Proliferation and constantly increasing concentrations make ozone one of the constituents of the ambient air causing considerable devastation among wild-growing plants, reducing annual tree increments and negatively influencing species biodiversity. In addition, ozone reduces the yield of sensitive crops and decreases their commercial value (Black et al. 2000). Ozone concentration in the troposphere has increased fourfold since the beginning of the industrial era and its peak values in the most industrialized countries achieved 100-400 ppb (Kley et al. 1999). Mean ozone concentrations in Europe and North America during summer months continue to grow (by 0.2-1% annually) and are sufficient to cause damage in ozone sensitive plants (Drzewiecka et al. 2012; Vingarzan 2004).

Tobacco plants (Nicotiana tabacum L.) of both cultivars were exposed to ambient air of the city of Poznan (a city located in west-central Poland) and surrounding rural areas according to the standard VDI methodology (2000) applied in the biomonitoring of air contamination with ozone in European countries in years 1999-2002 (Klumpp et al. 2006) . The exposure of the Bel-W3 plants to ozone caused a significant increase, on average fourfold, in the content of free salicylic acid (SA) and a nearly 20-fold increase in the total salicylic acid (TSA - free and in the form of glucoside) in leaves showing ozone-caused injuries (compared with control plants). The exposure of the ozone-tolerant Bel-B cultivar resulted in a slight, insignificant increase in the salicylic acid content only. SA observed for the tobacco leaves of the Bel-W3 cultivar exhibiting almost 100% injuries was at the concentration level close to nearly 20 mg g-1 FW. This was several times lower than the salicylic acid content in tobacco leaves inoculated with TMV 6 hours after inoculation (approximately 75 mg g-1 FW) (Enyedi et al. 1992). It is possible that distinct threshold concentrations of salicylic acid are required to induce PCD in plants challenged with different stress factors (Thulke and Conrath 1998). The exposure of the Bel-W3 tobacco plants reduced the proportion of SA and TSA contents in leaves from about 30% before the exposure to about 12% after it, which confirms a regulatory function of salicylic acid in the biosynthesis of its glucoside. A strong direct proportional relation between BA2H activity and TSA content and a slightly weaker one with SA were found, proving that the hydroxylation of benzoic acid on the ortho position is catalyzed by benzoic acid 2-hydroxylase as a response to ozone presence. A weaker correlation of BA2H activity with the content of free salicylic acid may indicate a relatively rapid transformation of this compound into the glucoside followed by salicylic acid release from the glucoside (Fig. 23.4).

Enhanced biosynthesis of salicylic acid in leaves of the Bel-W3 plants confirms its involvement in PCD via the impact on hydrogen peroxide metabolism in response to tropospheric ozone. According to Durner et al. (1997), salicylic acid influences the cellular redox state triggering lesion formation. By contrast, plants unable to accumulate salicylic acid (e.g. NahG plants) or having reduced ability to perceive SA (e.g. hybrid poplar clone NE-388) show weaker induction of antioxidant enzymes and a lack of ozone-induced PCD (Koch et al. 2000; Samuel et al. 2000). Our results supplement earlier reports on two-stage accumulation of ROS, mainly H2O2, in the leaves of both tobacco cultivars exposed to ozone in fumigation chambers (Pellinen et al. 1999; Schraudner et al. 1998; Wohlgemuth et al. 2002). In the case of the Bel-B cultivar the first phase during the exposure to ozone only as a result of ozone reactions in the apoplast was observed. In the case of the Bel-W3 cultivar, also the second phase was observed after the exposure, as the result of the induction of defence pathways and a controlled intracellular response to ozone, including biosynthesis of salicylic acid. Furthermore, a highly significant positive linear correlation between the content of salicylic acid and ozone-induced injuries of tobacco leaves was observed. However, in the case of moderate injuries, SA was strongly correlated with the number of lesions observed on the leaf surface (Fig. 23.5). This confirms the highest rate of salicylic acid biosynthesis as the plants' earliest response to stress.

Fig. 23.4 Linear correlation between the activity of benzoic acid 2-hydroxylase (BA2H) and salicylic acid content in tobacco Bel-W3 leaves after exposure to tropo-spheric ozone (a) free salicylic acid (SA);

(b) salicylic acid released from glucoside (SAG);

(c) total salicylic acid (TSA)

Fig. 23.4 Linear correlation between the activity of benzoic acid 2-hydroxylase (BA2H) and salicylic acid content in tobacco Bel-W3 leaves after exposure to tropo-spheric ozone (a) free salicylic acid (SA);

(b) salicylic acid released from glucoside (SAG);

(c) total salicylic acid (TSA)

Lappteknik Duk

Fig. 23.5 Linear correlations between number oflesions the form of glucoside) content in tobacco Bel-W3 leaves (a, b) or leaf injury (c, d) and salicylic acid (SA - free after exposure to tropospheric ozone (for medium and salicylic acid; TSA - total salicylic acid, i.e. free and in full-scale injuries, respectively)

Fig. 23.5 Linear correlations between number oflesions the form of glucoside) content in tobacco Bel-W3 leaves (a, b) or leaf injury (c, d) and salicylic acid (SA - free after exposure to tropospheric ozone (for medium and salicylic acid; TSA - total salicylic acid, i.e. free and in full-scale injuries, respectively)

Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment