Secretion of acid phosphatase by the roots of several crop species under phosphorus-deficient conditions

Abstract

Nine crop species (rice, wheat, soybean, lupin, azuki bean, sugar beet, tomato, cabbage and radish) were grown in phosphorus-sufficient and phosphorusdeficient nutrient solutions. Then, the activities of acid phosphatase secreted by the roots for 24 h and of that eluted by 100 mM NaC1 were investigated. Both activities of acid phosphatase increased under low phosphorus conditions in all the crop species. The activity of acid phosphatase secreted by the roots under phosphorus-deficient conditions was remarkably high in lupin and tomato, high in cabbage and radish, moderate in soybean and sugar beet, and low in wheat and azuki bean. The increased rates of activity of acid phosphatase (on a root dry weight basis) secreted under phosphorus-deficient conditions when compared with those under phosphorus-sufficient conditions ranged from 1.5 times in azuki bean to 19.9 times in lupin. The activity of acid phosphatase in the NaC1 eluates was also higher in all the crop species under phosphorus-deficient conditions than that under phosphorus-sufficient conditions. Rice and lupin were grown in the nutrient solution with four phosphorus levels, and the activities of acid phosphatase secreted and eluted by 100 mM NaC1 were determined during the growth. The activities of acid phosphatase in lupin were much higher than those in rice. Both activities of acid phosphatase increased with the decrease of the phosphorus concentration in the nutrient solution, and were substantially high at the initial growth stage. Significance of the secretion of acid phosphatase under phosphorus-deficient conditions was discussed.Key Words: acid phosphatase, phosphorus deficiency, rhizosphere, secretion by roots.Plant roots secrete several organic compounds into the rhizosphere along with the uptake of water and nutrients. Both the kind and the amount of organic compounds secreted by plant roots are dependent on various factors, such as plant species, plant age, temperature, light, microorganism status in the rhizosphere, soil moisture, and root damage, etc. (Rovira 1969). For example, mycorrhizal plant species exude a larger amount of carbohydrates and amino acids from the roots than nonmycorrhizal plant species (Schwab et al. 1984).The nutritional status of plants also has a great influence on the root secretion. When plants grow under nutrient stress conditions, such as phosphorus deficiency, iron deficiency, and aluminum excess, plant roots secrete functional organic compounds in order to avoid these conditions. Therefore, the root secretion ability of plants plays an important role in growth under nutrient stress conditions. It is considered that root secretion is one of the adaptive mechanisms in response to nutrient deficiencies. 130T. TADANO and H. SAKAI Large amounts of citric acid have been shown to be secreted by lupin roots under phosphorus-deficient conditions, and it has been indicated that citric acid was able to release inorganic phosphate from phosphated ferric hydroxide (Gardner et al. 1983). It has also been reported that the s...

Extraradical hyphae of the mycorrhizal fungus Glomus intraradices can hydrolyse organic phosphate

“…Moreover, mycorrhizal colonization often results in an increased plant P concentration, which usually results in reduced root phosphatase activity (Tadano & Sakai, 1991 ;Tadano et al, 1993). It is therefore not surprising to find that mycorrhizal colonization can actually be associated with a reduction rather than an increase in extracellular root phosphatase activity (Azcon et al, 1982).…”

Differences in carboxylic acid exudation among p-starved leguminous crops in relation to carboxylic acid contents in plant tissues and phospholipid level in roots

“…Stimulation of phosphatase exudation under phosphorus starvation (Hirata et al 1982) and significant differences in phosphatase exudation among several crop species were reported (Tadano and Sasaki 1991) which may contribute to phosphorus absorption from organic forms.…”

Effect of exogenous phosphatase and phytase activities on organic phosphate mobilization in soils with different phosphate adsorption capacities

“…White lupin is well known to form a unique root structure, so-called cluster roots, under P-deficient conditions to mobilize unavailable P forms (Neumann and Martinoia 2002). Reportedly, huge amounts of organic acids and phosphatases were secreted from the cluster roots of white lupin under P deficient conditions (Tadano and Sakai 1991;Neumann et al 1999;Wasaki et al 2003). Further improvement in phytate-P mobilization is expected from the tandem genetic manipulation of phytase and a key protein for exudation of organic acids, such as citrate or malate.…”

“…Secretion of APase from roots is an important plant response to P deficiency. White lupin has an enhanced ability to secrete APase from roots under P-deficient conditions (Tadano and Sakai 1991;Wasaki et al 2003). Ozawa and co-workers (1995) purified and characterized the APase protein of white lupin, and its complementary DNA (cDNA) has been isolated and named LASAP2 (Wasaki et al 2000).…”