Potential Use of Extracts of Seaweeds Against Plant Pathogens 179
sterols, phenols, fatty acids, terpenoids, and polysaccharides present in different seaweed species, either
exert direct anti-microbial action or may act indirectly to elicit plant responses to pathogen infection
(Kamenarska et al. 2002; Khan et al. 2009; Sangha et al. 2011; Subramanian et al. 2011). The number of
compounds isolated from various seaweeds is indeed large, but by no means have all been tested against
plant pathogens. Some of the common and important components which are thought to have roles against
plant pathogens are considered in further detail here:
Polysaccharides: Polysaccharides are major components in seaweeds that are present to a varying extent
in brown, red, and green seaweeds. Cell wall polysaccharides in brown seaweeds, for example, alginates,
sulphated galactans and fucans, as well as the storage polysaccharide laminarin (Klarzynski et al. 2003;
Khan et al. 2009), can elicit defense responses in plants against pathogens (Klarzynski et al. 2000;
Klarzynsci et al. 2003). The polysaccharide fucoidan (obtained from Lessonia vadosa) is another example
of a bioactive polysaccharide that has demonstrated significant activation of defence responses when
applied to plants (Chandia and Matsuhiro 2008). Similarly, sulphated polysaccharides, obtained from red
and green algae, have also been reported to elicit anti-microbial activities. For example, λ-carrageenans
from red seaweeds, and ulvan the principle cell wall polysaccharide obtained from green seaweeds, have
been shown to induce resistance in several plant species after their application (Mercier et al. 2001;
Araújo et al. 2008; Cluzet et al. 2004; Paulert et al. 2011; Jaulneau et al. 2010; Hernández-Herrera et
al. 2014; Esserti et al. 2017; Abouraïcha et al. 2017; Van Oosten et al. 2017). Interestingly, the levels of
bioactivity of polysaccharides seems to vary as per the level of sulphation of the molecules (Mercier et al.
2001; Menard et al. 2005; Sangha et al. 2010) and should be given prior consideration when being used
to elicit plant defense responses.
Terpenes: Terpenes are highly volatile organic compounds produced as secondary metabolites in plants,
seaweeds, insects, and many other organisms. Seaweed terpenes are thought to be produced as a chemical
defense strategy to ward off herbivores and for competitive advantage over reef corals (Bianco et al.
2010; Rasher et al. 2012). Terpenes from brown seaweeds; mostly halogenated, have been tested for
microbial suppression (Katayama 1962; Fenical et al. 1973; Venkatesh et al. 2011; Peres et al. 2012).
Staphylococcus aureus and Eschericia coli were inhibited by seaweed terpenes (Katayama 1962).
Terpenes have been shown to exhibit anti-microbial activity against plant pathogens (Bassolé and Juliani
2012). Sesquiterpenes, such as, zonarol and isozonarol from the brown alga, Dictyopteris zonarioides,
were active against 10 species of plant fungi (Fenical et al. 1973). Similarly, the meroditerpenoid
metabolite, methoxybifurcarenone, from the brown alga Cystoseira tamariscifolia, showed anti-fungal
activity against three tomato pathogenic fungi, for example, Botrytis cinerea, Fusarium oxysporum f.sp.
lycopersici, and Verticillium alboatrum whereas in vitro anti-bacterial activity was also observed against
Agrobacterium tumefaciens and Escherichia coli (Bennanmara et al. 1999). Five meroditerpenes were
isolated from the brown alga Cystoseira spp. (Navarro et al. 2004), that also showed anti-microbial
activity. Peres et al. (2012) reported anti-microbial terpenes present in ethanolic extracts of various
seaweeds viz., the browns Stypopodium zonale, Ascophyllum nodosum, Pelvetia canaliculata, Fucus
spiralis, Sargassum muticum, S. filipendula, S. stenophyllum, and Laminaria hyperborea and the reds
Laurencia dendroidea and Gracilaria edulis were seen to be effective against two plant pathogens. These
reports suggested that seaweed terpenes could be used potentially against plant pathogens.
Fatty acids: Fatty acids have been shown to have strong anti-microbial activity against various
microorganisms (Desbois and Smith 2010). Seaweeds are a well-known source of various fatty acids,
and comprise mainly of polyunsaturated fatty acids (PUFAs) (Khotimchenko et al. 2002; Dawczynski
et al. 2007; Schmid et al. 2017). For some time, it has been known that lipid extracts, from various
brown, green and red seaweeds have been effective against different pathogens under in vitro conditions
(Caccamese et al. 1981) indicating that fatty acids can play a role in the suppression of plant pathogens.
