Few topics in ecology are as intriguing or bedeviling as comparative studies of different types of mycorrhizas formed in the same plant species. Attempts to determine the relative benefits from each fungal type to the host plant are fraught with difficulties (Jones et al., 1998), and for this reason plants that form tripartite associations with arbuscular and ectomycorrhizal fungi make ideal experimental systems. Just such a tripartite system is explored in a report in this issue by Chen et al. (pp. 545–556).

Why compare mycorrhizas? Although both ecto- and arbuscular mycorrhizas are generally known to increase the uptake of nutrients such as phosphorus and nitrogen in infertile soils, their functions and the benefits they provide to their host plants may not be equivalent (Jones et al., 1998). Hence the ecological benefits of the two types of symbiosis are of great interest to plant community ecologists and researchers studying nutrient cycling. Arbuscular mycorrhizas are formed as a symbiosis of plant roots with primitive fungi of the order Glomales (Zygomycetes), while ectomycorrhizas are formed with higher basidiomycetes and a few ascomycetes. Only a few plant genera and species are capable of forming both types of association, sometimes on the same root tip (Fig. 1), but it is these that are so useful in comparative studies.

A walk through the forest is a good way to become aquainted with the variety of terpenes emitted by plants. The most fragrant of these are the monoterpenes, which lend a distinctive smell to trees (e.g. α- and β-pinene from pines), mints (e.g. menthol from peppermint), fruits (e.g. limonene from citrus) and flowers (e.g. geraniol from roses). In addition to adding fragrance, phytogenic hydrocarbons play an important role in atmospheric chemistry. This was recognized as early as 1960 by Fritz Went who noted that blue hazes often formed above forested areas (Went, 1960). This idea was picked up by US President Ronald Reagan who accused trees of polluting the atmosphere, although in truth phytogenic hydrocarbons only contribute to pollution in air that is already contaminated by industry or automobile exhaust (Feshenfeld et al., 1992).

Terpenes serve a variety of functions in plants including deterring herbivores and attracting pollinators. Until recently, the function of terpene emission from some groups of plants, which do not store the terpenes in their tissues, has been a mystery. Following the discovery that isoprene, a hemiterpene, increases the thermotolerance of photosynthesis in some species (Sharkey & Singsaas, 1995; Singsaas et al., 1997), Loreto et al. (1998) discovered that thermotolerance of the monoterpene-emitting oak, Quercus ilex, increases when leaves are fumigated with monoterpenes. Now, a report by Delfine et al. in this issue (pp. 27–36) provides further support for the hypothesis that terpenes play a role in thermotolerance of leaves and suggests, intriguingly, that the mechanism by which this occurs is not limited to plants that make these compounds.

Recent years have seen a flurry of research activity concerning the hyperaccumulation of heavy metals by plants. Much of the interest in hyperaccumulation has been fueled by the commercial potential of phytoremediation, the use of plants to clean up contaminated soils (Baker et al., 1994; Salt et al., 1995; Chaney et al., 1997, 2000). These applications have in turn spurred many studies of the genetics and physiology of metal uptake (e.g. Krämer et al., 1996; Lasat et al., 1996; Salt & Krämer, 1999; Baker et al., 2000; see also Krämer, 2000; Lombi et al., 2000). Although phytoremediation provides an intriguing and potentially profitable backdrop, the ecology and evolution of hyperaccumulation in natural populations are interesting subjects in their own right. Two papers in this issue (Ghaderian et al., pp. 219–224; Davis & Boyd, pp. 211–217) are exciting contributions to the growing consensus that hyperaccumulation may act as a defense against herbivores and pathogens, and suggest that hyperaccumulation may become a model system for research in this area.

