Inorganic phosphate (Pi) is an essential macronutrient for all life, and vital for crop production. The flora in south-western Australia has evolved on some of the world’s most Pi-impoverished soils. This Kwongon region is a biodiversity hotspot of global significance, and the non-mycorrhizal plant family Proteaceae features prominently, particularly on the most P-deficient soils of the region. Our goal is to identify and characterise novel metabolic adaptations that contribute to remarkably high Pi-acquisition and -use efficiency of Hakea prostrata (harsh hakea), a ‘model’ Proteaceae of Western Australia.
1) Role of novel post-translational enzyme modifications in the control of carbon metabolism in harsh hakea cluster roots
Accumulating evidence indicates pivotal functions for phosphoenolpyruvate carboxylase (PEPC) in Pi-starved plants. This includes controlling the production of organic acid anions (malate, citrate) that are excreted in copious amounts by specialized proteoid ‘cluster’ roots of non-mycorrhizal species such as H. prostrata. This, in turn, enhances the bioavailability of mineral-bound Pi by solubilizing phosphates that are bound to oxides and hydroxides of Al3+, Fe3+, and Ca2+ in soils. Immature proteoid roots contained an equivalent ratio of monoubiquitinated 110-kD and phosphorylated 107-kD PEPC polypeptides (p110 and p107, respectively). Enzyme purification, immunoblotting, and mass spectrometry established that p110 and p107 are subunits of a 430-kD PEPC heterotetramer and that they both originate from the same PEPC gene. Proteoid root maturation triggered PEPC’s activation via deubiquitination of p110 to p107, and subsequent phosphorylation of the deubiquitinated subunits. This unprecedented mechanism of posttranslational enzyme control was hypothesized to contribute to the massive synthesis and excretion of organic acid anions that dominates the carbon metabolism of the mature proteoid roots.
2) Senescence-inducible cell-wall and intracellular hydrolases of harsh hakea
The up-regulation of ribonuclease (RNase) and purple acid phosphatase (PAP) activity is a ubiquitous response of senescing or Pi-starved plant tissues. This facilitates intra- and extracellular Pi scavenging from organic P sources (e.g., RNA, P-esters). We have shown striking up-regulation of cell-wall localized and intracellular PAPs and RNases in senescing leaves and proteoid ‘cluster’ roots of H. prostrata. This likely contributes to the remarkable P-remobilization efficiency (~85%) of senescing H. prostrata tissues. The apparent contribution of cell-wall targeted hydrolases to remobilizing key macronutrients such as Pi during plant senescence has not been previously documented.
Our research is supported by grants from the Australian Research council (ARC) to MWS (DP1092856) an Australian Research Fellow, and the Natural Sciences an
d Engineering Research Council of Canada and the Queen’s Research Chairs program to WCP.
- Professor William C. Plaxton, Queen’s University, Kingston, Ontario, Canada