A Robust Auxin Response Network Controls Embryo and Suspensor Development through a bHLH Transcriptional Module

Land plants can reproduce sexually by developing an embryo from a fertilized egg cell. However, embryos can also be formed from other cell types in many plant species. A key question is thus how embryo identity in plants is controlled, and how this process is modified during non-zygotic embryogenesis. The Arabidopsis zygote divides to produce an embryonic lineage and an extra-embryonic suspensor. Yet, normally quiescent suspensor cells can develop a second embryo when the initial embryo is damaged, or when response to the signaling molecule auxin is locally blocked. Here we have used auxin-dependent suspensor embryogenesis as a model to determine transcriptome changes during embryonic reprogramming. We find that reprogramming is complex and accompanied by large transcriptomic changes prior to anatomic changes. This analysis revealed a strong enrichment for genes encoding components of auxin homeostasis and response among misregulated genes. Strikingly, deregulation among multiple auxin-related gene families converged upon re-establishment of cellular auxin levels or response. This suggests a remarkable degree of feedback regulation to create resilience in auxin response during embryo development. Starting from the transcriptome of auxin-deregulated embryos, we identify an auxin-dependent bHLH transcription factor network that mediates the activity of this hormone in suppressing embryo development from the suspensor. Wild-type, bHLH49-mutant and bHLH49-overexpressing plants were grown for 4 days on MS, whereafter roots were harvested and total RNA was isolated.