204 R. Asokan et al.
under normal circumstances, but permit quick
upregulation of genes when pathogens attack.
Studies have shown that silencing two bacterial
genes, iaaM and ipt reduced the formation of
crown gall tumors by Agrobacterium. This opens
a new avenue for engineering trees and woody
ornamental plants against crown gall formation
(Auer and Frederick 2009 ).
MicroRNAs in Imparting Plant
Resistance to Diseases
The first identified miRNA to play an active
role in pathogen associated molecular pattern
triggered immunity Pattern triggered immunity
(PTI) was in Arabidopsis. The bacterial infection
induces expression of miR393 which negatively
regulates the mRNAs for the auxin receptor,
including the transport inhibitor response1 for
the degradation. It has been observed that over-
expression of miR393 suppressed auxin signal-
ing and restricted the growth of the bacterium,
Pseudomonas syringae pv. Tomato. In addition to
the above, miR167 and miR160 that target auxin
response factors were also found to be induced
by the Pseudomonas syringae pv. tomato (Pst).
Hence several miRNAs that repress auxin signal-
ing are induded by bacterial pathogens and con-
tribute to the PTI (Padmanaban et al. 2009 ).
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