Issue No. 7, Article 3/May 6, 2005
Expression of Corn Rootworm Transgenic Cry3Bb1 Protein Reported to Diminish from V4 to V9 Stages in Five Hybrids
In a recent issue of Crop Science (March 28, 2005), scientists from Monsanto Company published a paper ("A Method of Controlling Corn Rootworm Feeding Using a Bacillus Thuringiensis Protein Expressed in Transgenic Maize") in which they described the expression level of Cry3Bb1 (Bt protein targeted against corn rootworm larvae) in five corn hybrids (MON 863) at two growth stages (V4 and V9) in a growth chamber study. The sample size for each hybrid was 14. Protein levels were determined by using an enzyme-linked immunosorbent assay (ELISA) technique. The entire root system at both V4 and V9 stages was utilized.
No significant difference in expression level of the Cry3Bb1 protein because of hybrid was detected. Nor was there any significant interaction between hybrids and growth stages with respect to protein expression. Expression of the Cry3Bb1 protein in root tissue for each of the hybrids at the V4 stage was reported as hybrid 1--60.21 ppm, hybrid 2--61.98 ppm, hybrid 3--69.67 ppm, hybrid 4--75.17 ppm, and hybrid 5--80.67 ppm. As corn plants reached the V9 stage, the expression of the Cry3Bb1 protein decreased across each of the five hybrids: hybrid 1--49.34 ppm, hybrid 2--40.50 ppm, hybrid 3--42.88 ppm, hybrid 4--42.33 ppm, and hybrid 5--45.81 ppm. Averaged across each of the five hybrids for both plant stages, expression of the Cry3Bb1 protein decreased from 69.8 ppm to 44.0 ppm. The lethal concentration (LC50) of the Cry3Bb1 protein (necessary to kill 50% of a corn rootworm larval population) is estimated to be approximately 75 ppm. The authors acknowledged that the effect of growth stage on the expression of the Cry3Bb1 protein was significant, on average declining by 25.8 ppm across hybrids from V4 to V9.
Despite the decline in the expression of the Bt protein across all five hybrids (V4 to V9), the root protection afforded by each transgenic hybrid was excellent, with only minor feeding scars on the root systems reported. The control (nontransgenic hybrid) had over two nodes of roots completely destroyed.
What are some potential implications of these findings? If a cool spring leads to a very late corn rootworm larval hatch (early to mid-June) and subsequent extended larval feeding period (late July through early August), we might anticipate more pressure being exerted on transgenic corn rootworm hybrids. Very early planting (early April) of corn could exacerbate this situation. Transgenic corn rootworm hybrids also tend to delay development and emergence of male and female western corn rootworm beetles (Journal of Economic Entomology, April 2005, pages 534-551). We will continue to learn how transgenic corn rootworm hybrids perform under different environmental conditions. To date, we only have two growing seasons under our belt with this technology.
Just last week we planted an experiment near Urbana with many different transgenic corn rootworm hybrids in which we will evaluate root injury later this summer. We'll keep you informed regarding the results.--Mike Gray