Biomedical Engineering Reference
In-Depth Information
FUNGAL ANTIOXIDANT GENES
Plants defend themselves from invading pathogens by activating a number
of defense responses and one rapidly activated resistance mechanism of
plants is the HR response (Jones and Dangl, 1996). HR or programmed cell
death (PCD) is an outcome of an incompatible interaction between the host
and the pathogen preceding a fl urry of generation of host defense responses
at the site of infection (Mittler et al. 1997) including cell wall reinforcement
(Dixon and Lamb, 1990) and induction of pathogenesis-related (
PR
) genes
(Shirasu et al. 1996). The HR has been postulated to be an outcome of the
oxidative burst or rapid production of active oxygen species (AOS) such
as superoxide anions (O
2
-
), hydroxy radicals (OH
·
) and hydrogen peroxide
(H
2
O
2
) (Levine et al. 1994; Baker and Orlandi, 1995; Singh et al. 2011). H
2
O
2
is a bifunctional signalling molecule associated with most environmental
stresses, which at higher concentration triggers PCD, but at sub-lethal
concentration it functions as a secondary messenger and evokes stress
tolerance in plants (Neill et al. 2002). Levine et al. (1994) were the fi rst
to show the involvement of H
2
O
2
in signal transduction in soybean cell
cultures. The generation of AOS is among the earliest detectable responses
of plant cells treated with pathogen elicitors and precedes plant cell death
(Baker et al. 1995). Accumulation of H
2
O
2
during the oxidative burst not
only directly inhibits microbes (Tzeng and De Vay 1993), but also plays
a key role in the oxidative cross-linking of cell wall proteins (Brisson et
al. 1994), lignifi cation (Olson and Varner 1993) and in the orchestration
of HR (Levine et al. 1994, Tenhaken et al. 1995). Several enzyme systems,
including plasma membrane NADPH oxidases (Desikan et al. 1996),
peroxidases and lipoxygenases (Low and Merida 1996) and alternative
oxidase (Wagner 1995), have been implicated in the production of AOS.
In addition to the induction of the HR, H
2
O
2
has also been implicated
in a number of other plant defense responses, including construction
of defensive barriers (Brisson et al. 1994), activation of phytoalexins
(Apostol et al. 1989) and the production of the defense regulator salicylic
acid (SA) (León et al. 1995). Recently, H
2
O
2
has been further implicated
as a secondary signal in the induction of systemic acquired resistance
(SAR) by SA (Chen et al. 1993) and 2, 6-dichloro isonicotinic acid (INA)
(Conrath et al. 1995). Being a systemic signal, H
2
O
2
can diffuse through
the cell membrane with relative ease (Neill et al. 2002). From the results
of those studies it is evident that increases in endogenous H
2
O
2
leads to
plant cell death and the activation of defense genes. In the recent years,
several studies have unequivocally proved that exogenous pretreatment
of plants with micromole concentrations of H
2
O
2
could induce tolerance
to low temperature stress (Hung et al. 2007), heat (Uchida et al. 2002),
light (Karpinski et al. 1999), salinity (Wahid et al. 2007) and heavy metals
