Agriculture Reference
In-Depth Information
mutate in order to overcome the host's defense mechanisms. Pyramiding
several resistance genes into one genotype is becoming a more widely
used to develop durable resistance, with the hopes that the target pathogen
will not undergo mutations that overcome all the resistant genes [103].
Several genotypes have been developed with improved resistance to eco-
nomic pathogens via gene pyramiding including barley, rice, wheat, and
tomato [104-107]. Liu et al. [107] developed durable and broad-spectrum
powdery mildew resistance in wheat using several resistance genes,
Pm2,
Pm4a,
and
Pm21
. By pyramiding the resistance genes
xa5, xa13, Xa21,
bacterial blight resistance was developed in rice [105]. However research
has shown that to develop superior genotypes with durable resistance, al-
leles are necessary at more than one QTL [104,105]. Singh et al. [105]
demonstrated that multiple members and combinations of resistant genes
condition different responses to pathogen infection, with some genes and
combinations being more effective at offering resistance. Pyramiding for
disease tolerance will likely occur for specifi c host-pathogen interactions.
By comparing Solanaceous crops (tomato, potato, pepper), Grube et al.
[106] identifi ed 12 cross-generic disease resistant genes (R genes). How-
ever, R genes with specifi city to the same pathogen were only found twice
at corresponding locations in different hosts. Improving the resistance of
common agricultural pathogens via pyramiding will be laborious and re-
quire additional research, but will offer breeders an opportunity to develop
genotypes with durable resistance, providing low-input producers with va-
rieties less susceptibility to pathogen infection, less reliance on pesticide
applications, while potentially improving yields.
11.4.5 PHOSPHORUS
Plants require three major mineral macronutrients (N-P-K) and a host of
other essential micronutrients in order to develop properly. High-input ag-
riculture relies heavily on fertilization with macronutrients, and fertilizer
production industry supplies farmers with mostly inorganic macronutri-
ents. Nitrogen (N) is captured from the atmosphere and reduced to ammo-
nium using the Haber-Bosch process previously described. Potassium (K)
is mined, with the current reserves expected to last for several centuries,
