Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 466

Considerable efforts have been made to improve the yield of plants subjected to
these stresses by modifying the soil, selecting and breeding genotypes and by ge-
netic modification.
Mineral deficient conditions influence differential regulation of hormones and
growth promoting substances. Altered metabolism of PAs in response to K + , Mg +
and PO 4 + deficiencies are reported (reviewed in Groppa and Benavides 2008 ).
K + deficiency-induced PA metabolism is mostly studied. Young and Galston (1984)
showed that accumulation of put under K-deficiency by long term growing of oat
plants on a low-K nutrient medium. ADC and ODC activities in entire K-deficient
shoots were 6-fold and 2-fold greater than in the K-sufficient grown plants respec-
tively (Young and Galston 1984 ). Arabidopsis thaliana responded to K-deficiency
by increasing ADC activity upto 10-fold over unstressed plants with a correspond-
ing increase in put levels up to 20-fold (Watson and Malmberg 1996 ). Important
function of put in maintaining cation-anion balance in plant tissues is well known
(reviewed in Bouchereau et al. 1999 ). In birch leaves, increased amounts of put and
DAP, a product of PAO activity was found in K-deficient situations (Sarjala and
Kaunisto 2002 ). Put accumulation is considered as a good indicator of K-deficiency
of forest trees (Kaunisto and Sarjala 1997 ; Sarjala and Kaunisto 2002 ). In tobacco
plants a short-term boron deficiency caused an increase in free put in roots and con-
jugated put in leaves indicating a possible link between boron and PA levels (Cama-
cho-Cristóbal et al. 2005 ). Source of Nitrogen dependent PA accumulation has been
reported in wheat and pepper. Significant increase in put content was observed in
ammonium nutrition and to a lesser extent in urea nutrition (Houdusse et al. 2005 ).
However, put content was significant reduced with nitrate nutrition (Houdusse et al.
2005 ). Proline content of the plants also showed a similar pattern as put content but
to a lesser degree (Houdusse et al. 2005 ). They hypothesized that put biosynthesis
might be related to proline degradation by a specific pathway related to ammonium
detoxification. It is also reported that Mg and N deficiencies induced changes in PA
content in grapevine (Evans and Malmberg 1989 ).
Changes in PA metabolism occur by metal toxicity (Groppa et al. 2001 , 2003 ,
2007 ; Lin and Kao 1999 ). Increased accumulation of total PAs was observed when
tobacco BY-2 cells were exposed to 0.05 mM Cd 2 + for 3 days (Kuthanová et al.
2004 ). Among all the PAs, put content was significantly higher and DAO activity
was also stimulated (Kuthanová et al. 2004 ). Treating mungbean seedlings with
CdCl 2 (0.1-1.5 mmol/L) resulted decreased DAO activities and subsequently re-
stricted the accumulation of endogenous PAs (Choudhary and Singh 2000 ). Groppa
et al. ( 2003 ) reported that PA metabolism is differentially regulated by Cd 2 + and
Cu 2 + in sunflower and wheat leaves. Both metals increased put content in wheat
leaves but reduced put content in sunflower leaves. Treating wheat leaves with Cd 2 +
raised the ADC and ODC activities while Cu 2 + raised only ODC activity (Groppa
et al. 2003 ). Groppa et al. ( 2007 ) showed that both ADC and ODC activities in
shoots of sunflower plants were increased by 1 mM Cd, whereas 1 mM Cu en-
hanced only ADC activity. A protective role of PAs in Cd 2 + and Cu 2 + -induced oxi-
dative stress in sunflower leaves is described (Groppa et al. 2001 ). Pre-treatment