Abstract
Symbiosis between legume plants and soil rhizobia culminates in the formation of a novel root organ, the ‘nodule’, containing bacteria differentiated as facultative nitrogen-fixing organelles. MtNF-YA1 is a Medicago truncatula CCAAT box-binding transcription factor (TF), formerly called HAP2-1, highly expressed in mature nodules and required for nodule meristem function and persistence. Here a role for MtNF-YA1 during early nodule development is demonstrated. Detailed expression analysis based on RNA sequencing, quantitiative real-time PCR (qRT-PCR), as well as promoter–β-glucuronidase (GUS) fusions reveal that MtNF-YA1 is first induced at the onset of symbiotic development during preparation for, and initiation and progression of, symbiotic infection. Moreover, using a new knock-out mutant, Mtnf-ya1-1, it is shown that MtNF-YA1 controls infection thread (IT) progression from initial root infection through colonization of nodule tissues. Extensive confocal and electronic microscopic observations suggest that the bulbous and erratic IT growth phenotypes observed in Mtnf-ya1-1 could be a consequence of the fact that walls of ITs in this mutant are thinner and less coherent than in the wild type. It is proposed that MtNF-YA1 controls rhizobial infection progression by regulating the formation and the wall of ITs.
Highlights
Plants within the legume family possess the remarkable property of endosymbiotic interaction with a group of bacteria collectively referred to as ‘rhizobia’
Ethylene-insensitive ein2 mutants of M. truncatula show a dramatic increase in the number of sustained infections (Penmetsa et al, 2008) and a corresponding substantial up-regulation of MtNF-YA1 was observed in skl plants relative to WT M. truncatula A17 (Penmetsa and Cook, 1997)
MtNF-YA1 induction was not detected in either dmi1-1 or the double mutant dmi1-1 skl. These results demonstrate that MtNF-YA1 expression is up-regulated early (6 h) upon rhizobial inoculation, that this up-regulation is Nod factors (NFs) dependent, that superinduction in skl is dependent on the NF pathway, and that there are temporal and quantitative correlations between rhizobial infection and the level of MtNF-YA1 expression
Summary
Plants within the legume family possess the remarkable property of endosymbiotic interaction with a group of bacteria collectively referred to as ‘rhizobia’. Cell divisions signify the nascent nodule primordium that subsequently develops into a fully active meristematic tissue. In legumes such as Medicago truncatula, cell divisions are initially observed in pericycle cells (Timmers et al, 1999) but predominantly in inner cortical cells, and the resulting indeterminate meristem drives nodule growth. In their mature state, indeterminate nodules are composed of characteristic zones of development, with each zone composed of specialized tissues and cell types (Vasse et al, 1990). Zone 3 is the fixation zone or nodule ‘central tissue’, where host cells and rhizobia complete differentiation processes that were initiated in the proximal part of zone 2, including endoreduplication and the acquisition of morphological features characteristic of the nitrogen-fixing organelle, the ‘bacteroid’
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