188 Marine Macro- and Microalgae: An Overview
potato virus X (PVX) in the leaves. Fucans purified from the fucoid alga Pelvetia canaliculata activated
both local and systemic resistance in tobacco against TMV resulting in suppression of multiplication of
the virus (Klarzynski et al. 2003). Similar results were reported with the use of oligosaccharides obtained
from Chilean marine macroalgae (Laporte et al. 2007). The application of oligosaccharides, for example,
guluronic acid (Poly-Gu) extracted from the blades of Lessonia trabeculata, mannuronic acid (Poly-Ma)
from L. vadosa and sulphated galactan (Poly-Ga) from the blades of Schizymenia binderi to tobacco
plants, stimulated plant defences against tobacco mosaic virus, seven and 15 days after spray treatment.
Vera et al. (2011a) also studied the anti-viral effect of the Poly-Ga in tobacco leaves infected with TMV.
The number of necrotic lesions was lower in the treatment than the control, with increasing number of
applications and concentrations of Poly-Ga. The levels of TMV-capsid protein (CP) transcripts decreased
in the distal leaves, indicating that Poly-Ga induced systemic protection against TMV. It was shown
that increased activity of the defence enzymes correlated with decreased numbers of necrotic lesions
and TMV-CP transcript levels. Pre-treatment of tomato plants with λ-carrageenan significantly reduced
the symptoms of the Tomato Chlorotic Dwarf Viroid (TCDVd) eight weeks after inoculation (Sangha et
al. 2015). Taken together, the above studies suggested that certain seaweed extracts have considerable
potential to protect plants against virii and viroids.
Mechanisms of seaweed extract-induced plant defences to pathogens
The response of plants to pathogens involves various pathways that can be regulated by molecules such
as salicylic acid (SA), jasmonic acid (JA), and ethylene (ETH) (Grant and Lamb 2006). Plant disease
responses to various seaweed extracts can be due to the elicitor-like abilities of peptides, proteins, lipids,
cell wall debris, oligo- or polysaccharides, carrageenans, ulvans, laminarin, β 1–3 glucans or sulphated
fucans, or some combination of these, within the extract (Kobayashi et al. 1993; Mercier et al. 2001;
Subramanian et al. 2011). Different mechanisms have been reported behind disease resistance associated
with elicitation with various seaweed extracts. Using the model plant Arabidopsis, it has been shown that
the jasmonic acid pathway plays a critical role in plant defence responses elicited by a commercial extract
of Ascophyllum nodosum (Subramanian et al. 2011). Inoculation of Pst DC3000 on A. nodosum extracttreated plants (by root irrigation), showed a JA-dependent, induced systemic resistance. This was evident
when the genes associated in JA-signaling, such as allene oxide synthase (AOS) and ‘plant defensin 1.2’
(PDF1.2) were strongly expressed in the A. nodosum-treated plants. Pathogens which are inhibited by
the JA-pathway are most often necrotrophs. Many devastating diseases in agricultural crops are caused
by necrotrophic fungal pathogens such as Alternaria, Fusarium, Botrytis, Verticillium, Phytophthora
and Pythium and bacterial pathogens such as Xanthomonas, Pseudomonas, and Erwinia. Thus, seaweed
extracts such as A. nodosum which can elicit JA-response in plants may offer protection to many of these
phytopathogens.
The oligo- and polysaccharides of seaweed extracts are reported to affect a broad spectrum and multiple
plant defence responses to pathogens (Klarzynsci et al. 2003; Cluzet et al. 2004; Stadnik and Freitas
2014; de Freitas and Stadnik 2015; Van Oosten et al. 2017). Phytohormones have been detected in some
seaweed extracts which might modulate the bio-stimulatory and stress tolerance plant responses (Górka
and Wieczorek 2017). The seaweeds also activate a variety of phytohormone responses in the treated plants
to the different stressors. Ulvans from green algae have been shown to activate plant defences through the
JA-signaling pathway, but not via a SA-dependent pathway, as shown in Nicotianae tabacum (Solanaceae),
Arabidopsis thaliana (Brassicaceae), and Medicago truncatula (Cluzet et al. 2004; Jaulneau et al. 2010).
The response of ulvan-induced gene expression in Medicago truncatula was similar to that observed with
methyl jasmonate (Me-JA)-treatment; there was increased proteinase inhibitory activity, which is a marker
for the Me-JA response. Although, exogenous application of λ-carrageenan at low concentrations, increased
salicylic acid (SA) levels in the plant, the JA and ethylene levels were also increased (Mercier et al. 2001).
