Abstract
Legume nodules are plant tissues with an exceptionally high concentration of phosphorus (P), which, when there is scarcity of P, is preferentially maintained there rather than being allocated to other plant organs. The hypothesis of this study was that nodules are affected before the P concentration in the organ declines during whole-plant P depletion. Nitrogen (N₂) fixation and P concentration in various organs were monitored during a whole-plant P-depletion process in Medicago truncatula. Nodule gene expression was profiled through RNA-seq at day 5 of P depletion. Until that point in time P concentration in leaves reached a lower threshold but was maintained in nodules. N₂-fixation activity per plant diverged from that of fully nourished plants beginning at day 5 of the P-depletion process, primarily because fewer nodules were being formed, while the activity of the existing nodules was maintained for as long as two weeks into P depletion. RNA-seq revealed nodule acclimation on a molecular level with a total of 1140 differentially expressed genes. Numerous genes for P remobilization from organic structures were increasingly expressed. Various genes involved in nodule malate formation were upregulated, while genes involved in fermentation were downregulated. The fact that nodule formation was strongly repressed with the onset of P deficiency is reflected in the differential expression of various genes involved in nodulation. It is concluded that plants follow a strategy to maintain N₂ fixation and viable leaf tissue as long as possible during whole-plant P depletion to maintain their ability to react to emerging new P sources (e.g. through active P acquisition by roots).
Highlights
Phosphorus (P) is a major plant nutrient but it is estimated that inexpensive and accessible rock phosphate deposits will be depleted during the 21st century (Runge-Metzger, 1995; Cordell et al, 2009)
The fact that nodule formation was strongly repressed with the onset of P deficiency is reflected in the differential expression of various genes involved in nodulation
It is concluded that plants follow a strategy to maintain N2 fixation and viable leaf tissue as long as possible during whole-plant P depletion to maintain their ability to react to emerging new P sources
Summary
Phosphorus (P) is a major plant nutrient but it is estimated that inexpensive and accessible rock phosphate deposits will be depleted during the 21st century (Runge-Metzger, 1995; Cordell et al, 2009). Given that P is crucially important for plant growth, plants have evolved strategies for increasing soil P availability and accessibility through intensive root formation and root turnover under low-P conditions (Steingrobe et al, 2001), making use of the benefits of mycorrhizal symbiosis (Bolan, 1991; Marschner and Dell, 1994; Küster et al, 2007) and the root exudation of compounds that increase P availability (Shane and Lambers, 2005), either directly or through bacteria-mediated effects (Schilling et al, 1998; Marschner et al, 2011). Comparative transcriptome and proteome studies reveal complex internal reactions and acclimations of plant organs to low-P concentrations (Hernández et al, 2009; Ramirez et al, 2013), including longdistance signalling processes involving miRNA and sugars as signal carriers (Vance, 2010)
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