Submitted to: Plant Physiology Supplement
Publication Type: Abstract Only
Publication Acceptance Date: 8/3/1997
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Proteoid roots are short, densely clustered lateral roots characteristic of most members of the Proteaceae. Proteoid roots are abundant in many members of this family and are involved in the solubilization of mineral bound nutrients. White lupin (Lupinus albus L.) also forms proteoid roots in response to stress. This adaptation is accompanied by developmental and biochemical responses that increase the efficiency of nutrient uptake. The formation of proteoid roots in white lupin is affected by the phosphorus (P) status of the plant, induction occurring at low P levels, and repression at high levels. When grown in -P conditions, proteoid root meristems are present by 5 days after emergence (DAE), and by 8 DAE meristems have emerged through the epidermis. By 22 DAE, over 50% of the root dry mass in the -P treatment occurs as proteoid roots, as compared to 5% for the +P control plants. Proteoid roots excrete phenomenal amounts of organic acids (such as citrate and malate), which assist in the mobilization of mineral bound P and other nutrients. Approximately one third of the carbon exuded as organic acids comes from root CO2 fixation. Lupin root phosphoenolpyruvate carboxylase (PEPC) is a key enzyme in this process, contributing carbon for organic acid synthesis. In a time course study, amounts of PEPC mRNA, PEPC specific activity, and enzyme protein were found to be greater in proteoid roots than in normal roots, beginning 10 DAE. An additional mechanism for efficient P acquisition by lupin involves the secretion of acid phosphatase (APase), which hydrolyzes soil organic P. We have demonstrated that there is a 4 fold increase in APase secretion by proteoid roots, and the timing of this localized secretion corresponds to the timing of organic acid secretion.