Cotton Transgenics

A number of genes of economic importance have been isolated and integrated into cotton, potatoes, tomatoes, rice, and other crops using genetic engineering. These include genes which affect plant size, earliness, yield, quality of grain, fruit and fiber, genes which confer resistance to insect and disease, and genes which confer tolerance to herbicides. What may have seemed impossible a few years ago is rapidly becoming common place.

Many of these genes have commercial value to the cotton industry. If managed appropriately, insect resistant transgenic cotton plants could save Texas cotton growers $million/year in reduced damage and decreased insecticide use.

If these genes are not managed properly, growers would likely see an overall net loss in profits within the foreseeable future. Although the potential is there to decrease damage from fruit and leaf feeding pests, there is also an increased chance for resistant pest biotypes to develop rapidly. Unlike insecticide applications which only expose a part of the pest population during relatively brief periods (some pests are protected in fruit, some are sheltered on other structures), insects feeding on resistant transgenic plants are exposed to toxins throughout most of their development and potentially throughout several generations.

In 1996, Roberta Smith, Kamal El-Zik, and Ted Wilson received funding through the TexCot Initiative to develop bollworm resistant Texas cottons, using Dr. Smith's Agrobacterium transformation method. As part of this project, we evaluated whether the Agrobacterium transformation method could be used to successfully transform the Texas cultivar, Spinx, developed by Dr. El-Zik. Neonate Helicoverpa zea (Boddie), (Lepidoptera: Noctuidae) were placed on leaf disks containing putatively Bt constructs and allowed to feed until about the third instar.

Typically, a healthy neonate Helicoverpa zea will consume 50% or more of a cotton leaf disk, in the absence of a toxin. The proportion of leaf disks that experienced no discernible feedings was not affected by either plant age or Agrobacterium strain. The proportion of leaf disks that experienced either no discernible feeding or which were only partially consumed was significantly affected by plant age, explaining 42% of the total data variability (Table 1).

Similar results were obtained for the estimated proportion of leaf mass consumed, with plant age being highly significant and explaining 48% of the total data variability (Table 2). For both response variables, the older the plant, the lesser the injury. While 56% of the leaf disks had either no discernible feeding or were only partially consumed for the 5/19 treatment, this had dropped to 13% by 6/1. Similarly, while an estimated 74% of the leaf mass was consumed for the 5/19 treatment, this had increased to 95% by 6/1.

Table 1. Effect of Agrobacterium strain and plant age on the proportion of leaf disks that experienced either no discernible feeding or which were only partially consumed.

Source	Nparm	DF	Sum of Squares	F Ratio	Prob > F
Ag. Strain	2	2	0.0862519	0.6636	0.5177
Date	6	6	1.8392777	4.7167	0.0003
Ag. Strain * Date	12	12	0.7739356	0.9924	0.4632

 

Table 2. Effect of Agrobacterium strain and plant age on the proportion of leaf mass consumed consumed.

Source	Nparm	DF	Sum of Squares	F Ratio	Prob > F
Ag. Strain	2	2	0.01666692	0.6203	0.5402
Date	6	6	0.47680484	5.9152	<.0001
Ag. Strain * Date	12	12	0.21974012	1.3630	0.2001

The results are tantalizing and strongly point out the need to control plant age when conducting biological assays. The important unanswered questions are 1) how do the constructs perform compared with control plants, 2) how well do putative resistant constructs perform over time or across generations of selection, and 3) how easy will it create transgenic constructs which contain genes which express multiple toxins having different modes of action.

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