suppressing blackheart expression. As Dietzgen and Mitter (2006) explain, post-transcriptional gene silencing can be induced through diverse pathways but it is invariably triggered by the sequencespecific processing of long double-stranded RNA into small interfering RNA duplexes of 21-24 nucleotides in length. These small RNAs guide RNA-induced silencing complexes to specifically degrade cognate transcripts or inhibit translation. When sense or antisense transgenes are used they are recognized by the plant as structurally ‘aberrant’ and with the assistance of plant RNA-dependent RNA polymerase they can be converted to doublestranded RNA using the single-stranded transcript as a template. However transgenes such as the antisense-sense, and especially the intron hairpin, are designed in a way that their transcripts are readily capable of folding into double-stranded RNA structures and are therefore more efficient in silencing genes (Dietzgen and Mitter, 2006; Smith et al., 2000; Waterhouse et al., 1998). In the case of blackheart resistance, during posttranscriptional gene silencing the ppo messenger RNA from which the PPO enzyme is translated, is degraded (silenced); therefore there is a much reduced amount of ppo mRNA available for translation and therefore much less PPO enzyme as shown in this study to lead to less/no blackheart. Certainly there was very little PPO activity in unaffected fruit, but this is to be expected in pineapple as PPO is synthesised de novo in response