In addition, ulvan-induced expression of JA-dependent genes, such as PDF1.2 (‘defensin’) in Arabidopsis
thaliana and the lipoxygenase (‘NtLOX1’) promoter in Nicotiana tabacum, fucan, and carrageenans, also
induced lipoxygenase gene expression in tobacco (Mercier et al. 2001; Klarzynski et al. 2003), suggesting
potato virus X (PVX) in the leaves. Fucans purified from the fucoid alga Pelvetia canaliculata activated
both local and systemic resistance in tobacco against TMV resulting in suppression of multiplication of
the virus (Klarzynski et al. 2003). Similar results were reported with the use of oligosaccharides obtained
from Chilean marine macroalgae (Laporte et al. 2007). The application of oligosaccharides, for example,
guluronic acid (Poly-Gu) extracted from the blades of Lessonia trabeculata, mannuronic acid (Poly-Ma)
from L. vadosa and sulphated galactan (Poly-Ga) from the blades of Schizymenia binderi to tobacco
plants, stimulated plant defences against tobacco mosaic virus, seven and 15 days after spray treatment.
Vera et al. (2011a) also studied the anti-viral effect of the Poly-Ga in tobacco leaves infected with TMV.
The number of necrotic lesions was lower in the treatment than the control, with increasing number of
applications and concentrations of Poly-Ga. The levels of TMV-capsid protein (CP) transcripts decreased
in the distal leaves, indicating that Poly-Ga induced systemic protection against TMV. It was shown
that increased activity of the defence enzymes correlated with decreased numbers of necrotic lesions
and TMV-CP transcript levels. Pre-treatment of tomato plants with λ-carrageenan significantly reduced
the symptoms of the Tomato Chlorotic Dwarf Viroid (TCDVd) eight weeks after inoculation (Sangha et
al. 2015). Taken together, the above studies suggested that certain seaweed extracts have considerable
potential to protect plants against virii and viroids.
Mechanisms of seaweed extract-induced plant defences to pathogens
The response of plants to pathogens involves various pathways that can be regulated by molecules such
as salicylic acid (SA), jasmonic acid (JA), and ethylene (ETH) (Grant and Lamb 2006). Plant disease
responses to various seaweed extracts can be due to the elicitor-like abilities of peptides, proteins, lipids,
cell wall debris, oligo- or polysaccharides, carrageenans, ulvans, laminarin, β 1–3 glucans or sulphated
fucans, or some combination of these, within the extract (Kobayashi et al. 1993; Mercier et al. 2001;
Subramanian et al. 2011). Different mechanisms have been reported behind disease resistance associated
with elicitation with various seaweed extracts. Using the model plant Arabidopsis, it has been shown that
the jasmonic acid pathway plays a critical role in plant defence responses elicited by a commercial extract
of Ascophyllum nodosum (Subramanian et al. 2011). Inoculation of Pst DC3000 on A. nodosum extracttreated plants (by root irrigation), showed a JA-dependent, induced systemic resistance. This was evident
when the genes associated in JA-signaling, such as allene oxide synthase (AOS) and ‘plant defensin 1.2’
(PDF1.2) were strongly expressed in the A. nodosum-treated plants. Pathogens which are inhibited by
the JA-pathway are most often necrotrophs. Many devastating diseases in agricultural crops are caused
by necrotrophic fungal pathogens such as Alternaria, Fusarium, Botrytis, Verticillium, Phytophthora
and Pythium and bacterial pathogens such as Xanthomonas, Pseudomonas, and Erwinia. Thus, seaweed
extracts such as A. nodosum which can elicit JA-response in plants may offer protection to many of these
phytopathogens.
The oligo- and polysaccharides of seaweed extracts are reported to affect a broad spectrum and multiple
plant defence responses to pathogens (Klarzynsci et al. 2003; Cluzet et al. 2004; Stadnik and Freitas
2014; de Freitas and Stadnik 2015; Van Oosten et al. 2017). Phytohormones have been detected in some
seaweed extracts which might modulate the bio-stimulatory and stress tolerance plant responses (Górka
and Wieczorek 2017). The seaweeds also activate a variety of phytohormone responses in the treated plants
to the different stressors. Ulvans from green algae have been shown to activate plant defences through the
JA-signaling pathway, but not via a SA-dependent pathway, as shown in Nicotianae tabacum (Solanaceae),
Arabidopsis thaliana (Brassicaceae), and Medicago truncatula (Cluzet et al. 2004; Jaulneau et al. 2010).
The response of ulvan-induced gene expression in Medicago truncatula was similar to that observed with
methyl jasmonate (Me-JA)-treatment; there was increased proteinase inhibitory activity, which is a marker
for the Me-JA response. Although, exogenous application of λ-carrageenan at low concentrations, increased
salicylic acid (SA) levels in the plant, the JA and ethylene levels were also increased (Mercier et al. 2001).
In addition, ulvan-induced expression of JA-dependent genes, such as PDF1.2 (‘defensin’) in Arabidopsis
thaliana and the lipoxygenase (‘NtLOX1’) promoter in Nicotiana tabacum, fucan, and carrageenans, also
induced lipoxygenase gene expression in tobacco (Mercier et al. 2001; Klarzynski et al. 2003), suggesting