Agriculture Reference
In-Depth Information
Increased activity of certain enzymes may help improve a crop's uti-
lization of nutrients. Malate dehydrogenase (MDH) is the enzyme used
in the biosynthesis of malate, which is a requirement for respiration in
nitrogen-fi xing bacteria. Overexpression of MDH in alfalfa and subse-
quent malate exudation by roots via specifi c membrane transporters may
improve nitrogen availability by providing substrates needed for respira-
tion of the rhizosphere microfl ora, resulting with an increase in nitrogen
fi xation, thus improving the availability of nutrients to crops [62]. Malate
export to the soil comes as an energetic expense to the plant, since photo-
synthates are released to the rhizosphere, and this activity must be tightly
controlled to be energetically and nutritionally benefi cial to the plant. Glu-
tamine synthase (GS) metabolizes glutamate to glutamine, with ammonia
as a required substrate for this reaction. Gallais and Hirel [64] showed that
GS activity is positively correlated to yield increases. GS may ultimately
determine rate of translocation of stored nitrogen to developing fruits, re-
sulting in yield increases under low-N conditions. Gallais and Coque [65]
suggested that high GS activity is a mechanism used by crop plants to pre-
vent embryo abortion, specifi cally in limited nitrogen systems, resulting in
increased the potential yield.
Improved interaction with soil microorganisms can signifi cantly in-
crease the effi ciency of nitrogen use in crop plants. Arbuscular mycorrhi-
zal (AM) fungi have been shown to improve nutrient uptake and potential
yield of crops, although most attention has been given to phosphorus up-
take. In addition, nitrogen plant nutrition can also be improved via mycor-
rhization [74,75]. The advantages of mycorrhizal colonization are most
observed when N-input is limited, implying that the symbiosis may not
signifi cantly aid in the uptake of nitrogen from fertilizer [62,75]. Coloni-
zation by AM fungi among wheat cultivars started to decline during the
1950's, coinciding with the increased availability and heavy application
of synthetic fertilizers. In a study by Hetrick et al. [76] found that wild
traditional farm varieties, or heirlooms by today's standards, had increased
AM symbioses which subsequently led to better growth compared to mod-
ern varieties. There is also indication that selecting varieties under low-N
conditions may inadvertently result in a genetic shift toward AM asso-
ciations and improved use of soil nutrients [77]. As an extension of the
root system, mycorrhizal association is also thought to improve drought
