Soil science. Soil and human security in the 21st century.

Human security has and will continue to rely on Earth's diverse soil resources. Yet we have now exploited the planet's most productive soils. Soil erosion greatly exceeds rates of production in many agricultural regions. Nitrogen produced by fossil fuel and geological reservoirs of other fertilizers are headed toward possible scarcity, increased cost, and/or geopolitical conflict. Climate change is accelerating the microbial release of greenhouse gases from soil organic matter and will likely play a large role in our near-term climate future. In this Review, we highlight challenges facing Earth's soil resources in the coming century. The direct and indirect response of soils to past and future human activities will play a major role in human prosperity and survival.

Evolutionary patterns in auxin action.

This review represents the first effort ever to survey the entire literature on auxin (indole-3-acetic acid, IAA) action in all plants, with special emphasis on the green plant lineage, including charophytes (the green alga group closest to the land plants), bryophytes (the most basal land plants), pteridophytes (vascular non-seed plants), and seed plants. What emerges from this survey is the surprising perspective that the physiological mechanisms for regulating IAA levels and many IAA-mediated responses found in seed plants are also present in charophytes and bryophytes, at least in nascent forms. For example, the available evidence suggests that the apical regions of both charophytes and liverworts synthesize IAA via a tryptophan-independent pathway, with IAA levels being regulated via the balance between the rates of IAA biosynthesis and IAA degradation. The apical regions of all the other land plants utilize the same class of biosynthetic pathway, but they have the potential to utilize IAA conjugation and conjugate hydrolysis reactions to achieve more precise spatial and temporal control of IAA levels. The thallus tips of charophytes exhibit saturable IAA influx and efflux carriers, which are apparently not sensitive to polar IAA transport inhibitors. By contrast, two divisions of bryophyte gametophytes and moss sporophytes are reported to carry out polar IAA transport, but these groups exhibit differing sensitivities to those inhibitors. Although the IAA regulation of charophyte development has received almost no research attention, the bryophytes manifest a wide range of developmental responses, including tropisms, apical dominance, and rhizoid initiation, which are subject to IAA regulation that resembles the hormonal control over corresponding responses in seed plants. In pteridophytes, IAA regulates root initiation and vascular tissue differentiation in a manner also very similar to its effects on those processes in seed plants. Thus, it is concluded that the seed plants did not evolve de novo mechanisms for mediating IAA responses, but have rather modified pre-existing mechanisms already operating in the early land plants. Finally, this paper discusses the encouraging prospects for investigating the molecular evolution of auxin action.