It is of growing concern internationally to capitalize on the enormous diversity of plant form and function across the globe to unearth novel adaptive traits that might be eventually genetically-engineered into modern crop varieties so they can effectively exploit the significant amounts of insoluble phosphate and organic-P compounds that remain mineral-bound in many agricultural soils. Recent evidence suggests that Australian and Chilean Proteaceae exhibit significant differences in their phosphate (P)-use efficiency and hence their P metabolism. The Proteaceae are a fascinating group of plants, with a Gondwanan distribution and numerous endemic species in south-western Australia. Their goal is to scrutinize these native plants for P-efficient metabolic traits.
Proteaceae are non-mycorrhizal and thrive on extremely phosphorus (P)-impoverished sand or P-fixing soils, where most crops need large inputs of superphosphate. The Proteaceae are well-known for their development of specialised proteoid roots that synthesise and secrete large amounts of malate and citrate to increase P-availability to the roots by solubilising otherwise inaccessible sources of soil P, while making P-esters more accessible to hydrolysis by secreted acid phosphatases. It is abundantly clear that Proteaceae display a wide range of plasticity in their proteoid-root functioning for phosphate (P) acquisition, and in their P-use efficiency. This likely contributes to the range of habitats they can successfully colonise. In Southern Chile, Proteaceae are acclimated to relatively young soils of volcanic origin with relatively high amounts of soil P compared with companion species endemic to the ancient, severely P-impoverished soils of south-western Australia. For example, the upregulation of key acid phosphatases in cell-wall and intercellular compartments correlate with extremely high phosphorus remobilisation efficiency during tissue senescence in Australian Proteaceae (i.e. Hakea prostrata). However, we expect lower levels of activity since for Chilean Proteaceae because their leaves have comparatively lower P resorption efficiency. Interestingly, the southern South American Embothrium coccineum, which occurs on young volcanic soils that contain vast amounts of relatively unavailable P, invests considerably less biomass in cluster roots that release organic anions at a faster rate and during a longer period when compared with Hakea prostrata from south-western Australia.
Professor Shane and Lambers therefore, are examining metabolic differences related to carbon metabolism in their proteoid roots. Phosphoenolpyruvate carboxylase is a tightly controlled cytosolic enzyme located at the core of plant carbon metabolism that catalyzes the irreversible ß-carboxylation of the glycolytic intermediate PEP to form the tricarboxylic acid intermediate oxaloacetate and phosphate. PEPC fulfils wide-ranging and essential non-photosynthetic functions, particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed during biosynthesis of e.g., organic anions (citrate, malate) exuded by mature proteoid roots. If metabolic pathways are to be manipulated for the development of grain crops having improved P-acquisition and P-use efficiency, it is essential to understand how the pathways are controlled. Proteaceae offer a relevant model to contrast metabolic adaptations/mechanisms deployed to survive in soils that contrast greatly in plant available P.
Their research is supported by grants from the Australian Research Council (MWS, HL), an Australian Research Fellowship (DP1092856) to MWS, and from the Chilean Science Council (FONDECYT 1130440).
- Dra. Alejandra Zuniga Feest