Evaluation of tubers from cassava transgenic lines tolerant to African cassava mosaic virus

Abstract

Cassava (Manihot esculenta Crantz) is a crop plant that produces starchy tuberous roots. The cassava plant is resilient to abiotic stress and very efficient at producing carbohydrates, making it a valuable crop in the production of starch, for industrial uses. Some of the main causes of yield loss in cassava plants are viral diseases, such as cassava mosaic diseases (CMD), caused by cassava mosaic geminiviruses (CMG). One of the most promising means to combat CMGs is the development of transgenic crops. AMM2-52, CMM6-2 and CMM6-6 transgenic lines were developed previously and have been proven tolerant to African cassava mosaic virus (ACMV), a CMG. After transgenic cassava lines are produced, it is necessary to perform tests to evaluate the potential effects of the transgenes on the tubers of the plant. There are many potential sources of variation in the process of developing transgenic plants, that can result in altered tuber production or composition. For preliminary studies, before field trials are performed, cassava is grown in highly controlled laboratory environments with sub-optimal conditions (such as soil, lighting, temperature, and water availability) for tuber production. Consequently, the transgenic ACMV tolerant cassava plants in this study were grown under non-field, controlled, growth room conditions, with pot size and salicylic acid (SA) foliar spray sub-treatments, that could potentially improve tuber production and growth. The growth characteristics and resulting tubers were also analysed, and modelled with nonlinear models, in comparison with the dsAC1 line (proven resistant to ACMV), cv.60444 cultivar (model cultivar that the transgenic lines are transformed from) and TME3 (CMD tolerant) landrace. At the end of the study, the three ACMV tolerant transgenic lines did not display any noticeable irregularities in starch composition and growth characteristics. The starch content of the tubers was justifiably low, due to the young age (six months) of the plants when the tubers were harvested, where smaller tubers correlated with a lower starch content. Amylose content of the tubers was below 30% for the most part, as is expected in cassava tubers. Starch granule morphology and size was uniform among the transgenic lines and control landraces tested. A method to reliably produce tubers in indoor growth facilities was not successful, but it was found that bigger pots resulted in a greater chance of tubers being produced and often resulted in an increased growth rate in the cassava plants. The salicylic acid treatment had no strong effect on tuber production, or growth of any of the cassava plants. Early growth, based on the height of cassava, in controlled environments, was successfully modelled, with the logistic model having the best fit. Leaf number was found to be unsuitable for nonlinear modelling, and leaf area index (LAI) or yield of leaves should be used instead