Insights into the history of the legume‐betaproteobacterial symbiosis

The interaction between legumes and rhizobia has been well studied in the context of a mutualistic, nitrogen‐fixing symbiosis. The fitness of legumes, including important agricultural crops, is enhanced by the plants’ ability to develop symbiotic associations with certain soil bacteria that fix atmospheric nitrogen into a utilizable form, namely, ammonia, via a chemical reaction that only bacteria and archaea can perform. Of the bacteria, members of the alpha subclass of the protebacteria are the best‐known nitrogen‐fixing symbionts of legumes. Recently, members of the beta subclass of the proteobacteria that induce nitrogen‐fixing nodules on legume roots in a species‐specific manner have been identified. In this issue, Bontemps et al. reveal that not only are these newly identified rhizobia novel in shifting the paradigm of our understanding of legume symbiosis, but also, based on symbiotic gene phylogenies, have a history that is both ancient and stable. Expanding our understanding of novel plant growth promoting rhizobia will be a valuable resource for incorporating alternative strategies of nitrogen fixation for enhancing plant growth.     

Agrarian legacy in soil nutrient pools of urbanizing arid lands

Today's worldwide expansion of dry land cities consumes cultivated and native ecosystems, providing laboratories for investigating imprints of former land use in places where people now live. Around Phoenix, USA, we compared soil nutrient pools in residential yards converted from farms with nutrient pools in yards developed on native desert. Organic matter, carbon (C), nitrogen (N), and soluble ions were >2-fold greater in yards that were previously agrarian than in yards that were not. These pools remain elevated 40 years after land conversion to residential use. Present N accumulation (1.5 g m⁻² yr⁻¹) is not affected by prior land use, suggesting that rates of residential fertilizer application and retention are not affected by antecedent soil fertility. Bioavailable, inorganic phosphorus (P_av) is elevated in soil with a recent agrarian past, but this signal disappears after 10–30 years of residential use owing to an accumulation of P_av in never-farmed yards. Our results indicate a 'direct agrarian legacy,' wherein agrarian amendment of nutrient pools endures urbanization, more so than an 'indirect legacy,' wherein present land management is molded by former land use. Agriculture in dry lands thus sequesters material in soils, and—as we also found higher material contents in residential soils than in contemporary agrarian soils—residential land use simply adds to the agrarian legacy these soils already bear. Intense human use of arid lands may cause increases in material pools in soils, a condition with potential global consequence.     

Reflections on Organisms: Examples from the Neurobiology of Insect Rhythmic Movements

Consideration of organisms is necessary for neurobiologistsinterested in the neural bases of behavior, because a behavioralact performed by an intact animal defines the problem to besolved and also constitutes the context in which the nervoussystem operates. In addition, observations of behaving organismscan introduce new preparations for study of general questions.Examples are given from rhythmic gill movements of Corydaluscornutus, insect flight, and development of the moth flightmotor. In a concluding section, questions are raised about possiblephilosophical and judgmental influences on the current tensionsbetween holistic approaches expressed in organismic biologyand reductionist approaches expressed in biotechnology.     

Seasonal decline in plant defence is associated with relaxed offensive oviposition behaviour in the viburnum leaf beetle Pyrrhalta viburni

1. Plant defence of Viburnum shrubs against oviposition by its specialist herbivore, the viburnum leaf beetle [VLB Pyrrhalta viburni(Paykull)], involves an egg‐crushing wound response in twigs. Although the response is variable among Viburnum species, it can have a strong impact on egg survivorship. Beetles typically aggregate egg masses with conspecifics along infested twigs, forming clusters that can overwhelm the twig response. It was investigated whether twig responses and beetle oviposition behaviour vary seasonally. 2. In a field experiment, twig defences decreased towards the end of the VLB oviposition period: wound response of the North American Viburnum dentatum L. and the European V. opulus L. was reduced by 100% and 54%, respectively, in September compared with the July to August period. 3. Oviposition trials demonstrated a corresponding behavioural change: VLB females displayed aggregative oviposition in August, but not in September. 4. Further tests revealed that late‐season VLB females reverted to aggregative oviposition after being kept on uninfested twigs, whereas females kept on heavily infested twigs did not. This behavioural change suggests that relaxation of aggregative oviposition originates from cues associated with high densities of egg masses. 5. Relaxation of aggregative oviposition may be adaptive (and beneficial for invasion) on shrubs with low levels of defences by reducing intra‐specific competition.     

Evaluating Residual Tree Patches as Stepping Stones and Short-Term Refugia for Red-Legged Frogs

Abstract: Temperate pond-breeding amphibians are vulnerable to forest fragmentation because they must access upland terrestrial sites during the nonbreeding season but are prone to desiccation in hot, dry environments without canopy cover. Harvesting techniques that retain live trees in the cut block are advocated for sustaining forest biodiversity, but the effects of these practices on amphibians are unknown. We studied red-legged frogs (Rana aurora) in movement trials to assess: 1) how short-term use of residual trees was affected by tree patch size, streams, and neighborhood features; 2) whether residual tree patches were used as stepping stones in negotiating cut blocks; 3) the effects of patch size and patch proximity in altering movement paths; and 4) the effects of retention level and patch size on interpatch distance. Residual tree patches were potentially valuable short-term refugia but their value was size dependent. Virtually all frogs released at the base of single trees or inside small tree clusters left within 72 hours, but the proportion leaving decreased curvilinearly with increasing patch size. Frogs were less likely to leave tree patches with a running stream or where neighborhood stream density was high. Residual tree patches did not systematically alter movement paths. Frogs intercepted residual tree patches mostly at random and had to be within 5-20 m of a tree patch before moving to it in greater proportions than expected by chance. However, amphibian movements were biased toward large (0.8 ha) patches and away from small (0.3 ha) patches 50 m away. Our results indicated that residual trees should not be retained singly but should be aggregated in groups between 0.8 ha and 1.5 ha, preferably in stream locations.     

DISAPPEARANCE OF NITRATE REDUCTASE ACTIVITY FROM CHLAMYDOMONAS REINHARDI

Nitrate reductase activity was induced in Chlamydomonas reinhardi following addition of nitrate. Enzymic activity was assayed in cell-free supernatants and in whole cells whose permeability had been increased by freezing. Nitrate reductase activity in cells decreased rapidly when CO₂-fixation was prevented by (a) darkening cultures, (b) aerating cultures with CO₂-free air, or (c) addition of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). A smaller loss of nitrate reductase activity from darkened cells occurred when (a) acetate-adapted cells were supplied with acetate, or (b) cells were allowed to accumulate carbon reserves by nitrogen starvation before darkening. It was concluded that maintenance of nitrate reductase activity was dependent upon the availability of a suitable carbon source.