Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/12/1996
Publication Date: N/A
Citation: Interpretive Summary: Phosphorous (P), one of the primary nutrients required for plant growth, is a costly farm input. World P supply is dwindling and will become critically low by 2050. It is therefore important to understand how plants adapt to low soil P and how to increase P uptake efficiency. Roots of P stressed white lupin are known to excrete organic acids, which increases the availability of P in soil. To further assess how white lupin adapts t P stress, we grew plants in the presence and absence of P. Root morphology and root exudates and gene expression were evaluated over a 23 day period. Lateral root development was altered in P stressed lupin. Clustered tertiary roots called proteoid roots formed over 60% of the root mass on P stressed plants. Proteoid root zones could be identified at 9 days after planting (DAP). The organic acids malate and citrate were exuded by proteoid roots beginning at 10 DAP. The P-stressed plants exuded 25 fold more citrate and malate as compared to P-sufficient plants. The activity and amount of gene expression for an enzyme (protein catalyst) involved in organic acid was also measured. The activity and gene expression for this enzyme increased coincident with organic acid exudation and proteoid root development. These results are important because they show that white lupin has an efficient mechanism to obtain P under stress conditions. This mechanism involved altered root morphology and gene expression. Information gained in these studies will be useful in developing strategies to improve plant P acquisition.
Technical Abstract: The development of clustered tertiary lateral roots (proteoid roots) and the expression of phosphoenolpyruvate carboxylase (PEPC, EC 126.96.36.199) in roots were studied in white lupin (Lupinus albus L.) grown with either 1 mM P (+P treated) or zero P (-P treated). The +P treated plants initiated fewer clustered tertiary meristems and emergence of those meristems appeared to be delayed compared to -P treated plants. Proteoid root zones can be identified 9 days after emergence (DAE) in both P treatments. Amounts of PEPC mRNA, PEPC specific activity, and enzyme protein were greater in proteoid roots compared to normal roots beginning at 10, 12, and 14 DAE, respectively. The increases in PEPC mRNA, PEPC enzyme, and PEPC specific activity suggest that this enzyme is in part under transcriptional regulation. Recovery of organic acids from root exudates coincided with the increases in PEPC specific activity. The -P treated plants exuded 40, 20, and 5 fold more citrate, malate, and succinate, respectively, compared to +P treated plants. Data presented support the hypothesis that white lupin has concerted regulation of proteoid root development, transcriptional regulation of PEPC, and biosynthesis of organic acids for exudation in response to P deficiency.