Agriculture Reference
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effi cient absorption [138]. Benefi ts of mycorrhizal colonization have been
observed mostly in organic and low-input systems with P defi ciencies
[79,139,140]. Studies have shown that maize produced under P defi cient
conditions has increased P acquisition and plant growth. However, this
was not sustained as P concentrations were increased [141]. Additionally,
biodiversity of AM fungi is greater in low-input production systems com-
pared to high-input, likely due to the availability of nutrients making mi-
crobial symbiotic relationships obsolete and energy expensive to the crop
[142]. Furthermore, Xavier and Germida [143] suggested that coloniza-
tion by mycorrhizal fungi is correlated to yield responses in wheat, depen-
dent on genotype and other advantageous root traits. The specifi c genetic
mechanisms promoting symbioses between AM fungi and crop plants are
not fully understood, although genotypic differences have been observed
in maize, rice, and wheat [77,140,144]. Hetrick et al. [77] determined that
landraces and traditional varieties developed prior to 1950 had greater reli-
ance on mycorrhizal relationships than modern varieties. This implies that
landraces and traditional varieties possess specifi c traits and genotypes
that are benefi cial in the development of symbiotic relationships with soil
microbes. By reintroducing these favorable alleles into modern variet-
ies, nutrient acquisition could improve, which may ultimately reduce the
amount and need for phosphorus fertilizers.
The activity of certain enzymes may also prove to be valuable when
selecting varieties for low P conditions. Acid phosphatases are ubiquitous
enzymes present in various plant organs throughout development. They
are responsible for providing phosphate to growing tissues during ger-
mination from stored phytate, remobilizing internal phosphate. Organic
phosphate can breakdown reserves in the soil through exudation from
roots into the rhizosphere when under low-input conditions [123,129].
Marschner et al. [145] found that P-effi cient genotypes grown in P-deplet-
ed soils had greater phosphatase activity, which correlated to improved
plant growth and nutrient uptake. Intracellular phosphatase activity when
under P-stress, primarily functions to remobilize P from stored phytate and
senescing tissues [123,146]. By breeding for increased phosphatase activ-
ity or any combination of the P-effi ciency traits, crops produced in low-P
soils will signifi cantly improve the ability to acquire P whether from soil
reserves or through remobilization of internally stored supplies, leading to
 
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