Biochemical, metabolic and developmental adaptations in white lupin roots in response to phosphorus stress

White lupin (Lupinus albus L.) develops proteoid or cluster root morphology when subjected to low phosphorus (P) stress. These densely clustered, short lateral roots are the site of substantial citrate and other organic acid exudation, resulting in increased P availability. The goal of these experiments was to advance the knowledge of developmental, biochemical and physiological adaptations of lupin roots to P stress. The activities of citrate synthase (CS), malate dehydrogenase (MDH) and phosphoenolpyruvate carboxylase (PEPC) were measured as indices of metabolic adaptation. In vitro specific activities of CS, MDH and PEPC were higher in excised proteoid roots compared to normal roots. Organic acid concentrations in plant tissues and root exudates, respiration rates, and nonphotosynthetic C fixation were measured in excised and intact plants grown with 1 mM P or without P. In vivo nonphotosynthetic {dollar}rmsp{lcub}14{rcub}COsb2{dollar} fixation was higher in excised proteoid roots compared to normal roots. The majority of the {dollar}sp{lcub}14{rcub}{dollar}C fixed in excised and intact roots was detected in citrate and malate. Shoots or roots of intact plants were labeled with {dollar}rmsp{lcub}14{rcub}COsb2{dollar} to determine the relative contribution of C fixed in shoots and roots to exuded citrate. About 25-fold more acid-stable {dollar}sp{lcub}14{rcub}{dollar}C, primarily in citrate and malate, was exuded from the roots of {dollar}-{dollar}P-treated plants compared to +P-treated plants. Nonphotosynthetic C fixation in roots contributed about 25 and 34% of the C exuded as citrate and as malate, respectively, from {dollar}-{dollar}P-treated plants. Proteoid root meristems could be seen arising from the pericycle six days after shoot emergence (DAE) and were emerged by nine DAE. The development of proteoid roots preceded the increases in PEPC mRNA concentration, PEPC enzyme and PEPC specific activity in proteoid roots compared to normal roots. The increase in PEPC mRNA was prior to, and the increase in PEPC enzyme was coincident with increased PEPC specific activity, indicating that PEPC is in part under transcriptional regulation. Citrate and malate were detected in root exudates coincidentally with increased specific activity of PEPC in the proteoid roots compared to the normal roots. White lupin has an integrated P acquisition strategy that includes developmental, biochemical and metabolic adaptations.