Respiratory and photosynthetic C and N metabolism of nodulated Lupin roots during phosphorus deficiency

• Main Author: Le Roux, Marcellous R
• Other Authors: Valentine, AJ
• Language: en
• Published: University of the Western Cape 2021
• Subjects: Bacteroids; C-metabolism; Dicarboxylic acids; Nodules; Photosynthesis; Pi deficiency; Nitrogen; N2-fixation; Respiration; Lupinus; Malate
• Online Access: http://hdl.handle.net/11394/8427

Philosophiae Doctor - PhD === Growth of symbiotic legume hosts is P limited, because of the high energetic requirements associated with N2 fixation. Attempts to overcome P deficiency in soils where legumes are grown involve addition of P-based fertilisers. However, these are produced from fmite, non-renewable resources that could be exhausted in the next 50-80 years. For this and other prudent reasons, viable alternatives are sought that include producing genetically enhanced plants with better P use efficiency (PUE). There exist some inter- and intraspecific genetic variation for associated traits of PUE in various legumes and these will have to be exploited to realize the development of P efficient cultivars. With the advent of sophisticated molecular tools, good progress has been made to understand the molecular response of some common physiological and morphological functions observed under LP. The research aims here were to investigate the energy costs and the alternative metabolic routes associated with C and N metabolism under LP in legumes, which is very scant in literature. We also investigated the recovery responses of nodulated roots upon P alleviation. Consequently, improvement strategies to produce legume varieties for better adaptation in poor P soils are envisaged. We have demonstrated varying degrees of sensitivity between the amide and ureide legume systems being investigated under short-term LP. The species-specific responses were ascribed to differences related to the agro-climatic origins, nodule morphologies and the type of N containing export product of the different legume types. These different responses also underscore possible different regulatory mechanisms under LP. Lupins were probed further, because of its apparent tolerance to P deficiency. Lupin nodules had between 3 to 5-fold higher Pj concentrations compared with soybeans under LP and HP, respectively. The maintenance of Pj levels, as oppose to a decline in the total P pool, is discussed in relation to its role in maintaining N2 fixation in lupins. Under LP, an effective Pj recycling mechanism in nodules is proposed to occur via the induction of the PEPc- MDH-ME route. This route also enhanced the capacity of root nodules to procure high malate concentrations that are used to fuel bacteroid respiration and N2 fixation. Two distinctly different cMDH proteins, one corresponding to HP and another corresponding to LP, were identified. The high malate concentrations reported here are speculated to have arisen through LP-induced cMDH. Metabolically available Pj decline developed gradually as P deficiency progressed. This coincided with a 15% decline in the %Ndfa. Moreover, under prolonged P deficiency the disproportionate synthesis of organic acids, most notably malate, that occurred at the expense of amino acids was proposed to account for this decline. The recovery in response to alleviation from LP involved alterations in the allocation of respiratory costs to growth and nutrient acquisition. Under LP, smaller nodules were formed and nodule metabolism revolved around accentuating PUE. Thus, there is considerable potential for improvement of P efficiency in legumes through manipulation of root: shoot partitioning.