Agriculture Reference
In-Depth Information
5. Combination of microbial ecology of P-enhancing microorganisms with plant breed-
ing or insertion of transgenic genes to develop roots with architectures, exudates, and
signaling that optimize root and microbial responses to increase P uptake by crops.
Bacteria have been isolated that can enhance the ability of crops to withstand water stress
by increasing seedling root elongation (promotes stand establishment) and various crop
physiological responses (reduction in cell elasticity and osmotic potential and rise in
apoplastic water fractions) (Creus et al., 1998; Alvarez et al., 1996). Also, autochthonous
arbuscular mycorrhizae (AM) can increase plant resistance to water stress (Marulanda et
al., 2007; Kohler, 2008). Barea and coworkers (2002) have shown that this could be further
improved with coinoculation with bacteria (
Bacillus thuringiensis
) because of the reduced
amount of water required to produce shoot biomass (Vivas et al., 2003).
Abiotic stress such as water stress causes oxidative damage at the cellular level. To
overcome this, plant cells can produce antioxidant enzymes (CAT, various peroxidases,
and superoxide dismutase) that inactivate reactive free radicals (Simova-Stoilova et
al., 2008)
.
Interestingly, it was shown by
Kohler
(2008) on
lettuce (
Lactuca sativa
L.) that
Pseudomonas mendocina
could assist this response by boosting plant CAT under severe
drought conditions.
Paenibacillus polymyxa
was shown by Timmusk and Wagner (1999) to
induce drought tolerance in
Arabidopsis thaliana
by induction of a drought-responsive gene.
In wheat (
Triticum aestivum
) under drought stress, Creus et al. (2004) showed that
inocula-
tion with
Azospirillum
brasilense
improved water status and grain yield. Plants inoculated
with bacteria producing exopolysaccharide (EPS) can increase resistance to water stress
because EPS forms biofilm on the surface of roots (Bensalim et al., 1998).
The inoculated
seedlings showed improved soil aggregation and root-adhering soil and higher relative
water content in the leaves (Sandhya et al.,
2010)
.
Sorghum inoculated with
Azospirillum
(Sarig et al., 1988; Fallik et al., 1994) resulted in a
larger root system that improves water uptake in water-deficient soils. These field experi-
ments showed inoculation increased leaf water potential, lowered canopy temperatures,
and increased stomatal conductance and transpiration. In addition, inoculated sorghum
increased total water extraction from soil over a control (by about 15%) and obtained water
from deeper soil layers as compared with noninoculated controls. Similarly, Hamaoui et
al. (2001) showed that inoculation of sorghum with
A. brasilense
significantly reduced the
negative effects of saline irrigation water, which has been attributed to the stimulation
of root development, delayed leaf senescence, and improved water uptake in saline soils
(Sarig et al., 1990).
Sandhya et al. (2010) found five
Pseudomonas
spp. (
P.
entomophila
strain BV-P13,
P.
stutzeri
strain GRFHAP-P14,
P. putida
strain GAP-P45,
P. syringae
strain GRFHYTP52, and
P. monteilli
strain WAPP53) that improved plant biomass, relative water content, leaf water
potential, root-adhering soil/root tissue ratio, aggregate stability, and mean weight diam-
eter and decreased leaf water loss. The inoculated plants showed higher levels of proline,
sugars, and free amino acids under drought stress. They also showed that inoculation
significantly decreased electrolyte leakage and activities of antioxidant enzymes, ascor-
bate peroxidase (APX), CAT, and glutathione peroxidase (GPX) under drought stress, indi-
cating that inoculated seedlings felt less stress compared to uninoculated seedlings. The
strain GAP-P45 was the best in terms of influencing growth and biochemical and physi-
ological status of the seedlings under drought stress.
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