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Land-based biological N-fixation amounts to approx. 150 million tonnes per year, of which the symbiosis between legumes and rhizobia accounts for about 25%. We have made great progress in understanding the biochemistry of N-fixation and in elucidating some aspects of the relationship between the plant host and bacterial symbiont. The tools of modern genetics have been very useful in this research, as is illustrated by the work of Lohar et al., at the Universities of Missouri, USA and Queensland, Australia (pp. 277–285). It is already known that ethylene inhibits nodule formation while ethylene inhibitors may promote it. To ‘dissect’ further the role of ethylene in nodule formation, the authors have transferred the Arabidopsis thaliana dominant etr-1-1 allele (which encodes a mutated non-functional ethylene receptor) into Lotus japonicus. This resulted in plants that were clearly ethylene-insensitive, as shown both by the absence of the well-known ‘triple response’ and by their ability to grow and nodulate in the presence of the ethylene precursor ACC. However, the degree of ethylene-insensitivity varied between different transgenic lines. Those with highest levels of insensitivity exhibited spectacular effects on nodulation with up to 3·5 times as many nodules/nodule foci as wild-type plants. This increase in nodulation was ascribed both to increased infection and to a seven-fold increase in nodules developing between the xylem poles, a location in which nodule formation is usually very low. This suggests that in wild-type plants ethylene has a role in mediating positional information. Further, there was evidence that ethylene may affect the activity of the symbiont itself: transgenic plants had on average 1·7 times as many bacteroids per ‘symbiosome’ as wild-type. Despite these major differences between the wild-type and the ethylene-insensitive plants, it was also clear that the latter had retained the auto-regulation and the nitrate inhibition of nodulation. These two aspects of regulation thus lie outside the influence of ethylene.
As I write this, the governments of the world's most powerful nations are being urged to take drastic steps in order to keep global warming to less than +2°C. We hope that they (and we) succeed but in the meantime the ranges of several species of animals (and to a lesser extent of plants) are moving slowly towards the poles. So, which factors will actually affect the ability of plants to become established? Milbau et al. (University of Antwerp, Belgium and Umeå University, Sweden; pp. 287–296) have asked this question in respect of sub-arctic regions. Summer soil temperatures will be higher but winter soil temperatures will be lower because the snow cover is likely to be thinner. The authors stratified seeds at ‘real’ sub-arctic winter soil temperatures that occur under a thick or a thin blanket of snow. The seeds were then germinated either under optimum conditions or at real sub-arctic spring/early temperatures, or at real temperatures plus 2·5°C. Firstly, to state the obvious, seeds germinated most rapidly and attained the highest germination percentage under optimum conditions. Secondly, in comparisons between present and probable future conditions, germination of most species was faster in the warmer future conditions than in current conditions; germination percentages were unaffected. Thirdly, comparison of the two stratification conditions gave less clear-cut results. In ten species, exposure to lower winter soil temperature caused delayed germination; in four of those species there was also a reduction in germination percentage, most marked (–33%) in Vaccinium uiliginosum. By contrast, the beautiful Silene acaulis showed a 19% increase in germination percentage and a decrease of 2 days in mean germination time when stratified at lower temperature. Despite the negative effects for some species of the lower winter soil temperatures, the authors consider that earlier germination and longer growing seasons will have an overall benefit for seedling establishment in sub-arctic regions.
I first became aware of artemisinin in the 1980s when reading a student dissertation on traditional Chinese medicine. It was already clear that the compound, synthesized by Artemesia annua (sweet wormwood), was effective against malaria. Since then it has been characterized as a sesquiterpene lactone and has entered mainstream medicine, especially for use against Plasmodium that is resistant to other drugs. It thus joins the large number of plant-derived products that comprise about 25% of drugs prescribed in Western medicine. However, there are problems in maintaining an adequate supply of artemisinin, not least because the yields from the source plant are relatively low and somewhat variable. There is now therefore a programme of breeding and agronomic research aimed not only at obtaining higher yields but also at producing varieties of A. annua that perform well in new areas, including the temperate regions of northern Europe. The work described by Davies et al. (a large UK-based multi-centre group; pp. 315–323) forms part of this research. Plants were raised in a glasshouse and then, whilst still in pots, transferred outside. Rain was excluded. Pots were supplied with different concentrations of N and K; plant biomass and artemisinin concentrations were measured. Biomass increased with increasing N supply up to 106 mg N L–1 (although leaf N continued to increase with higher concentrations of applied N). Total leaf artemisinin content also increased significantly with increasing N and, as with biomass, plateaued at 106 mg N L–1. However, because of the increased biomass the actual concentration of artemisinin on a dry weight basis decreased slightly. Plant biomass was also increased following application of K at concentrations up to 153 mg L–1, but without added N there was no increase in artemisinin content. Artemisinin accumulation in leaves is thus linked with N-induced growth and it will be both fascinating and important to explore this linkage further.
In human society it is often said that problems rarely come in ones. This saying can equally well be applied to the environmental stresses that are experienced by plants, as is illustrated by the work of Xuecheng Sun et al., at Wuhan, China (pp. 345–356). They have studied the interaction between molybdenum (Mo) deficiency and cold-responsiveness in two cultivars of winter wheat (Triticum aestivum). One cultivar was Mo-efficient and one was Mo-deficient and both were grown with and without added Mo. Cold stress was applied by transferring plants from 15/12°C to 5/2°C. The authors’ analyses of the responses to Mo application and cold were very comprehensive and here we present a brief overview of their data. In the absence of added Mo the Mo-efficient cultivar was more freeze-tolerant than the Mo-deficient cultivar. Addition of Mo improved the freeze-tolerance of both cultivars, and in cold-stressed plants led to an increase in Mo-dependent aldehyde oxidase activity. This enzyme is involved in the synthesis of ABA, and thus the increase in ABA content during cold-stress was much higher in plants supplied with Mo than in those not supplied. This was again true of both cultivars. ‘Downstream’ from ABA, exposure to cold led to transient increases in transcription of genes encoding bZIP-type transcription factors and in transcription of ABA-dependent cold-response (COR) genes. As before, these responses were greater in the Mo-fed plants. This was also seen, although to a lesser extent, later in the sequence of responses, in transcription of the genes encoding CBF/DREB transcription factors and of the ABA-independent COR genes. The greater effect of Mo on the ABA-dependent pathway may possibly be ascribed to the metal ion's direct effect on synthesis of ABA via its role as a co-factor for aldehyde oxidase. Direct interactions in the ABA-independent pathway are not so clear, but nevertheless the interplay between cold-tolerance, photosynthesis and Mo provides food for thought.