1. PEPC activation in leaves: TCA activation (organic acids) for K
+ malate transportation to roots
2. Rhizosphere activation: Malate exudation, VAM
Acid Phosphatase Exudation from Roots
Phosphorus deficiency induces the synthesis of acid phosphatases (APases) at both
the molecular and cellular levels in the roots of lupin and other plant species (Tadano
et al. 1993). As APase decomposes organic P in soil, roots can absorb Pi from
decomposing organic P.
Lupin plants can grow even in extremely low-P soil, as shown in a -P plot (no P
fertilizer application and removal of all aboveground plant parts from the field) at
Hokkaido University since 1904. Among field crops and pasture plants, as only lupin
plants can grow under P-deficient conditions, lupin plants have been used as model
plants to study P deficiency mechanisms. For example, lupin plants showed
increased levels of total acid phosphatase activity within 2–5 days after P was
withheld, and levels approximately doubled in 15 days. Lateral roots, not the main
tap roots, were responsible for this increase in acid phosphatase activity (Wasaki
et al. 1997, 1999a, b, 2000, 2003a, b; Yamagishi et al. 2011; Shinano et al. 2013).
Organic Acid Exudation from Roots
In acidic mineral soils, for example, soils in Kohong (Kh: Typic Paleudults; coarseloamy siliceous), Hat Yai (Hy: Typic Paleudults; clayey-skeletal, kaolinitic), and
Ban Thon (Bh: Typic Haplorthods; sandy, noncemented), the insoluble P portion in
soil is very high at approximately 1–14% in aluminum phosphate (AI-P) and iron
phosphate (Fe-P), and a small portion of approximately 0–5% of P is in calcium
phosphate (Ca-P). In the Bh soil, AI-P was dominant (25–51%), followed by organic
P (22–45%). For these soils, citrate and oxalate application was effective in solubilizing the insoluble P from Al-P and Fe-P (Onthong et al. 1999).
Root exudation of organic acids is also effective in solubilizing insoluble P
compounds, including organic P (Maejima et al. 2014; Yamamura et al. 2004).
Wu et al. (2018) found that the beneficial effects of organic acid excretion from
plant roots were first proposed as being associated with the superior phosphorus
utilization mechanism in the cluster roots of white lupin (Lupinus albus L.). These
effects are now widely accepted as pleiotropic effects that are associated with plant
stress tolerance. Excreted organic acids detoxify rhizotoxic aluminum, recruit beneficial bacteria for induced systemic resistance, and modify root architecture to
alleviate phosphorus starvation.
Cluster Root Formation
The importance of cluster roots for Pi acquisition is well known, and acid phosphatase (APase) produced by the cluster roots of white lupin (Lupinus albus L.) plays an
important role in inorganic phosphate (Pi) acquisition. Strong APase activity in the
epidermal tissues of normal roots and cluster rootlets and in the root hairs of cluster
rootlets under P deficiency has been detected (Wasaki et al. 2003a, b, 2008; Zhou
et al. 2008).
1 Basic Information About Tropical Peatland Ecosystems
43
+ malate transportation to roots
2. Rhizosphere activation: Malate exudation, VAM
Acid Phosphatase Exudation from Roots
Phosphorus deficiency induces the synthesis of acid phosphatases (APases) at both
the molecular and cellular levels in the roots of lupin and other plant species (Tadano
et al. 1993). As APase decomposes organic P in soil, roots can absorb Pi from
decomposing organic P.
Lupin plants can grow even in extremely low-P soil, as shown in a -P plot (no P
fertilizer application and removal of all aboveground plant parts from the field) at
Hokkaido University since 1904. Among field crops and pasture plants, as only lupin
plants can grow under P-deficient conditions, lupin plants have been used as model
plants to study P deficiency mechanisms. For example, lupin plants showed
increased levels of total acid phosphatase activity within 2–5 days after P was
withheld, and levels approximately doubled in 15 days. Lateral roots, not the main
tap roots, were responsible for this increase in acid phosphatase activity (Wasaki
et al. 1997, 1999a, b, 2000, 2003a, b; Yamagishi et al. 2011; Shinano et al. 2013).
Organic Acid Exudation from Roots
In acidic mineral soils, for example, soils in Kohong (Kh: Typic Paleudults; coarseloamy siliceous), Hat Yai (Hy: Typic Paleudults; clayey-skeletal, kaolinitic), and
Ban Thon (Bh: Typic Haplorthods; sandy, noncemented), the insoluble P portion in
soil is very high at approximately 1–14% in aluminum phosphate (AI-P) and iron
phosphate (Fe-P), and a small portion of approximately 0–5% of P is in calcium
phosphate (Ca-P). In the Bh soil, AI-P was dominant (25–51%), followed by organic
P (22–45%). For these soils, citrate and oxalate application was effective in solubilizing the insoluble P from Al-P and Fe-P (Onthong et al. 1999).
Root exudation of organic acids is also effective in solubilizing insoluble P
compounds, including organic P (Maejima et al. 2014; Yamamura et al. 2004).
Wu et al. (2018) found that the beneficial effects of organic acid excretion from
plant roots were first proposed as being associated with the superior phosphorus
utilization mechanism in the cluster roots of white lupin (Lupinus albus L.). These
effects are now widely accepted as pleiotropic effects that are associated with plant
stress tolerance. Excreted organic acids detoxify rhizotoxic aluminum, recruit beneficial bacteria for induced systemic resistance, and modify root architecture to
alleviate phosphorus starvation.
Cluster Root Formation
The importance of cluster roots for Pi acquisition is well known, and acid phosphatase (APase) produced by the cluster roots of white lupin (Lupinus albus L.) plays an
important role in inorganic phosphate (Pi) acquisition. Strong APase activity in the
epidermal tissues of normal roots and cluster rootlets and in the root hairs of cluster
rootlets under P deficiency has been detected (Wasaki et al. 2003a, b, 2008; Zhou
et al. 2008).
1 Basic Information About Tropical Peatland Ecosystems
43