Agriculture Reference
In-Depth Information
tively increased and decreased after drought treatment. Water withholding applied
to pepper (
Capsicum annuum
L.) seedlings for one week resulted in elevated levels
of Cad and Put in leaves (Sziderics et al.
2010
), whereas concentration of PAs was
reduced in roots. Authors suggested that PAs might be involved in the stress protec-
tion of pepper leaves rather than in the osmotic adaptation to drought, since PAs
level was somewhat low. On the other hand, because proline may be synthesized
from ornithine via ornithine aminotransferase (Delauney and Verma
1993
), thus
competing for the substrate (ornithine) with the PA biosynthetic pathway (Theiss
et al.
2002
), reduced levels of Put, Spd and Spm could be a consequence of a pref-
erential proline synthesis in roots.
Up and downregulation by drought of enzymes involved in PAs metabolism has
been studied in several crops. In a largescale study on changes in transcript abun-
dance (Ozturk et al.
2002
), drought-induced transcripts of two ADCs were detected
in leaves and roots of barley plants subjected to water deficit. By means of a micro-
array analysis, a decrease in the enzyme SAMDC2 (TM00041253; Tian et al.
2004
)
was observed in the reproductive organs of maize (
Zea mays
L.), at an early stage of
water deficit (Zhuang et al.
2008
). With the aim of identifying drought-responsive
compounds in potato, Evers et al. (
2010
) analyzed transcriptomic and targeted me-
tabolites of two potato clones (
Solanum tuberosum
L.) of the Andean cultivar group,
Sullu and SS2613. These clones presented different drought-tolerance phenotypes,
as exposed to a continuously increasing drought stress in a field trial. Upon drought,
genes encoding for PAs biosynthesis ADC and SAMDC enzymes were upregulated
in both clones. In grape seedlings grown
in vitro
, ADC and Spm synthase inductions
were observed between within one week, after 350 mM of mannitol treatment (Liu
et al.
2011
).
Addition of exogenous PAs to intact plants has earlier attracted the attention
of several researchers as the observed growth promotion effect resembled that of
phytohormones (Rastogi and Davies
1991
). The binding of Spd and Spm to proteins
or nucleic acids protects last compounds from degradation and provides them with
a higher conformational stability under stress conditions. The exogenous applica-
tion of Spd to osmotically-stressed oat (
Avena sativa
) plants stabilized the structure
of thylakoid proteins Dl and D2, cytochromes and Rubisco (Tiburcio et al.
1994
;
Besford et al.
1993
). When treated with Spd, water-stressed cucumber (
Cucumis
sativus
L.) seedlings showed enhanced guaiacol peroxidase activity and a reduction
of SOD and catalase activities compared to untreated, water-stressed ones (Kubis'
2008
). The author suggested that PAs are able to moderate the activity of scaveng-
ing system enzymes and influence the oxidative stress intensity. Likewise, exog-
enously applied PAs increased drought tolerance of rice by improving leaf water
status, photosynthesis and membrane properties (Farooq et al.
2009
). Recently, in
vitro
Citrus
plants pre-treated with Spm showed improved tolerance to dehydra-
tion stress through less water loss and lower electrolyte leakage (Shi et al.
2010
).
Pre-treatment with Spm led to higher endogenous PAs content and the activation
of antioxidant enzymes. The authors assigned the reduced water loss to increased
stomatal closure. On the other hand, they attributed the lower electrolyte leakage to
inhibition of lipid peroxidation and biomembranes stabilization due to diminution
