Phylloremediation of Air Pollutants: Exploiting the Potential of Plant Leaves and Leaf-Associated Microbes

Abstract

Air pollution is air contaminated by anthropogenic or naturally occurring substances in high concentrations for a prolonged time, resulting in adverse effects on human comfort and health as well as on ecosystems. Major air pollutants include particulate matters (PMs), ground-level ozone (O₃), sulfur dioxide (SO₂), nitrogen dioxides (NO₂), and volatile organic compounds (VOCs). During the last three decades, air has become increasingly polluted in countries like China and India due to rapid economic growth accompanied by increased energy consumption. Various policies, regulations, and technologies have been brought together for remediation of air pollution, but the air still remains polluted. In this review, we direct attention to bioremediation of air pollutants by exploiting the potentials of plant leaves and leaf-associated microbes. The aerial surfaces of plants, particularly leaves, are estimated to sum up to 4 × 10⁸ km² on the earth and are also home for up to 10²⁶ bacterial cells. Plant leaves are able to adsorb or absorb air pollutants, and habituated microbes on leaf surface and in leaves (endophytes) are reported to be able to biodegrade or transform pollutants into less or nontoxic molecules, but their potentials for air remediation has been largely unexplored. With advances in omics technologies, molecular mechanisms underlying plant leaves and leaf associated microbes in reduction of air pollutants will be deeply examined, which will provide theoretical bases for developing leaf-based remediation technologies or phylloremediation for mitigating pollutants in the air.

Keywords: air pollution; nitrogen dioxides; ozone; particulate matter; phylloremediation; phyllosphere; sulfur dioxide; volatile organic compounds.

Figure 1

A schematic illustration of phyllosphere. The middle panel represent an aerial part of a plant. Right panel shows a magnified schematic cross section of a leaf where leaf surface and trichomes can retain particulate matter (PMs) and stomata adsorb or absorb PMs as well as how leaves can assimilate SO₂, NO₂, and CH₂O (formaldehyde) to simple organic compounds, amino acids, or proteins. The left panel depict a magnified leaf surface with bacteria, which can biodegrade or transform volatile organic compounds to less toxic or nontoxic ones like benzene and its derivatives that can be degraded through Ortho pathway or Meta pathway.

Figure 2

A general outline for developing phylloremediation technologies. Plants species and microbes should be selected from air polluted areas. Selected plants should be evaluated for their ability to adsorb or absorb air pollutants, and concurrently microbes are screened for biodegradation or biotransformation of pollutants. The selected plants and microbes are tested for synergistic effects on the reduction of particular air pollutants. Based on the test results, specific plant-microbe combinations that can remove one or more air pollutants are identified, and protocols are formulated for evaluating their effectiveness in removal pollutants indoors and outdoors. Effective protocols will be developed into phylloremediation technologies for use in reducing air pollutants.