Aging in Legume Symbiosis. A Molecular View on Nodule Senescence in Medicago truncatula    

Abstract

Rhizobia reside as symbiosomes in the infected cells of legume nodules to fix atmospheric nitrogen. The symbiotic relation is strictly controlled, lasts for some time, but eventually leads to nodule senescence. We present a comprehensive transcriptomics study to understand the onset of nodule senescence in the legume Medicago truncatula. Distinct developmental stages with characteristic gene expression were delineated during which the two symbiotic partners were degraded consecutively, marking the switch in nodule tissue status from carbon sink to general nutrient source. Cluster analysis discriminated an early expression group that harbored regulatory genes that might be primary tools to interfere with pod filling-related or stress-induced nodule senescence, ultimately causing prolonged nitrogen fixation. Interestingly, the transcriptomes of nodule and leaf senescence had a high degree of overlap, arguing for the recruitment of similar pathways.

EFD Is an ERF Transcription Factor Involved in the Control of Nodule Number and Differentiation inMedicago truncatula

“…Among the 34 upregulated genes in efd-1 10-d-old nodules, genes coding for peptidases and Cys proteinases were identified, some of which are highly homologous to the SAG2-like senescence-specific genes (Noh and Amasino, 1999), such as As NODf32, activated at the onset of nodule senescence (Naito et al, 2000). This is highly reminiscent of Cys proteinases found by cDNA-amplified fragment length polymorphism in the senescent zone IV in M. truncatula (Van de Velde et al, 2006). These microarray results were validated by Q-RT-PCR analyses on 48 genes, of which 40 gave qualitatively similar results (see Supplemental Table 5 online).…”

Sinorhizobium meliloti lsrB is involved in alfalfa root nodule development and nitrogen-fixing bacteroid differentiation

“…These findings demonstrate that the S. meliloti lsrB gene is involved in alfalfa root nodule development and bacteroid differentiation by suppressing oxidative bursts or defense responses in host cells. Root nodules begin senescence when a host developmental program is activated at the beginning of pod filling or under several different kinds of environmental stresses including darkness, drought, and temperature stress [4]. Leguminous root nodules exhibit differential senescence processes.…”

“…Root nodules begin senescence when a host developmental program is activated at the beginning of pod filling or under several different kinds of environmental stresses including darkness, drought, and temperature stress [4]. Leguminous root nodules exhibit differential senescence processes.…”

“…Senescence of determinant root nodules (round root nodules lacking persistent meristems) on Glycine max (soybean) initiates from the inside and spreads towards the exterior. Senescence of indeterminate root nodules (com-posed of apical meristem zone, infection zone, and nitrogen fixing zone) on Medicago sativa (alfalfa) originates in the nitrogen-fixing zone and forms a senescent zone that moves gradually to the apical zone [4]. However, senescent root nodules have several key features as described below: (1) a color shift in the nitrogen fixation zone, appearing white or turning from pink to green due to degradation or oxidization of leghemoglobin; (2) central vacuole differentiated into multiple vesicles; (3) cytoplasm that appears less electronically dense; (4) disintegrated symbiosome membranes; (5) degraded bacteroids; and (6) resorbed symbiosomes.…”

Transcriptomic profiles of nodule senescence in <i>Lotus japonicus</i> and <i>Mesorhizobium loti</i> symbiosis

“…Indeterminate nodules, as observed in Medicago truncatula, form cylindrical nodules that consist of a gradient of developmental zones with a persistent apical meristem; an infection zone, a fixation zone, and a senescence zone. Aged nodules with those three zones of M. truncatula were monitored by microarray (Van de Velde et al 2006). In the case of Lotus japonicus, determinate nodules which are round organs with a central fixation zone were formed.…”