Dissecting cassava root bulking

Timing, mechanisms, and signal pathways of storage root organs formation in cassava (Manihot esculenta) is poorly understood. Importantly, only little information on root formation originates from field-grown cassava plants. An extensive analysis of cassava plants has been conducted in the field to daily-monitor root growth and development, and the final formation of storage roots. Morphological, structural, and root count parameters were monitored and recorded by destructive root sampling. Several genotypes of cassava have been studied, including some characterized by high and low yielding, early and late flowering, and stay-green phenotypes. Our observations revealed that at the early time points, all roots look morphologically alike. However, starting from 5 weeks after planting, some of the roots appear slightly ticker and darker than before. Preliminary experimental results indicate that storage roots develop from such modified roots whereas fibrous roots never bulk. Root and leaf samples collected from plants before, during, and after root bulking were processed for transcriptome analysis. As expected, in leaf tissues gene expression is regulated between different organs (sink and source leaves) and time points. However, root genes are more significantly regulated among different time points and, therefore, gene expression of storage and fibrous roots cluster together at each time points. This suggests that regulation of only few genes can be responsible for development of root bulking. The implications of single gene/gene pathways in root bulking formation as well as the development of molecular markers for selection of early bulking genotypes will be discussed in the presentation.