The symbiosis with nitrogen-fixing bacteria leading to root nodules is a relatively recent evolutionary innovation and limited to a distinct order of land plants. It has long been a mystery how plants have invented this complex trait. However, recent advances in molecular genetics of model legumes has elucidated genes involved in the development of root nodules, providing insights into this process. Here we discuss how the de novo assembly of transcriptional networks may account for the predisposition to nodulate. Transcriptional networks and modes of gene regulation from the arbuscular mycorrhizal symbiosis, nitrate responses and aspects of lateral root development have likely all contributed to the emergence and development of root nodules.
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