Phytoremediation is emerging as a potential cost-effective solution for the remediation of contaminated soils. Because contaminants such as lead (Pb) have limited bioavailability in the soil, a means of solubilizing the Pb in the soil and facilitating its transport to the shoots of plants is vital to the success of phytoremediation. Indian mustard (Brassica juncea) was used to demonstrate the capability of plants to accumulate high tissue concentrations of Pb when grown in Pb-contaminated soil. Concentrations of 1.5% Pb in the shoots of B. juncea were obtained from soils containing 600 mg of Pb/kg amended with synthetic chelates such as EDTA. The accumulation of Pb in the tissue corresponded to the concentration of Pb in the soil and the concentration of EDTA added to the soil. The accumulation of Cd, Cu, Ni, and Zn from contaminated soil amended with EDTA and other synthetic chelators was also demonstrated. The research indicates that the accumulation of metal in the shoots of B. juncea can be enhanced through the application of synthetic chelates to the soil, facilitating high biomass accumulation as well as metal uptake.

Strigolactones (SLs) have been proposed as a new group of plant hormones, inhibiting shoot branching, and as signaling molecules for plant interactions. Here, we present evidence for effects of SLs on root development. The analysis of mutants flawed in SLs synthesis or signaling suggested that the absence of SLs enhances lateral root formation. In accordance, roots grown in the presence of GR24, a synthetic bioactive SL, showed reduced number of lateral roots in WT and in max3-11 and max4-1 mutants, deficient in SL synthesis. The GR24-induced reduction in lateral roots was not apparent in the SL signaling mutant max2-1. Moreover, GR24 led to increased root-hair length in WT and in max3-11 and max4-1 mutants, but not in max2-1. SLs effect on lateral root formation and root-hair elongation may suggest a role for SLs in the regulation of root development; perhaps, as a response to growth conditions.

Indian mustard (Brassica juncea) plants exposed to Pb and EDTA in hydroponic solution were able to accumulate up to 55 mmol kg The synthetic chelate EDTA forms a soluble complex with many metals, including Pb (Kroschwitz, 1995), and can solubilize Pb from soil particles (Means and Crerar, 1978). Recently, application of EDTA to Pb-contaminated soils has been shown to induce the uptake of Pb by plants (Jøgensen, 1993; Huang and Cunningham, 1996;Blaylock et al., 1997;Huang et al., 1997), causing Pb to accumulate to more than 1% (w/w) of shoot dry biomass (Huang and Cunningham, 1996;Blaylock et al., 1997;Huang et al., 1997). For the in situ remediation of Pb-contaminated soils it appears that this chelate-assisted phytoextraction strategy (Salt et al., 1998) may be more effective than a strategy based on the natural ability of certain wild plant species for metal hyperaccumulation (Chaney, 1983; Baker et al., 1988).For more than 40 years, synthetic chelates have been used to supply plants with micronutrients in both soil and hydroponics. Yet the mechanisms by which chelates enhance metal accumulation are still not well characterized (Wallace and Wallace, 1992), and what is known appears contradictory. For example, some evidence suggests that the Fe-chelate EDTA can be absorbed by plants and translocated to shoots (Weinstein et al., 1954; Llyod-Jones, 1961, 1965). However, Tiffin et al. (1960) concluded that Fe-chelates are excluded from root tissue, and this was supported by Chaney et al. (1972), who demonstrated that Fe is taken up by plants only after first being split from the Fe-chelate complex by the action of a specific plasma membrane-bound Fe-chelate reductase.To optimize the process of chelate-assisted phytoextraction, it is important to understand the biological mechanisms responsible for this process. Because of the stimulatory role of chelate application in the uptake of Pb and other metals by plants, we have investigated the role of EDTA in Pb accumulation in plants. In this study we have demonstrated that the previously described EDTAenhanced Pb accumulation in Indian mustard (Brassica juncea) is based on the ability of EDTA to chelate and transport Pb from soil into shoot tissue. MATERIALS AND METHODSRoots of Indian mustard (Brassica juncea [L.] Czern. var 426308) (Kumar et al., 1995) were grown under microbiologically controlled conditions as follows. Seeds were surface sterilized in 2.6% (w/v) bleach for 30 min, rinsed four times in autoclaved deionized water, and transferred onto sterile 1.2% (w/v) agar plates containing 3.0% (w/v) Suc. Plates were held vertically and the seeds allowed to germinate and grow in the dark at 22°C for 72 h. Etiolated seedlings that did not show microbial contamination on the agar plates were transferred individually into small glass vials (29 ϫ 65 mm) containing 23 mL of sterile nutrient solution. Soft Styrofoam stoppers used to cap the vials were incised radially to provide support for the hypocotyls.

