Numerous reports have been published on the occurrence, isolation, and characterization of phenolic compounds in plant-soil systems. The low molecular weight phenolics are of great interest because of their effects as allelopathic compounds and plant growth regulators, and they have traditionally been considered as defense molecules in plant-pathogen interactions. More recently, their role as signal molecules in plantmicrobe systems has become evident. Specific molecules can act either as inducers for virulent genes in plantpathogen systems, such as Agrobacterium, or as inducers and "repressors" in Rhizobium-legumc and probably other symbiosis. The overall regulation role of these compounds in compatible and incompatible host-microbe interactions is discussed in this review.

Arbuscular mycorrhizal fungi (AMF) were surveyed for species richness and abundance in sporulation in six distinct land uses in the western Amazon region of Brazil. Areas included mature pristine forest and sites converted to pasture, crops, agroforestry, young and old secondary forest. A total of 61 AMF morphotypes were recovered and 30% of them could not be identified to known species. Fungal communities were dominated by Glomus species but Acaulospora species produced the most abundant sporulation. Acaulospora gedanensis cf., Acaulospora foveata, Acaulospora spinosa, Acaulospora tuberculata, Glomus corymbiforme, Glomus sp15, Scutellospora pellucida, and Archaeospora trappei sporulated in all land use areas. Total spore numbers were highly variable among land uses. Mean species richness in crop, agroforestry, young and old secondary forest sites was twice that in pristine forest and pasture. fungal communities were dominated in all land use areas except young secondary forest by two or three species which accounted for 48% to 63% of all sporulation. Land uses influenced AMF community in (1) frequency of occurrence of sporulating AMF species, (2) mean species diversity, and (3) relative spore abundance. Conversion of pristine forest into distinct land uses does not appear to reduce AMF diversity. Cultural practices adopted in this region maintain a high diversity of arbuscular mycorrhizal fungi.

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Rare earth elements (REE) are a homogenous group of 17 chemical elements in the periodic table that are key to many modern industries including chemicals, consumer electronics, clean energy, transportation, health care, aviation, and defense. Moreover, in recent years, they have been used in agriculture. One of the consequences of their worldwide use is the possible increase of their levels in various environmental compartments. This review addresses major topics concerning the study of REE in the soil environment, with special attention to the latest research findings. The main sources of REE to soils, the contents of REE in soils worldwide, and relevant information on the effects of REE to plants were explored. Ecological and human health risk issues related to the presence of REE in soils were also discussed. Although several findings reported positive effects of REE on plant growth, many questions about their biological role remain unanswered. Therefore, studies concerning the actual mechanism of action of these elements on cellular and physiological processes should be further refined. Even more urgent is to unveil their chemical behavior in soils and the ecological and human health risks that might be associated with the widespread use of REE in our modern society.

SUMMARYThe effects of flavonoid compounds on VA mycorrhiza root colonization and growth of w-hite clover {Trifolium repens L.) plants under growth chamber conditions are reported. The isoflavonoids, formononetin and biochanin A, previously identified from clover roots, stimulated colonization and growtb of clover, while several other flavonoid compounds were inactive when tested at concentrations of 5 mg 1"', The flavone, chr\'sin, wben applied 3t concentrations higher than those tested for formononetin and biochanin A, also increased root colonization and plant growth. The stimulatory effects of the isoflavonoids on p\ant growth were mediated by VA mycorrhiza) fungi and were dependent on concentration, period of growth and soil spore density. Maximum responses were found when 5 mg 1"' solutions were applied to soi! containing 2 to 4 VA mycorrhiza spores g"^ of soil. Tbese results may provide insights on the molecular mechanisms of bost-fungus interaction and for the development of technology to exploit the potential of the indigenous VA mycorrhizai fungi in field soil.

Arbuscular mycorrhiza (AM) associations are of great importance in forest ecology and land rehabilitation in the tropics, but information on AM susceptibility, host dependence, and host responsiveness to the fungi is scarce. The present study was carried out under greenhouse conditions in a low-fertility soil with 29 woody species. There were very large differences between plant species in AM colonization, responsiveness to inoculation, mycorrhizal dependency and efficiency of phosphorus (P) uptake. All of these parameters were influenced by available soil P in solution. AM colonization ranged from zero in several non-mycotrophic species to >60% in the highly mycotrophic ones. Ten species (34% of the total) were found to be mycorrhizaindependent or non-mycotrophic, whereas the rest were highly to very highly dependent. The level of P above which there was no AM effect, defined here as the T′ value, allowed distinction between AM dependence and responsiveness of the host and was very efficient for separating species according to these traits. Mycorrhizal responsiveness and dependency were not related and some species were responsive to increased P in the soil solution only when mycorrhizal. Efficiency of P uptake was affected by AM and by P levels. Some species exhibited a high efficiency independent of AM, while others were very inefficient even at high P. Despite differences between species, in most cases AM growth enhancement was nutritionally mediated. Differences in AM responsiveness and dependency as well as the importance of these concepts for reforestation technology in the tropics are discussed.

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Brazilian name canga refers to the ecosystems associated with superficial iron crusts typical for the Brazilian state of Minas Gerais (MG) and some parts of Amazon (Flona de Carajas). Iron stone is associated with mountain plateaux and so, in addition to high metal concentrations (particularly iron and manganese), canga ecosystems, as other rock outcrops, are characterized by isolation and environmental harshness. Canga inselbergs, all together, occupy no more than 200 km2 of area spread over thousands of km2 of the Iron Quadrangle (MG) and the Flona de Carajas, resulting in considerable beta biodiversity. Moreover, the presence of different microhabitats within the iron crust is associated with high alpha biodiversity. Hundreds of angiosperm species have been reported so far across remote canga inselbergs and different micro-habitats. Among these are endemics such as the cactus Arthrocereus glaziovii and the medicinal plant Pilocarpus microphyllus. Canga is also home to iron and manganese metallophytes; species that evolved to tolerate high metal concentrations. These are particularly interesting to study metal homeostasis as both iron and manganese are essential plant micro-elements. Besides being models for metal metabolism, metallophytes can be used for bio-remediation of metal contaminated sites, and as such are considered among priority species for canga restoration. “Biodiversity mining” is not the only mining business attracted to canga. Open cast iron mining generates as much as 5–6% of Brazilian gross domestic product and dialog between mining companies, government, society, and ecologists, enforced by legal regulation, is ongoing to find compromise for canga protection, and where mining is unavoidable for ecosystem restoration. Environmental factors that shaped canga vegetation, canga biodiversity, physiological mechanisms to play a role, and ways to protect and restore canga will be reviewed.