Potato Transformation with Modified Nucleotide Sequences of the cry9Aa2 Gene Improves Resistance to Potato Tuber Moth

The efficacy of four versions of a cry9Aa2 gene was evaluated against larvae of potato tuber moth (PTM; Phthorimaea operculella Zeller) following transfer to potato (Solanum tuberosum L.). A truncated native cry9Aa2 gene and three modified versions (G7, G10 and G14) were used, each with progressively more nucleotide substitutions to produce up to 79 codon changes to more closely resemble the codon bias of dicotyledonous plants. All four cry9Aa2 genes, under the transcriptional control of a CaMV 35S promoter, were independently transformed into the potato S. tuberosum L. cv. Iwa. A total of 100 independently derived cry9Aa2-transgenic plants were regenerated and their transgenic status confirmed by PCR and Southern analysis. Replicated plants were transferred to a containment greenhouse for bioassays against PTM larvae. Higher efficacy of the cry9Aa2 gene against PTM larval growth on greenhouse-grown plants was associated with more changes to the codon usage. This was evident in both the frequency of independently derived lines exhibiting a significant growth inhibition of PTM larvae as well as a lower larval growth index.