How plants gather their resources, harvest solar energy and convert raw materials into an increased biomass is one of the enduring fascinations of biological science. Just about every step has been charted by biochemists and few processes have escaped the determination of a rate constant by physiologists. Even the molecular biologist is busy identifying the genes that govern the processes of synthesis and assembly. Nevertheless, at higher levels of organization, of building populations and, especially, communities, the prediction of growth continues to be problematic. Thus, the development by Bright & Walsby (pp. 301–316 in this issue) of a method for calculating the daily growth of a photoautotroph in its natural environment is bound to command our attention. Their approach is ingenious and the veracity of their calculations compelling. This is exciting science.

For wood mycologists, the work of Savory (1954a,b) marked an important step in the understanding of decay processes by lignolytic fungi. His description of decay by ascomycete and deuteromycete fungi in wood from industrial water cooling towers revealed a particular pattern observed previously by Bailey & Vestal (1937), Tamblyn (1937) and Barghoorn & Linder (1944) in wood from other situations. This type of decay was termed ‘soft rot’ because of the spongy texture of the wood surface. Soft rot, characterized by hyphal penetration of the wood cell walls in a fashion similar to sap-stain fungi, results in the formation of decay cavities in the central zone of wood cell walls, which leads to their ultimate destruction. Soft rot was therefore described as distinct from the brown and white rot forms of wood decay normally attributed to lignolytic basidiomycetes and some of the larger ascomycetes, such as members of the Xylariaceae. Indeed, members of the genera Daldinia, Hypoxylon and Xylaria have usually been considered to be white rot fungi. However, the boundaries between these three types of fungal decay have since become less clear-cut and a report in this issue, of a study into hyphal penetration of the reaction zone in beechwood (Schwarze and Baum, pp. 129–140), identifies different penetration mechanisms.

Several generations of soil microbiologists and plant pathologists have attempted to unravel the microbial composition, population dynamics and various life-sustaining processes in the soil. However, until recently, this basic resource for plant growth and nutrient recycling has to a large extent remained a black box. Studies have been made of the behavior of individual microorganisms or their population dynamics, but the relationship between the growth of individual organisms or parts of organisms (such as fungal hyphae) and that of populations and communities has been largely unknown. In particular, the spread of fungal colonies in soil has been an enigma, but now Bailey et al. (pp. 000–000 in this issue) have clearly demonstrated that spatial behavior of Rhizoctonia colonies can switch from finite to invasive expansion dependent on the non-linear relationship between the distance among substrate particles and the probability of colonization.

