Understanding plant-microbe interactions for phytoremediation of petroleum-polluted soil.

Plant-microbe interactions are considered to be important processes determining the efficiency of phytoremediation of petroleum pollution, however relatively little is known about how these interactions are influenced by petroleum pollution. In this experimental study using a microcosm approach, we examined how plant ecophysiological traits, soil nutrients and microbial activities were influenced by petroleum pollution in Phragmites australis, a phytoremediating species. Generally, petroleum pollution reduced plant performance, especially at early stages of plant growth. Petroleum had negative effects on the net accumulation of inorganic nitrogen from its organic forms (net nitrogen mineralization (NNM)) most likely by decreasing the inorganic nitrogen available to the plants in petroleum-polluted soils. However, abundant dissolved organic nitrogen (DON) was found in petroleum-polluted soil. In order to overcome initial deficiency of inorganic nitrogen, plants by dint of high colonization of arbuscular mycorrhizal fungi might absorb some DON for their growth in petroleum-polluted soils. In addition, through using a real-time polymerase chain reaction method, we quantified hydrocarbon-degrading bacterial traits based on their catabolic genes (i.e. alkB (alkane monooxygenase), nah (naphthalene dioxygenase) and tol (xylene monooxygenase) genes). This enumeration of target genes suggests that different hydrocarbon-degrading bacteria experienced different dynamic changes during phytoremediation and a greater abundance of alkB was detected during vegetative growth stages. Because phytoremediation of different components of petroleum is performed by different hydrocarbon-degrading bacteria, plants' ability of phytoremediating different components might therefore vary during the plant life cycle. Phytoremediation might be most effective during the vegetative growth stages as greater abundances of hydrocarbon-degrading bacteria containing alkB and tol genes were observed at these stages. The information provided by this study enhances our understanding of the effects of petroleum pollution on plant-microbe interactions and the roles of these interactions in the phytoremediation of petroleum-polluted soil.

Plants' use of different nitrogen forms in response to crude oil contamination.

In this study, we investigated Phragmites australis' use of different forms of nitrogen (N) and associated soil N transformations in response to petroleum contamination. 15N tracer studies indicated that the total amount of inorganic and organic N assimilated by P. australis was low in petroleum-contaminated soil, while the rates of inorganic and organic N uptake on a per-unit-biomass basis were higher in petroleum-contaminated soil than those in un-contaminated soil. The percentage of organic N in total plant-assimilated N increased with petroleum concentration. In addition, high gross N immobilization and nitrification rates relative to gross N mineralization rate might reduce inorganic-N availability to the plants. Therefore, the enhanced rate of N uptake and increased importance of organic N in plant N assimilation might be of great significance to plants growing in petroleum-contaminated soils. Our results suggest that plants might regulate N capture under petroleum contamination.

Do plants modulate biomass allocation in response to petroleum pollution?

Biomass allocation is an important plant trait that responds plastically to environmental heterogeneities. However, the effects on this trait of pollutants owing to human activities remain largely unknown. In this study, we investigated the response of biomass allocation of Phragmites australis to petroleum pollution by a ¹³CO2 pulse-labelling technique. Our data show that plant biomass significantly decreased under petroleum pollution, but the root-shoot ratio for both plant biomass and ¹³C increased with increasing petroleum concentration, suggesting that plants could increase biomass allocation to roots in petroleum-polluted soil. Furthermore, assimilated ¹³C was found to be significantly higher in soil, microbial biomass and soil respiration after soils were polluted by petroleum. These results suggested that the carbon released from roots is rapidly turned over by soil microbes under petroleum pollution. This study found that plants can modulate biomass allocation in response to petroleum pollution.

Phenylalanine ammonia lyase functions as a switch directly controlling the accumulation of calycosin and calycosin-7-O-beta-D-glucoside in Astragalus membranaceus var. mongholicus plants.

Previously it had been shown that calycosin and calycosin-7-O-beta-D-glucoside (CGs) accumulate in whole plants, mainly in leaves, of Astragalus membranaceus Bge. var. mongholicus (Bge.) Hsiao (A. mongholicus) plants in response to low temperature. In this work, it was demonstrated that the influences of different conditions on CGs biosynthesis, by examining the changes in CGs content, as well as the expression of related genes, including phenylalanine ammonia lyase (PAL1), cinnamic acid 4-hydroxylase (C4H), chalcone synthase (CHS), chalcone reductase (CHR), chalcone isomerase (CHI), isoflavone synthase (IFS), and isoflavone 3'-hydroxylase (I3'H). The seven gene mRNAs accumulated in leaves of A. mongholicus upon exposure to low temperature in a light-dependent manner, though they exhibited different expression patterns. Transcriptions of CHS, CHR, CHI, IFS, and I3'H of the calycosin-7-O-beta-D-glucoside pathway were all up-regulated when plants were transferred from 16 degrees C to 2 degrees C or 25 degrees C or from 2 degrees C (kept for 24 h) to 25 degrees C. However, fluctuations in temperature influenced differently the transcriptions of PAL1 and C4H of the general phenylpropanoid pathway in leaves. Moreover, the amount of PAL1 expression changed sharply up and down, consistent with the variation of the content of CGs. PAL enzyme activity appears to be the limiting factor in determining the CGs levels. The inhibitor of PAL enzyme, L-alpha-aminooxy-beta-phenylpropionic acid, almost entirely shut down CGs accumulation at low temperature. All these results confirmed that PAL1, as a smart gene switch, directly controls the accumulation of CGs in A. mongholicus plants, in a light-dependent manner, during low temperature treatment.