Direct degradation by plants:
There is some evidence of direct degradation of petroleum hydrocarbons by plants, although much of it is dated. In 1977, Durmishidze summarized various studies on the degradation pathways of hydrocarbons in plants. The  pathway of the conversion for alkanes in plants was generalized as:

Durmishidze also reported that BTEX compounds were metabolized quickly,  in 2-6 days, depending on the plant species. More recent research has indicated PAHs can also be degraded directly by plants. It has been documented that [¹⁴C]anthracene and [¹⁴C]benz[a]anthracene were metabolized by bush bean grown in a nutrient solution containing two PAHs. Within the plant, parent compounds were transformed into polar and non-polar metabolites. ( Edwards, 1988).

Indirect degradation by plants:
Plant exudates link plants to microbes and are responsible for the rhizosphere effect, discussed below. It has also been shown that plants exude enzymes which are capable of transforming organic contaminants by catalyzing chemical reactions in the soil. Schnoor et al. (1995) identified plant enzymes as the causative agents in the transformation of contaminants mixed with sediments and soil. (Frick et al., 1999). These findings suggest that a plant has an effect on hydrocarbon degradation that extends beyond it's physical reaches. These effects may continue after the plant has died, as residual enzymes may remain in the soil (Cunningham et al., 1996).

Plants and their roots influence the properties of soil. Plant roots increase contact between microbes and the contaminant by their exploration of the soil. Plants also increase the organic matter content of a soil as they die or as the roots are sloughed through growth. Organic matter in the soil often reduces the bioavailability of many petroleum hydrocarbons (See When is phytoremediation appropriate?), thus resulting in phytostabilization.

The Rhizosphere Effect:
The rhizosphere is the region of the soil closest to the roots of the plants and is, therefore, under the direct influence of the root system. (Frick et al.,1999). The rhizosphere extends about 1 mm around the root tissue. Plants release root exudates into the rhizosphere which can account for 10-20% of photosynthesis annually (Frick et al.,1999; Schooner et al., 1995).  These exudates consist of sugars, acids, alcohols, enzymes and often oxygen, and support large numbers of microbes. There are approximately 10⁸ - 10⁹ vegetative microbes per gram of soil in the rhizosphere (Frick et al.,1999; Erickson et al., 1995). Studies have shown that microbial populations in the rhizosphere are 5 - 100 times greater then populations in bulk soil.

As indicated by the graph above, the number of microrganisms decrease dramatically as the distance from the root zone increases. Jordahl et al. (1997) reported that populations of microorganisms capable of degrading BTEX compounds were 5 times more abundant in the soil around the root zone of poplar trees than in bulk soil.
Thus it seems plants are invaluable in supporting a healthy microbial population with which to degrade petroleum hydrocarbons; phytostimulation is occurring.

Microbial degradation:
Microbes degrade organic contaminants to use for their own growth and reproduction. Organic pollutants provide a carbon source, which is the basic building block for new cell constituents; as well as provide electrons which microbes use as an energy source (Committee on In situ Bioremediation et al., 1993).

Basic metabolism of contaminants is achieved through aerobic respiration. This means the microbes break down the pollutants by utilizing O² as an electron acceptor and they gain energy, and often carbon, as electrons are transferred. There are variations in microbial metabolism of organics which involve anaerobic respiration or fermentation, among other processes. In general, the metabolic processes of microorganisms act on a wider range of compounds, carry out more difficult degradation reactions, and transform contaminants into more simple molecules than those of plants (Frick et al., 1999; Cunningham and Berti, 1993.) However, the pathways in which different microbes breakdown different pollutants are extremely variable. There are as many different transformation pathways as there are microbes and pollutants. The microbial community will vary in size and composition with rhizosphere conditions,  which include plant species, plant age, soil type. The history of a given site may also affect the types and numbers of microbe present. With this degree of variability it is easy to see why phytoremediation of organics is such a complex process. For more information on the metabolic pathways involved in the degradation of BTEX compounds see http://umbbd.ahc.umn.edu/BTEX/BTEX_map.html.

Microbes and reduction of phytotoxicity:
Microbes also play a role in reducing phytotoxicity of contaminants and thereby allowing for plant growth. This enables the rhizosphere effect to enhance the degradation of non-phytotoxic contaminants. Plant and microbes have developed a mutually beneficial strategy for dealing with phytotoxicity, where microorganisms benefit from the plant exudates while the plants benefit from the ability of microorganisms to break down toxic chemicals (Frick et al., 1999).