Summary 185

I. INTRODUCTION 186

II. EFFECT OF ZINC ON PRODUCTION OF REACTIVE OXYGEN SPECIES 186

1. Superoxide-generating NADPH oxidase 186

2. Zinc deficiency potentiates iron-mediated free radical production 189

(a) Iron accumulation in zinc-deficient plants 189

(b) Iron-induced production of free radicals 189

3. Zinc deficiency-enhanced photooxidation 191

(a) Decrease in photosynthesis 191

(b) Light-induced leaf chlorosis 192

(c) Decrease in indole-3-acetic acid 192

III. MEMBRANE DAMAGE BY REACTIVE OXYGEN SPECIES 193

1. Impairments in membrane structure 193

2. Phospholipids and –SH groups 195

3. Alterations in ion absorption 195

(a) Membrane-bound ATPases 195

(b) Nutrient uptake 197

(c) Changes in activity of ion channels 197

IV. DETOXIFICATION OF REACTIVE OXYGEN SPECIES 198

1. Superoxide dismutases 198

2. H2O2-scavenging enzymes 198

V. INVOLVEMENT OF ZINC IN PLANT STRESS TOLERANCE 199

VI. CONCLUSIONS 199

Acknowledgements 200

References 200

Zinc deficiency is one of the most widespread micronutrient deficiencies in plants and causes severe reductions in crop production. There are a number of physiological impairments in Zn-deficient cells causing inhibition of the growth, differentiation and development of plants. Increasing evidence indicates that oxidative damage to critical cell compounds resulting from attack by reactive O2 species (ROS) is the basis of disturbances in plant growth caused by Zn deficiency. Zinc interferes with membrane-bound NADPH oxidase producing ROS. In Zn-deficient plants the iron concentration increases, which potentiates the production of free radicals. The Zn nutritional status of plants influences photooxidative damage to chloroplasts, catalysed by ROS. Zinc-deficient leaves are highly light-sensitive, rapidly becoming chlorotic and necrotic when exposed to high light intensity. Zinc plays critical roles in the defence system of cells against ROS, and thus represents an excellent protective agent against the oxidation of several vital cell components such as membrane lipids and proteins, chlorophyll, SH-containing enzymes and DNA. The cysteine, histidine and glutamate or aspartate residues represent the most critical Zn- binding sites in enzymes, DNA-binding proteins (Zn-finger proteins) and membrane proteins. In addition, animal studies have shown that Zn is involved in inhibition of apoptosis (programmed cell death) which is preceded by DNA and membrane damage through reactions with ROS.

Of the three component organs of the grass leaf – blade, sheath and ligule – the ligule is the least studied and the least understood. Traditionally, it has been assumed to act in a passive way in protecting the culm and leaves that it encloses from the entry of water, dust and harmful spores. However, ultrastructural and cytochemical studies of the membranous ligules of several taxa, particularly Lolium species, have challenged that rather simplistic view and suggest that these ligules play a more active role in the life of the grass plant as a secretory tissue. This review summarizes the evidence for the latter notion, assesses the validity of both the passive and active hypotheses of membranous ligule function and notes similarities between membranous grass ligules, root caps and lycopsid ligules.

Cadmium accumulation and tolerance are discussed in a New Phytologist article by Krämer (2000), which comments on a paper by Lombi et al. (2000). In this context, a number of additional points should be made, putting the role of humans in the evolution of metal resistance into context and emphasizing what is the ‘hype’ of phytoremediation.

It is important that sites created by humans should not be overemphasized in considering the evolution of metal resistance. Plants resistant to heavy metals have their primary sites not on these, but on soils where ores are outcropping, the so-called metalliferous or orogenic soils (Ernst, 1974). Over thousands of years, natural exposure to a surplus of various metals, depending on the mineralization process, has driven the evolution of metal resistance in many plant species under the local environmental conditions. Many publications have shown that Thlaspi caerulescens can hyperaccumulate Zn (e.g. Vázquez et al., 1992), and accumulate other heavy metals such as Cu and Pb depending on soil chemistry (e.g. Baker et al., 1994). It has been known for more than 30 years that T. caerulescens gives a good response to experimentally supplied high Zn levels (Ernst, 1968). One of the ecological effects of hyperaccumulation of heavy metals is a defence against herbivorous insects (Boyd & Martens, 1994). This effect is enhanced by a preferential accumulation of heavy metals in the epidermal leaf layer (Heath et al., 1997).

