Most miRNAs target multiple targets belonging to the
same gene family in plants. Emerging data indicate that
miRNAs can selectively regulate the expression of specific
target genes under specific conditions. For example, decreased
miR398 expression under abiotic stresses leads to
upregulation of both CSD1 and CSD2, whereas its downregulation
by P. syringae pv. tomato DC3000 AvrRpt2 or
AvrRpm1 infection affects CSD1 but not CSD2 [60]. Similarly,
miR393 that is upregulated in response to nitrogen
treatment affects AFB3 transcript levels, but not its three
other targets [14]. It is unclear how the other targets of
these two miRNAs evade regulation. It is possible that
miRNA and its target gene are expressed in a non-overlapping
manner, as has been shown for miR395 and AST68
in Arabidopsis [48]. There are other possibilities, such as
miRNA-loaded RNA-induced silencing complex (RISC) being
unable to recognize certain mRNA targets because the
miRNA recognition site is inaccessible on the target transcript
because of binding of an RNA-binding protein, or the
presence of local secondary RNA structures, as suggested
in the case of human RISC