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Legume and rhizobium species can establish a nitrogen-fixing nodule symbiosis. Previous studies have shown that several transcription factors that play a role in (lateral) root development are also involved in nodule development. Chromatin remodeling factors, like transcription factors, are key players in regulating gene expression. However, studies have not investigated whether chromatin remodeling genes that are essential for root development are also involved in nodule development. Here, we studied the role of Medicago (Medicago truncatula) histone deacetylases (MtHDTs) in nodule development. Arabidopsis (Arabidopsis thaliana) orthologs of HDTs have been shown to play a role in root development. MtHDT expression is induced in nodule primordia and is maintained in the nodule meristem and infection zone. Conditional, nodule-specific knockdown of MtHDT expression by RNAi blocks nodule primordium development. A few nodules may still form, but their nodule meristems are smaller, and rhizobial colonization of the cells derived from the meristem is markedly reduced. Although the HDTs are expressed during nodule and root development, transcriptome analyses indicate that HDTs control the development of each organ in a different manner. During nodule development, the MtHDTs positively regulate 3-hydroxy-3-methylglutaryl coenzyme a reductase 1 (MtHMGR1). Decreased expression of MtHMGR1 is sufficient to explain the inhibition of primordium formation.

Legume and rhizobium can establish a nitrogen-fixing nodule symbiosis. Previous studies have shown that several transcription factors that play a role in (lateral) root development are also involved in nodule development. Chromatin remodelling factors, like transcription factors, are key players in regulating gene expression. However, it has not been studied whether chromatin remodelling genes that are essential for root development get involved in nodule development. Here we studied the role of Medicago histone deacetylases (MtHDTs) in nodule development. Their Arabidopsis orthologs have been shown to play a role in root development. The expression ofMtHDTsis induced in nodule primordia and is maintained in nodule meristem and infection zone. Conditional knock-down of their expression in a nodule-specific way by RNAi blocks nodule primordium development. A few nodules still can be formed but their nodule meristems are smaller and rhizobial colonization of the cells derived from the meristem is markedly reduced. Although the HDTs are expressed during nodule and root development, transcriptome analyses indicate that HDTs control the development of these organs in a different manner. During nodule development the MtHDTs positively regulate3-hydroxy-3-methylglutaryl coenzyme a reductase 1(MtHMGR1). The decreased expression ofMtHMGR1is sufficient to explain the block of primordium formation.ONE SENTENCE SUMMARYPlant-specific histone deacetylases regulate the expression of3-hydroxy-3-methylglutaryl-coenzyme A reductasesto control root nodule development.

Root growth is modulated by environmental factors and depends on cell production in the root meristem (RM). New cells in the meristem are generated by stem cells and transit-amplifying cells, which together determine RM cell number. Transcription factors and chromatin-remodeling factors have been implicated in regulating the switch from stem cells to transit-amplifying cells. Here, we show that two

International audience; Legume root nodules are induced by N-fixing rhizobium bacteria that are hosted in an intracellular manner. These nodules are formed by reprogramming differentiated root cells. The model legume Medicago truncatula forms indeterminate nodules with a meristem at their apex. This organ grows by the activity of the meristem that adds cells to the different nodule tissues. In Medicago sativa it has been shown that the nodule meristem is derived from the root middle cortex. During nodule initiation, inner cortical cells and pericycle cells are also mitotically activated. However, whether and how these cells contribute to the mature nodule has not been studied. Here, we produce a nodule fate map that precisely describes the origin of the different nodule tissues based on sequential longitudinal sections and on the use of marker genes that allow the distinction of cells originating from different root tissues. We show that nodule meristem originates from the third cortical layer, while several cell layers of the base of the nodule are directly formed from cells of the inner cortical layers, root endodermis and pericycle. The latter two differentiate into the uninfected tissues that are located at the base of the mature nodule, whereas the cells derived from the inner cortical cell layers form about eight cell layers of infected cells. This nodule fate map has then been used to re-analyse several mutant nodule phenotypes. This showed, among other things, that intracellular release of rhizobia in primordium cells and meristem daughter cells are regulated in a different manner.