Physiological Response of the Target Stemborer <i>Chilo suppressalis</i> to Elevated CO₂ as Reared with Transgenic <i>Bt</i> Rice during Different Plant Growth Stages

Transgenic <i>Bt</i> rice (abbr. <i>Bt</i> rice) has provided a powerful tactics to control the striped stemborer <i>Chilo suppressalis</i> as one key lepidopteran pest in the paddyfields of China. Globally rising carbon dioxide (i.e., CO₂) concentration has been predicted to affect the Cry protein contents in plant tissues of <i>Bt</i> rice and thus might affect its control efficiency to target insect pests. To reveal the resistance ability and the corresponding mechanism of <i>C. suppressalis</i> to <i>Bt</i> rice during different growth stage under elevated CO₂ (eCO₂), we carried out this experiment to measure the Bt toxin contents in <i>Bt</i> rice stems grown under ambient CO₂ (aCO₂) (400 ppm) and eCO₂ (800 ppm) at seedling, tillering and heading stages, and to observe the larval mortality and bioassay the activity of midgut protease and the expression levels of Bt-toxin-receptor genes, aminopeptidases (APNs) in <i>C. suppressalis</i> larvae. Compared with aCO₂, eCO₂ increased the Bt-toxin content of <i>Bt</i> rice at seedling stage (+6.66%), and decreased that at heading stages (−13.99%), and significantly reduced the Bt-toxin content at tillering stage (−15.21%). And the larval mortality of <i>C. suppressalis</i> was lower as reared with <i>Bt</i> rice stems during tillering stage grown under eCO₂ in contrast to aCO₂. In addition, eCO₂ significantly increased the activity of total protease, tryptase-like enzyme and aminopeptidase of <i>C. suppressalis</i> larvae fed on <i>Bt</i> rice during seedling stage, and significantly reduced the activity of tryptase-like enzyme and aminopeptidase of <i>C. suppressails</i> larvae fed on <i>Bt</i> rice during tillering and heading stages respectively. Moreover, eCO₂ significantly increased the expression level of APN1 and APN5 of <i>C. suppressails</i> larvae fed on <i>Bt</i> rice during seedling stage, and significantly reduced the expression level of APN5 of <i>C. suppressalis</i> larvae fed on <i>Bt</i> rice during tillering and heading stages respectively. In summary, the control efficiency of <i>Bt</i> rice to target insect pests under eCO₂ showed a downward trend during tillering and heading stages, and especially during tillering stage.