Abstract
BackgroundLegumes can utilize atmospheric nitrogen by hosting nitrogen-fixing bacteria in special lateral root organs, called nodules. Legume nodules have a unique ontology, despite similarities in the gene networks controlling nodule and lateral root development. It has been shown that Medicago truncatula NODULE ROOT1 (MtNOOT1) is required for the maintenance of nodule identity, preventing the conversion to lateral root development. MtNOOT1 and its orthologs in other plant species -collectively called the NOOT-BOP-COCH-LIKE (NBCL) family- specify boundary formation in various aerial organs. However, MtNOOT1 is not only expressed in nodules and aerial organs, but also in developing roots, where its function remains elusive.ResultsWe show that Mtnoot1 mutant seedlings display accelerated root elongation due to an enlarged root apical meristem. Also, Mtnoot1 mutant roots are thinner than wild-type and are delayed in xylem cell differentiation. We provide molecular evidence that the affected spatial development of Mtnoot1 mutant roots correlates with delayed induction of genes involved in xylem cell differentiation. This coincides with a basipetal shift of the root zone that is susceptible to rhizobium-secreted symbiotic signal molecules.ConclusionsOur data show that MtNOOT1 regulates the size of the root apical meristem and vascular differentiation. Our data demonstrate that MtNOOT1 not only functions as a homeotic gene in nodule development but also coordinates the spatial development of the root.
Highlights
Legumes can utilize atmospheric nitrogen by hosting nitrogen-fixing bacteria in special lateral root organs, called nodules
The primary root of the medicago Mtnoot1 mutant is longer According to Medicago truncatula Gene Expression Atlas [29], the Medicago truncatula NODULE ROOT1 (MtNOOT1) gene (Medtr7g090020) is highest expressed in root tips, surpassing the expression in many nodule samples that have been analysed (Additional file 1: Figure S1)
To obtain insight in the timing of the increase in primary root growth of the Mtnoot1 mutant, we measured the root length at different time points (2 Days After Germination (DAG), 4 Days after germination (DAG) and 6 DAG). This showed that the primary roots of the Mtnoot1 mutants are markedly longer at 4 and 6 DAG when compared to wild-type seedlings (Fig. 1a-b)
Summary
Legumes can utilize atmospheric nitrogen by hosting nitrogen-fixing bacteria in special lateral root organs, called nodules. In the legume model Medicago truncatula (medicago) LCO signalling induces cell divisions in the pericycle and endodermis, followed by a coordinated mitotic activation of cortical cells. This will give rise to nodule primordia [1]. Legumes recruited a BTB/POZankyrin domain containing protein of the NOOT-BOPCHOCLEATA-LIKE (NBCL) family to maintain nodule identity in the newly formed primordium [10,11,12] Knockout mutations in this gene – in medicago named NODULE ROOT1 (MtNOOT1)- cause a homeotic switch from nodule organogenesis towards lateral root formation [10].
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