Transcriptome Analysis Suggests Cytokinin and Gibberellin Signaling May Account for Differences Between Spring and Winter Canola (Brassica napus L.) Root Development

Spring-type canola produces significantly smaller and less vigorous roots than winter type at later growth stages. To identify the gene-regulatory networks and cross talks among phytohormones that may be responsible for the difference between the root system of the two growth types, we analyzed transcriptomes from the root samples collected at two time points, 30 and 60 days. Out of 169,646 transcripts that were analyzed, 582 and 555 transcripts were found to be significantly differentially expressed between spring and winter types at 30 and 60 days, respectively. Several auxin-responsive genes were significantly differentially expressed between spring and winter types at both time points, but no distinct pattern of differential regulation was observed in either of the growth types. Type-A and type-B ARABIDOPSIS RESPONSE REGULATOR (ARR) genes, and gene sets involved in cytokinin signaling pathway, were upregulated in the spring types at 60 days. This would be consistent with higher cytokinin activity, which is known to inhibit root growth, in spring type root system at the later growth stage. In addition, several gibberellin-responsive and gibberellin-signaling gene sets were also upregulated only in spring types at 60 days. We observed an elongated root system with fewer root branches in spring type compared to winter type at 60 days in this study, which is consistent with a role of gibberellin in root growth and development. Extensive gene expression data generated in this research will further assist in understanding the natural variation of root system in canola growth habits.