Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 122

mycorrhizae and rhizobia. Roots produce chemical signals that attract bacteria and induce chemo-
taxis (Thimmaraju et al ., 2008). Positive interactions mediated by root exudates include population
of growth facilitators that support growth of plants. Negative interactions mediated by root exu-
dates involve secretion of antimicrobial compounds (Bais et al ., 2006). Excretion of organic acids
and CO 2 production by roots may affect rhizosphere microorganisms and the buffering capacity
of the soil (Marschner, 1995). PGPR may (i) synthesize siderophores, which can solubilize and
sequester iron from the soil and provide it to plant (Rajkumar et al ., 2010), (ii) organic acids of root
exudates solubilize minerals such as phosphorus, which becomes readily available. Availability
of Fe may affect the iron-plaque formation by microbes in roots of rice plants and consequent As
adsorption. Being chemical analogs, both As and P have comparable dissociation constants for
their acids and solubility products for their salts, resulting in similar geochemical behavior of As
and P in soil (Adriano, 2001). Hence, it is reasonable to assume that carboxylate exudation could
play a role in the bioavailability of As in the rhizosphere and its further uptake by plants.
6.4.5 Plant microbe interactions
Interactions between plants and soil microbes are highly dynamic in nature and based on
co-evolutionary pressures (Morgon et al ., 2005; Reinhart and Callaway, 2006). Consequently,
it is not astonishing that rhizosphere microbial communities differ among plant species, and fur-
ther genotypes within species along with different developmental stages of a particular plant
(Batten et al ., 2006; Mougel et al ., 2006; Wei et al ., 2007) ( Table 6.4 ) . Pteris vittata ,an
As-hyperaccumultor, has generated interest in As-phytoremediation (Ma et al ., 2001). Applica-
tions of microbes for enhancing phytoremediation in P . vittata has been attempted with positive
results like plant biomass increased by >53% and As uptake by >44% (Agely et al ., 2005; Huang
et al ., 2010; Liu et al ., 2005; Vogel-Mikus et al ., 2006; Yang et al ., 2011). The phytoremediation
of As can be influenced by the variation of rhizospheric microbes; and the interactions between
microbes and plant roots, including microbial and plant root exudates (Tang et al ., 2001).
6.4.6 Mycorrhiza
Arbuscular mycorrhizal (AM) fungi have ubiquitous symbiotic associations in both natural and
contaminated sites (Wang et al ., 2007). AM fungi may stimulate metal phytoextraction by essen-
tially improving plant growth and consequent increase in total metal uptake (Agely et al ., 2005;
Barua et al ., 2010; Leung et al ., 2006). According to Leung et al . (2006), the infectious percentage
of mycorrhizas was 26.4, 30.3 and 40.6% upon 0, 50 and 100 mg kg − 1 As treatment/exposure,
respectively, compared to the control. The indigenous mycorrhizas enhanced As accumulation as
3.70, 58.3 and 88.1 mg kg − 1 , respectively. Mycorrhizal fungi may alleviate metal toxicity to the
host plant by acting as a barrier for metal uptake in some cases (Leyval et al ., 1997). AM fungi
have been shown to not only increase the nutrient status of their host plant, but also to improve the
plant's ability in tolerating toxic elements (Aggangan et al ., 1998). Zhang et al . (2005) noted that
AM fungi contributed to the resistance of upland rice grown in soil contaminated with metals.
Li et al . (2011) showed that rice/AMF combinations had beneficial effects on lowering grain As
concentration, improving grain yield and grain P uptake. Variation in transfer and uptake of As
and P reflect functional diversity in AM symbioses in different plants.
6.4.7 Iron plaque
Liu et al . (2004) have shown that As can be sequestered in iron plaque of root surface of plants,
thus reducing As uptake in plant tissues. Iron plaque is a precipitate of reddish-brown Fe oxides
and is formed on to the roots of paddy rice (Liu et al ., 2004). Iron plaque is formed by abi-
otic oxidation by iron oxidizing bacteria (Weiss et al ., 2003). Studies have demonstrated that
the microbial reduction of As-bearing Fe(III) (hydro)oxides results in a dissolution of the solid