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during the entire life span of the plants and plays key roles in regulating their
growth and productivity (Arberg
1981
). The role of SA in seed germination has
been debatable as there are inconsistent reports suggesting that it can either inhibit
germination or increase seed vigour. The reported contradictory effects can be
related to the SA concentrations employed. In Arabidopsis thaliana, SA concen-
trations [1 mM delay or even inhibit germination (Rajou et al.
2006
). In barley,
doses [0.250 mM SA inhibit seed germination (Xie et al.
2007
), while in maize
germination is completely inhibited by SA doses ranging from 3 to 5 mM (Guan
and Scandalios
1995
). The effect of SA as a negative regulator of seed germination
is probably due to an SA-induced oxidative stress. In Arabidopsis plants treated
with SA (1-5 mM), hydrogen peroxide (H
2
O
2
) levels increase up to 3-fold as a
result of increased activities of Cu, Zn-superoxide dismutase and inactivation of the
H
2
O
2
-degrading enzymes, catalase and ascorbate peroxidase (Rao et al.
1997
).
Enhanced germination and seedling growth were recorded in wheat, when the
grains were subjected to pre-sowing seed-soaking treatment in SA (Shakirova
2007
). Fariduddin et al. (
2003
) reported that the dry matter accumulation was
significantly enhanced in Brassica juncea, when lower concentrations of SA were
sprayed. However, higher concentrations of SA had an inhibitory effect.
In another study, Hayat et al. (
2005
) showed that the leaf number, fresh and dry
mass per plant of wheat seedlings raised from the grains soaked in lower con-
centration (10
-5
M) of SA, increased significantly. Similar growth promoting
responses were generated in barley seedlings sprayed with SA (Pancheva et al.
1996
). Khodary (
2004
) observed a significant increase in growth characteristics,
pigment contents and photosynthetic rate in maize, sprayed with SA. The exog-
enous SA application also enhanced the carbohydrate content in maize (Khodary
2004
). Hussein et al. (
2007
) in their pot experiment sprayed salicylic acid to the
foliage of wheat plants, irrigated with Mediterranean sea water and reported an
enhanced productivity due to an improvement in all growth characteristics
including plant height, number and area of green leaves, stem diameter and dry
weight of stem, leaves and of the plant as a whole.
In the year
1989
, Carswell et al. reported that acetyl SA can promote colony
formation in maize protoplasts suggesting a role for SA in the regulation of the cell
cycle. Xyloglucan endotransglucosylase/hydrolase (XTH) genes encode enzymes
that are implicated in cell wall loosening and cell expansion (Rose et al.
2002
).
Arabidopsis contains 33 XTH genes in its genome (Yokoyama and Nishitani
2001
). Among 33 XTH genes, expression levels of XTH8, XTH17 and XTH31 were
strongly down-regulated in both cpr5 and mpk4 but did not change in nahG (Miura
et al.
2010
). Reverse transcription-PCR (RT-PCR) results indicated that XTH8
and XTH31, but notXTH17, were down-regulated in siz1 and expression of
XTH8and XTH31 was recovered in nahGsiz1-2 (Miura et al.
2010
). Moreover,
expression level of XTH24 (MERI5), a potential target for ANGUSTIFOLIA (AN),
which regulates the width of leaves (Kim et al.
2002
) and may also play a role in
leafmorphogenesis at the early stage (Verica and Medford
1997
). Thus, SIZ1
regulates SA-dependent XTH8 and XTH31 expression, but may not be involved in
AN-dependent regulation of cell elongation (Miura et al.
2010
).
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