Yin et al. (2020) visualized the spatial distribution of recent
11 C-photoassimilates
translocated and released in the root system and soil of white lupin (Lupinus albus)
and soybean (Glycine max). The inputs of the recently assimilated C in the entire root
that were released into the soil were approximately 0.3–2.9% for white lupin within
90 min and 0.9–2.3% for soybean within 65 min, with no significant differences
between the two plant species; however, the recently assimilated C in lupin was
released at high concentrations in specific areas (hotspots), whereas that in soybean
was released uniformly in the soil. As white lupin forms a structure called cluster
roots in which the lateral roots are densely packed, white lupin secretes large
amounts of organic acids and enzymes into the surrounding soil to intensively take
up nutrients, including phosphorus and iron. The released C compounds have
important effects on microbial populations and nutrient solubility and availability
while also enhancing the plant’s ability to cope with adverse soil–chemical conditions (Li et al. 2010; Neumann et al. 2000; Walker et al. 2003; Jones et al. 2004).
Rhizobacterial Activity under P Stress
The inositol hexa-phosphate (IHP, phytate), as a form of organic P, is present at high
levels in soil because IHP, especially Al-IHP and Fe-IHP, is difficult for phosphatase
to solubilize; thus, IHP remains in the soil. To solubilize IHP, plant roots must exude
phytase; however, phytase exudation is very low from plant roots except that from a
few plants, such as white lupin (Lupinus albus L.). However, under low P and N
conditions, plants can acquire nutrients either by a shift to a more specialized
microbial community or by changes in microbial enzyme production (Wasaki
et al. 2018). The microbial community structure can be estimated by fingerprinting
(denaturing gradient gel electrophoresis) and sequencing techniques. P deficiency
induced the release of citrate and acid phosphomonoesters from cluster roots and
stimulated the production of microbe-derived alkaline phosphomonoesterase in the
rhizosphere. P deficiency decreased microbial diversity in the cluster root rhizosphere. The increased relative abundance of Burkholderiales in the rhizosphere of
P-deficient plants might be responsible for the degradation of different organic P
fractions, such as phytates. N deficiency induced an increase in the number of
nodules and in the P concentration in the shoots as well as the roots of white
lupin. The strong release of citrate from cluster roots might be the preferred mechanism for meeting the P demand of nodulated plants under N deficiency. In addition,
the high abundance of Rhizobiales and Rhodospirillales in the rhizosphere of cluster
roots showed the importance of N-fixing microorganisms under N deficiency
(Wasaki et al. 2018).
Rhizobacteria survive on carbon (C) exuded from roots and may contribute to
plant nutrition by liberating P from organic compounds such as phytates. Over
300 phytate (Na-inositol hexa-phosphate; Na-IHP)-utilizing bacterial strains were
isolated from the rhizosheath and rhizoplane of Lupinus albus L. (Unno et al. 2005).
Almost all of the isolates were classified as Burkholderia based on 16S rDNA
sequence analysis. Rhizosheath isolates cultured with Na-IHP as the only source
of C and P showed lower P uptake at the same extracellular phytase activity rate as
that in rhizoplane strains, suggesting that bacteria from the rhizosheath utilized
44
M. Osaki et al.
11 C-photoassimilates
translocated and released in the root system and soil of white lupin (Lupinus albus)
and soybean (Glycine max). The inputs of the recently assimilated C in the entire root
that were released into the soil were approximately 0.3–2.9% for white lupin within
90 min and 0.9–2.3% for soybean within 65 min, with no significant differences
between the two plant species; however, the recently assimilated C in lupin was
released at high concentrations in specific areas (hotspots), whereas that in soybean
was released uniformly in the soil. As white lupin forms a structure called cluster
roots in which the lateral roots are densely packed, white lupin secretes large
amounts of organic acids and enzymes into the surrounding soil to intensively take
up nutrients, including phosphorus and iron. The released C compounds have
important effects on microbial populations and nutrient solubility and availability
while also enhancing the plant’s ability to cope with adverse soil–chemical conditions (Li et al. 2010; Neumann et al. 2000; Walker et al. 2003; Jones et al. 2004).
Rhizobacterial Activity under P Stress
The inositol hexa-phosphate (IHP, phytate), as a form of organic P, is present at high
levels in soil because IHP, especially Al-IHP and Fe-IHP, is difficult for phosphatase
to solubilize; thus, IHP remains in the soil. To solubilize IHP, plant roots must exude
phytase; however, phytase exudation is very low from plant roots except that from a
few plants, such as white lupin (Lupinus albus L.). However, under low P and N
conditions, plants can acquire nutrients either by a shift to a more specialized
microbial community or by changes in microbial enzyme production (Wasaki
et al. 2018). The microbial community structure can be estimated by fingerprinting
(denaturing gradient gel electrophoresis) and sequencing techniques. P deficiency
induced the release of citrate and acid phosphomonoesters from cluster roots and
stimulated the production of microbe-derived alkaline phosphomonoesterase in the
rhizosphere. P deficiency decreased microbial diversity in the cluster root rhizosphere. The increased relative abundance of Burkholderiales in the rhizosphere of
P-deficient plants might be responsible for the degradation of different organic P
fractions, such as phytates. N deficiency induced an increase in the number of
nodules and in the P concentration in the shoots as well as the roots of white
lupin. The strong release of citrate from cluster roots might be the preferred mechanism for meeting the P demand of nodulated plants under N deficiency. In addition,
the high abundance of Rhizobiales and Rhodospirillales in the rhizosphere of cluster
roots showed the importance of N-fixing microorganisms under N deficiency
(Wasaki et al. 2018).
Rhizobacteria survive on carbon (C) exuded from roots and may contribute to
plant nutrition by liberating P from organic compounds such as phytates. Over
300 phytate (Na-inositol hexa-phosphate; Na-IHP)-utilizing bacterial strains were
isolated from the rhizosheath and rhizoplane of Lupinus albus L. (Unno et al. 2005).
Almost all of the isolates were classified as Burkholderia based on 16S rDNA
sequence analysis. Rhizosheath isolates cultured with Na-IHP as the only source
of C and P showed lower P uptake at the same extracellular phytase activity rate as
that in rhizoplane strains, suggesting that bacteria from the rhizosheath utilized
44
M. Osaki et al.