Most of the symplastic water transport in plants occurs via aquaporins, but the extent to which aquaporins contribute to plant water status under favorable growth conditions and abiotic stress is not clear. To address this issue, we constitutively overexpressed the Arabidopsis plasma membrane aquaporin, PIP1b, in transgenic tobacco plants. Under favorable growth conditions, PIP1b overexpression significantly increased plant growth rate, transpiration rate, stomatal density, and photosynthetic efficiency. By contrast, PIP1b overexpression had no beneficial effect under salt stress, whereas during drought stress it had a negative effect, causing faster wilting. Our results suggest that symplastic water transport via plasma membrane aquaporins represents a limiting factor for plant growth and vigor under favorable conditions and that even fully irrigated plants face limited water transportation. By contrast, enhanced symplastic water transport via plasma membrane aquaporins may not have any beneficial effect under salt stress, and it has a deleterious effect during drought stress.

Most studies on the reduction of disease incidence in soil treated with Trichoderma asperellum have focused on microbial interactions rather than on plant responses. This study presents conclusive evidence for the induction of a systemic response against angular leaf spot of cucumber (Pseudomonas syringae pv. lachrymans) following application of T. asperellum to the root system. To ascertain that T. asperellum was the only microorganism present in the root milieu, plants were grown in an aseptic hydroponic growth system. Disease symptoms were reduced by as much as 80%, corresponding to a reduction of 2 orders of magnitude in bacterial cell densities in leaves of plants pretreated with T. asperellum. As revealed by electron microscopy, bacterial cell proliferation in these plants was halted. The protection afforded by the biocontrol agent was associated with the accumulation of mRNA of two defense genes: the phenylpropanoid pathway gene encoding phenylalanine ammonia lyase (PAL) and the lipoxygenase pathway gene encoding hydroxyperoxide lyase (HPL). This was further supported by the accumulation of secondary metabolites of a phenolic nature that showed an increase of up to sixfold in inhibition capacity of bacterial growth in vitro. The bulk of the antimicrobial activity was found in the acid-hydrolyzed extract containing the phenolics in their aglycone form. High-performance liquid chromatography analysis of phenolic compounds showed a marked change in their profile in the challenged, preelicited plants relative to that in challenged controls. The results suggest that similar to beneficial rhizobacteria, T. asperellum may activate separate metabolic pathways in cucumber that are involved in plant signaling and biosynthesis, eventually leading to the systemic accumulation of phytoalexins.

Plants have had an essential role in the folklore of ancient cultures. In addition to the use as food and spices, plants have also been utilized as medicines for over 5000 years. It is estimated that 70–95% of the population in developing countries continues to use traditional medicines even today. A new trend, that involved the isolation of plant active compounds begun during the early nineteenth century. This trend led to the discovery of different active compounds that are derived from plants. In the last decades, more and more new materials derived from plants have been authorized and subscribed as medicines, including those with anti-cancer activity. Cancer is among the leading causes of morbidity and mortality worldwide. The number of new cases is expected to rise by about 70% over the next two decades. Thus, there is a real need for new efficient anti-cancer drugs with reduced side effects, and plants are a promising source for such entities. Here we focus on some plant-derived substances exhibiting anti-cancer and chemoprevention activity, their mode of action and bioavailability. These include paclitaxel, curcumin, and cannabinoids. In addition, development and use of their synthetic analogs, and those of strigolactones, are discussed. Also discussed are commercial considerations and future prospects for development of plant derived substances with anti-cancer activity.

This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000 Azospirilla are free-living rhizobacteria that are able to promote plant growth and increase yields in many crops of agronomic importance. It is assumed that the bacteria affect plant growth mainly by the production of plant growth promoting substances, which leads to an improvement in root development and an increase in the rate of water and mineral uptake. In the present review, we discuss the physiological responses of the plant roots to inoculation with Azospirillum, and report on field and greenhouse experiments carried out with these bacteria during 1994–2001 in Belgium, Uruguay, Mexico and Israel.

Strigolactones (SLs) are plant hormones that suppress lateral shoot branching, and act to regulate root hair elongation and lateral root formation. Here, we show that SLs are regulators of plant perception of or response to low inorganic phosphate (Pi) conditions. This regulation is mediated by MORE AXILLARY GROWTH2 (MAX2) and correlated with transcriptional induction of the auxin receptor TRANSPORT INHIBITOR RESPONSE1 (TIR1). Mutants of SL signaling (max2-1) or biosynthesis (max4-1) showed reduced response to low Pi conditions relative to the wild type. In max4-1, but not max2-1, the reduction in response to low Pi was compensated by the application of a synthetic strigolactone GR24. Moreover, AbamineSG, which decreases SL levels in plants, reduced the response to low Pi in the wild type, but not in SL-signaling or biosynthesis mutants. In accordance with the reduced response of max2-1 to low Pi relative to the wild type, several phosphate-starvation response and phosphate-transporter genes displayed reduced induction in max2-1, even though Pi content in max2-1 and the wild type were similar. Auxin, but not ethylene, was sufficient to compensate for the reduced max2-1 response to low Pi conditions. Moreover, the expression level of TIR1 was induced under low Pi conditions in the wild type, but not in max2-1. Accordingly, the tir1-1 mutant showed a transient reduction in root hair density in comparison with the wild type under low Pi conditions. Therefore, we suggest that the response of plants to low Pi is regulated by SLs; this regulation is transmitted via the MAX2 component of SL signaling and is correlated with transcriptional induction of the TIR1 auxin receptor.