Agriculture Reference
In-Depth Information
2 The Overlapping Regimes of Abiotic and Biotic
Challenges in Plants
Salicylic acid has established its worthiness not only in local and systemic pathogenic
and symbiotic responses but also surpassed diverse abiotic stresses such as salinity,
high temperature, chilling, drought, radiation and heavy metal stresses (Kranteve et al.
2008
; Choudhuri and Panda
2004
), and sometimes even par-systemically in adjacent
plant population (Huang et al.
2005
; Heil and Ton
2008
, Ortíz-Castro
2009
). There-
fore, SA appears to manage reactive oxygen species (ROS) signaling at its effective
threshold to regulate the balance of free radicals and antioxidation of cell(s) locally,
systemically and par-systemically to enable stimulated growth and/or defense. And at
the same time, it does not allow cell death, unless under specific conditions where its
accumulation is supposed to be regulated externally (Ortíz-Castro
2009
).
Converging points of abiotic and biotic signals' crosstalk are still rudimentary
(Fujita et al.
2006
). Although the role of abiotic stresses in facilitating primary in-
fection has been well recognized (Zahran
1999
; Castejón-Muñoz
2008
; Babu and
Devaraj
2008
; Çiçek and Çakırlar
2008
), it has not been elaborated in molecular
detail. Disease, the outcome of successful pathogenesis, denotes the consistent fail-
ure of the host to counter the forced metabolic and redox perturbation of tissue
to be invaded. The other causes that favour the microbial ingression include the
abiotic factors such as relative humidity, temperature, aeration (Chupp
1946
), in-
vader's factors such as virulence and inoculum density of microbe (Navas-Cort'es
et al.
2000
) and host factors such as genotype and physiological state/age (Millett
et al.
2009
; Staskawicz et al.
1995
,
2001
; Basim
1998
; Li et al.
2001
). The stress-
retention time marks the fate of host-microbe relation, and is the manifestation of
host resistance capability versus aggressiveness of invader (virulence) under a set of
abiotic conditions. If the prevailing abiotic factor favours the invader, a continuous
perturbation of redox metabolism triggers instant upsurge and burst of ROS and its
organic radical derivatives, favouring subsequent tissue necrosis and eventual plant
death (Greenberg and Yao
2004
). Therefore, specific mal-physiological symptoms
and reduced plant growth become obvious. The bulk diversion of photosynthates
and disturbed hormonal distribution could be hypothesized as the reasons for the
plant's death (Berger et al.
2007
). However, consistent prevailing environment may
support host physiology to attenuate microbe disfavouring the inoculum buildup,
hence, rendering its ingression to fail. Ingression could be checked though, at dif-
ferent levels of host defense (de Wit
2007
) claiming differential loss of tissue size
(manifested as intensity of symptoms) denoting eventual combat of host in this
tug of war of metabolic management. Apart from genotype, the threshold time of
perception and subsequent response of host are crucial determinants to ascertain
host defense response. The quick initial defense strategy against microbial inva-
sion includes instant upsurge of free radicals in adjoining cells (Bolwell et al.
2002
;
Wojtaszek
1997
; Lamb and Dixon
1997
) at the cost of cellular and microbial death
and the rise of neutralizing defense molecules in the vicinity, e.g., phenolics of
phenyl propanoid pathway (Bhattacharya et al.
2010
; Dixon and Paiva
1995
; Baron
