Phosphorus mobilization and localisation in the roots and nodules of a Cape Floristic Region legume, and its impact on nitrogen fixation

[摘要] ENGLISH ABSTRACT: During phosphorus (P) deficiency, plants can exhibit a wide array of morphological,physiological and biochemical responses. Legume plants are vulnerable to P deficiency,because it affects their ability to fix atmospheric nitrogen (N2) via their symbiotic interactionwith rhizobial bacteria. In particular, legumes from nutrient poor ecosystems, such as thefynbos in the Cape Floristic Region (CFR) would have evolved on P deficient soils and maytherefore display unique modifications. Moreover, since P distribution in soils isheterogenous, even less is known about the recovery from P deficiency responses in nodules.The aim of this research was to investigate P recycling and distribution in the nodules of thefynbos legume Virgilia divaricata, during low P supply and its recovery from P deficiency.The legume species was inoculated with a locally compatible N2 fixing bacterialstrain, Burkholderia, isolated from V. divaricata nodules grown in fynbos soil. Plants weregrown under glasshouse conditions, using a modified Long Ashton Nutrient Solution (LANS)to simulate the low nutrient conditions of the fynbos ecosystem. Plants were subsequentlyanalysed for growth kinetics, nutrient acquisition and distribution, nodule anatomy, Precycling and P metabolite composition.The results indicated that V. divaricata can experience P deficiency during exposure to low Psupply. Under low P conditions, plants experienced lower biomass and nodule production.Although biological N2 fixation (BNF) was lower during P deficiency as compared to duringconditions of optimal P supply, the nodules of plants grown under P deficient conditions hada greater BNF per nodule mass and unit P. In addition, low P nodules also showedhomogenous P tissue localisation and a greater concentration of Fe. The total P level waslower in nodule tissues, and the activities of phosphohydrolases (APase, RNase and phytase) higher. In addition, there was also a possible remobilization of membrane phospholipids, inorder to release additional Pi.Although V. divaricata experiences P deficiency in its biomass and P nutrition, it also has aremarkable physiological ability to recover from P deficiency during P resupply. In contrastto the observed perturbations in biomass and nutrition during P stress, the impact on thenodules was different to that of the roots. The underlying mechanisms for functionalmaintenance of the nodules during low P seems to be associated with an internal mechanism,related to P mobilization from organic sources, metabolic bypass mechanisms to conserve Pand a re-allocation of Fe to the infected cells. The higher enzyme activity of the internalphosphohydrolases (APase, RNase and phytase) favours the liberation of cellular P formetabolic reactions and internal P turnover.This research has generated knowledge regarding the physiological impact and flexibility ofmechanisms involving below-ground P recycling in legumes. It has demonstrated that alegume from a nutrient poor ecosystem, favours internal mechanisms of P recycling andconservation, in order to maintain the efficient functioning of nodules under P stress ratherthan improve acquisition from external sources.

[发布机构] Stellenbosch University