© Benaki Phytopathological Institute
Si increases plant resistance to pathogenic fungi
7
sociated with the reduced severity of the
blast disease (
Magnaporthe grisea
) in sus-
ceptible and partially resistant rice culti-
vars, although the thickness of the epider-
mal cell wall was not significantly affected
by the presence of silicon. For the cucum-
ber–
Podosphaera xanthii
pathosystem,
the
foliar applied silicon produced only phys-
ical barrier and osmotic effect. However,
the root applied silicon led to systemic ac-
quired resistance when plants were infect-
ed by the powdery mildew pathogen (Li-
ang
et al.,
2005). Moreover, Hayasaka
et al.
(2008) confirmed that silicon in the rice leaf
epidermis may confer resistance against
M.
grisea
(blast) appressorial penetration. How-
ever, the prophylactic effect against pow-
dery mildew was lost when silicon feeding
to cucumber plants was interrupted (Sam-
uels
et al.,
1991). Heine
et al.
(2007) report-
ed that the accumulation of silicon in root
cell walls did not represent a physical bar-
rier to the spread of
Pythium aphaniderma-
tum
in the roots of bitter gourd and tomato.
Although these authors concluded that silic-
ified epidermal cell walls in leaves could be
the main factor for the reduction in severi-
ty of plant diseases caused by fungal patho-
gens, they did not report that this was suf-
ficient evidence to explain the impediment
of fungal penetration in the leaves. Based
on these results, it was suggested that resis-
tance to fungal pathogens in plants treated
with silicon was much more complex than
a physical resistance, which was strongly
contested and doubted in recent years (Van
Bockhaven
et al.,
2013).
4.2. Biochemical defense
Regarding the second hypothesis of sil-
icon biochemical enhanced resistance, the
soluble silicon in plant tissue may be associ-
ated with an increase in resistance to fungal
diseases. In this model, the enhancement
of resistance is due to (1) increased activity
of defense-related enzymes in leaves, such
as polyphenoloxidase, peroxidase, pheny-
lalanine ammonia-lyase, and glucanase, (2)
increased production of antifungal com-
pounds, such as phenolic metabolism prod-
ucts (lignin), flavonoids, phytoalexins and
pathogenesis-related proteins in plants, and
(3) activation of some plant defense-relat-
ed genes (Fauteux
et al.,
2005; Datnoff
et al.,
2007; Van Bockhaven
et al.,
2013). When in-
fected with necrotizing pathogens, many
plants developed an enhanced resistance
against further pathogen attack, which is
referred to as systemic acquired resistance
(SAR) (Conrath, 2006). The two mechanisms
involved in increasing the activity of en-
zymes and antifungal compounds due to
silicon application on plants could induce
defense response similar to SAR (Cai
et al.,
2009). Moreover, there might be other bio-
chemical and physiological mechanisms in-
volved in the silicon-mediated resistance of
plants to diseases. For example, higher lev-
els of salicylic acid, jasmonic acid, and eth-
ylene have been reported to be induced by
silicon supplements in some host-pathogen
interactions: powdery mildew
of Arabidop-
sis
caused by
Golovinomyces cichoracearum
(
Vivancos
et al.,
2015) and rice-brown spot
caused by
Cochliobolus miyabeanus
(Van
Bockhaven
et al.,
2015).
4.2.1. Defense-related enzymes
Defense-related enzymes are impor-
tant in relation to disease resistance. Several
studies indicated that lower disease intensi-
ty in the silicon-treated plants was related to
higher activity of protective enzymes. Sili-
con has been demonstrated to stimulate ac-
cumulation of defense-related enzymes in
plant leaves after fungal infection (Fauteux
et al.,
2005; Datnoff
et al.,
2007; Van Bock-
haven
et al.,
2013).
Activities of chitinase, peroxidases and
polyphenoloxidases in cucumber plants in-
fected by
Pythium
spp. were enhanced as
a result of silicon root application (Cherif
et
al.,
1994; Liang
et al.,
2005). Increased activ-
ity of chitinase and
β
-1,3-glucanase in pea
seeds supplied with potassium silicate re-
duced incidence of
Mycosphaerella pinodes
(Dann and Muir, 2002). Enhanced peroxi-
dase activity in wheat leaves, due to silicon
treatment, decreased the severity of pow-
dery mildew caused by
Blumeria graminis
