Roots and Nodules Response Differently to P Starvation in the Mediterranean-Type Legume Virgilia divaricata.

Gary G Stevens,D Jacobus Brand,Rafael J L Morcillo,Alex J Valentine,Paul Hills,Emma T Steenkamp,Aleysia Kleinert,María A Pérez-Fernández

doi:10.3389/fpls.2019.00073

Abstract

Virgilia divaricata is a tree legume that grows in the Cape Floristic Region (CFA) in poor nutrient soils. A comparison between high and low phosphate growth conditions between roots and nodules was conducted and evaluated for the plants ability to cope under low phosphate stress conditions in V. divaricata. We proved that the plant copes with low phosphate stress through an increased allocation of resources, reliance on BNF and enhanced enzyme activity, especially PEPC. Nodules had a lower percentage decline in P compared to roots to uphold its metabolic functions. These strategies partly explain how V. divaricata can sustain growth despite LP conditions. Although the number of nodules declined with LP, their biomass remained unchanged in spite of a plant decline in dry weight. This is achieved via the high efficiency of BNF under P stress. During LP, nodules had a lower % decline at 34% compared to the roots at 88%. We attribute this behavior to P conservation strategies in LP nodules that imply an increase in a metabolic bypass that operates at the PEP branch point in glycolysis. The enhanced activities of nodule PEPC, MDH, and ME, whilst PK declines, suggests that under LP conditions an adenylate bypass was in operation either to synthesize more organic acids or to mediate pyruvate via a non-adenylate requiring metabolic route. Both possibilities represent a P-stress adaptation route and this is the first report of its kind for legume trees that are indigenous to low P, acid soils. Although BNF declined by a small percentage during LP, this P conservation was evident in the unchanged BNF efficiency per weight, and the increase in BNF efficiency per mol of P. It appears that legumes that are indigenous to acid soils, may be able to continue their reliance on BNF via increased allocation to nodules and also due to increase their efficiency for BNF on a P basis, owing to P-saving mechanisms such as the organic acid routes.

Highlights

The Cape Floristic Region (CFR), found in the south western area of South Africa can be regarded as one of the highest P-impoverished regions of the world and simultaneously a Global Biodiverse Hotspot (Lambers and Shane, 2007)
Legume species reliant on Biological Nitrogen Fixation (BNF) are highly dependant on P supply, more so than legumes growing on mineral N (Drevon and Hartwig, 1997)
To that end we investigated how Phosphoenolpyruvate Carboxylase (PEPC)-derived C is metabolized into amino acids and downstream organic acids of P-deficient nodules, using 13C Nuclear Magnetic Resonance (NMR) spectroscopy

Summary

Introduction

The Cape Floristic Region (CFR), found in the south western area of South Africa can be regarded as one of the highest P-impoverished regions of the world and simultaneously a Global Biodiverse Hotspot (Lambers and Shane, 2007). Legume species reliant on Biological Nitrogen Fixation (BNF) are highly dependant on P supply, more so than legumes growing on mineral N (Drevon and Hartwig, 1997). The tree species, V. divaricata (Adamson), is a native legume to the CFR and it is distributed over a wide range of P-poor soils from the relatively richer forest margins to poorer Fynbos soils (Coetsee and Wigley, 2013). This implies that the indigenous species may have a range of mechanisms to adapt to variable soil P supply

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Conclusion