I. INTRODUCTION 360

II. PHOTOINHIBITION AND ACTIVE OXYGEN 360

III. OXYGEN AS AN ELECTRON ACCEPTOR 362

1. Oxygen ‘poises’ electron transport and carbon assimilation 362

2. The role of oxygen in ATP synthesis 364

3. How fast is O2reduction at PSI? 364

4. Chloroplastic processing of H2O2 366

IV. REDOX REGULATION OF PHOTOSYNTHETIC METABOLISM 368

1. The thioredoxin system 368

2. Manipulating the expression of thiol-regulated enzymes 369

3. Modifying sensitivity to thiol regulation 369

V. PHOTORESPIRATION 369

1. The pathway and its genetic manipulation 369

2. Engineering plants that photorespire less? 371

3. Is photorespiration important in energy dissipation? 372

4. Production and processing of photorespiratory H2O2 373

5. Catalase and foliar H2O2levels 374

6. Catalase and non-photorespiratory H2O2generation 375

VI. RESPIRATION 376

1. ‘Photosynthetic’ respiration 376

2. AOS in the mitochondrion 376

3. AOX: regulation and significance to photosynthesis 377

VII. PHOTOSYNTHESIS AND REDOX SIGNAL TRANSDUCTION 378

1. The need for sensors, signals and transducers 378

2. Signal transduction at the local level 378

3. Remote signalling and responses leading to acclimation of photosynthesis? 379

4. Interactions between AOS, NO., and antioxidants 380

VIII. CONCLUSIONS 380

Acknowledgements 381

References 381

The gradual but huge increase in atmospheric O2 concentration that followed the evolution of oxygenic photosynthesis is one consequence that marks this event as one of the most significant in the earth's history. The high redox potential of the O2/water couple makes it an extremely powerful electron sink that enables energy to be transduced in respiration. In addition to the tetravalent interconversion of O2 and water, there exist a plethora of reactions that involve the partial reduction of O2 or photodynamic energy transfer to produce active oxygen species (AOS). All these redox reactions have become integrated during evolution into the aerobic photosynthetic cell. This review considers photosynthesis as a whole-cell process, in which O2 and AOS are involved in reactions at both photosystems, enzyme regulation in the chloroplast stroma, photorespiration, and mitochondrial electron transport in the light. In addition, oxidants and antioxidants are discussed as metabolic indicators of redox status, acting as sensors and signal molecules leading to acclimatory responses. Our aim throughout is to assess the insights gained from the application of mutagenesis and transformation techniques to studies of the role of O2 and related redox components in the integrated regulation of photosynthesis.

Physcomitrella patens, recently renamed Aphanoregma patens, has been transformed with the plasmid, pBI426. On selective medium approx. 30% of regenerants expressed the transformed phenotype transiently (transients). The remaining 70% (transformants) retained their transformed phenotype (GUS-positive and resistant to G418) indefinitely when subcultured repeatedly on selective medium. However, most lost this phenotype after one or two passages through nonselective medium (unstable transformants). Approximately 0.2% of transformants retained their transformed phenotype after numerous passages through nonselective medium (stable transformants). Using PCR methodology, it has been shown that loss of the transformed phenotype by unstable transformants is invariably accompanied by disappearance of the transgenic DNA. Southern blot analysis data argue strongly that unstable transformants cultured under selective conditions contain unintegrated pBI426 as circular concatenates consisting of 3–40 copies of the plasmid. Under selective conditions, it appears that replication and/or partitioning of these extrachromosomal concatemers might be growth rate-limiting. This is the first report of a transgenic, autonomously replicating extrachromosomal element in a photosynthetic plant. A single copy of pBI426 has been inserted into the moss genome in each of three stable transformants analysed.

We tested if fumigation with exogenous monoterpenes might induce thermotolerance in leaves of an oak species (Quercus suber) which does not form and emit isoprenoids. To understand if exogenous monoterpene fumigation results in internal accumulation of monoterpenes, a physical method of monoterpene extraction was used. The internal content of monoterpenes increased in concert with increasing fumigation doses. This unambiguously demonstrated acquisition of exogenous monoterpenes. We exposed fumigated Q. suber leaves to two cycles of increasing temperatures from 35 to 55°C at 5°C steps. When leaves were exposed to a low dose of exogenous monoterpenes, yielding an internal content similar to that endogenously formed in the leaves of the monoterpene-emitter Q. ilex, no clear improvement in thermotolerance was found. When leaves were exposed to a high dose of exogenous monoterpenes, yielding an internal content of about five fold the endogenous pool of Q. ilex, but comparable with the expected content following stress-induced stomatal closure, photosynthesis inhibition at high temperatures was attenuated. This effect was observed only at temperatures <45°C during the first cycle, but at all temperatures between 35 and 55°C when plants were exposed to two cycles of high temperatures. Monoterpenes were still found in the leaves of Q. suber 12 h after ending the fumigation. Monoterpenes were also found in non-fumigated leaves distant up to 45 cm from the fumigated leaves. If monoterpenes make the photosynthetic apparatus more resistant to high temperatures, the effect might not be limited to the fumigated leaves and might be persistent after fumigation.