Downstream fractionation of lipophilic extracts from the red Gracilaria edulis, the brown Sargassum
wightii, and the green Enteromorpha* flexuosa (*Enteromorpha are properly named Ulva), and testing
for anti-bacterial activity revealed that their fatty acids, predominantly palmitic acids, were anti-bacterial
against Xanthomonas oryzae pv. oryzae (Arunkumar and Rengasamy 2000). Fatty acids such as palmitic
acid, followed by oleic and myristic acids from the brown alga Padina pavonia have also shown
sterols, phenols, fatty acids, terpenoids, and polysaccharides present in different seaweed species, either
exert direct anti-microbial action or may act indirectly to elicit plant responses to pathogen infection
(Kamenarska et al. 2002; Khan et al. 2009; Sangha et al. 2011; Subramanian et al. 2011). The number of
compounds isolated from various seaweeds is indeed large, but by no means have all been tested against
plant pathogens. Some of the common and important components which are thought to have roles against
plant pathogens are considered in further detail here:
Polysaccharides: Polysaccharides are major components in seaweeds that are present to a varying extent
in brown, red, and green seaweeds. Cell wall polysaccharides in brown seaweeds, for example, alginates,
sulphated galactans and fucans, as well as the storage polysaccharide laminarin (Klarzynski et al. 2003;
Khan et al. 2009), can elicit defense responses in plants against pathogens (Klarzynski et al. 2000;
Klarzynsci et al. 2003). The polysaccharide fucoidan (obtained from Lessonia vadosa) is another example
of a bioactive polysaccharide that has demonstrated significant activation of defence responses when
applied to plants (Chandia and Matsuhiro 2008). Similarly, sulphated polysaccharides, obtained from red
and green algae, have also been reported to elicit anti-microbial activities. For example, λ-carrageenans
from red seaweeds, and ulvan the principle cell wall polysaccharide obtained from green seaweeds, have
been shown to induce resistance in several plant species after their application (Mercier et al. 2001;
Araújo et al. 2008; Cluzet et al. 2004; Paulert et al. 2011; Jaulneau et al. 2010; Hernández-Herrera et
al. 2014; Esserti et al. 2017; Abouraïcha et al. 2017; Van Oosten et al. 2017). Interestingly, the levels of
bioactivity of polysaccharides seems to vary as per the level of sulphation of the molecules (Mercier et al.
2001; Menard et al. 2005; Sangha et al. 2010) and should be given prior consideration when being used
to elicit plant defense responses.
Terpenes: Terpenes are highly volatile organic compounds produced as secondary metabolites in plants,
seaweeds, insects, and many other organisms. Seaweed terpenes are thought to be produced as a chemical
defense strategy to ward off herbivores and for competitive advantage over reef corals (Bianco et al.
2010; Rasher et al. 2012). Terpenes from brown seaweeds; mostly halogenated, have been tested for
microbial suppression (Katayama 1962; Fenical et al. 1973; Venkatesh et al. 2011; Peres et al. 2012).
Staphylococcus aureus and Eschericia coli were inhibited by seaweed terpenes (Katayama 1962).
Terpenes have been shown to exhibit anti-microbial activity against plant pathogens (Bassolé and Juliani
2012). Sesquiterpenes, such as, zonarol and isozonarol from the brown alga, Dictyopteris zonarioides,
were active against 10 species of plant fungi (Fenical et al. 1973). Similarly, the meroditerpenoid
metabolite, methoxybifurcarenone, from the brown alga Cystoseira tamariscifolia, showed anti-fungal
activity against three tomato pathogenic fungi, for example, Botrytis cinerea, Fusarium oxysporum f.sp.
lycopersici, and Verticillium alboatrum whereas in vitro anti-bacterial activity was also observed against
Agrobacterium tumefaciens and Escherichia coli (Bennanmara et al. 1999). Five meroditerpenes were
isolated from the brown alga Cystoseira spp. (Navarro et al. 2004), that also showed anti-microbial
activity. Peres et al. (2012) reported anti-microbial terpenes present in ethanolic extracts of various
seaweeds viz., the browns Stypopodium zonale, Ascophyllum nodosum, Pelvetia canaliculata, Fucus
spiralis, Sargassum muticum, S. filipendula, S. stenophyllum, and Laminaria hyperborea and the reds
Laurencia dendroidea and Gracilaria edulis were seen to be effective against two plant pathogens. These
reports suggested that seaweed terpenes could be used potentially against plant pathogens.
Fatty acids: Fatty acids have been shown to have strong anti-microbial activity against various
microorganisms (Desbois and Smith 2010). Seaweeds are a well-known source of various fatty acids,
and comprise mainly of polyunsaturated fatty acids (PUFAs) (Khotimchenko et al. 2002; Dawczynski
et al. 2007; Schmid et al. 2017). For some time, it has been known that lipid extracts, from various
brown, green and red seaweeds have been effective against different pathogens under in vitro conditions
(Caccamese et al. 1981) indicating that fatty acids can play a role in the suppression of plant pathogens.
Downstream fractionation of lipophilic extracts from the red Gracilaria edulis, the brown Sargassum
wightii, and the green Enteromorpha* flexuosa (*Enteromorpha are properly named Ulva), and testing
for anti-bacterial activity revealed that their fatty acids, predominantly palmitic acids, were anti-bacterial
against Xanthomonas oryzae pv. oryzae (Arunkumar and Rengasamy 2000). Fatty acids such as palmitic
acid, followed by oleic and myristic acids from the brown alga Padina pavonia have also shown