土壤中的生物多样性与生态系统功能

我们回顾了目前关于土壤生物多样性及其在生态系统过程中的作用、对于人类目标的重要性及对胁迫和扰动的恢复能力等的研究进展。现存物种的数量远远大于人们已经描述的数量,即使是对于肉眼可见的分类群也是如此。而且也普遍缺乏在生物地理学上的综合描述。迫切需要在分类学和对新一代系统科学家培训上做更大努力。尤其是应当集中致力于就我们所知。对生态系统功能起关键作用的土壤生物类群方面。为了确定这种类群,需要对土壤生物区系的影响圈(sphere of Influence,SOI),如根的生物区系、有机物质破碎者(shredder)和土壤生物扰动者(bioturbator)加以识别,这些影响圈比如可能通过与植物的相互作用而控制着生态系统的过程。在这些SOI中,我们分析出了土壤有机体的功能类群。最迫切需要研究的问题是将物种按功能类群进行区分,并确定功能类群中的物种冗余。这种迫切性是根据为阐述与集约化农业、森林扰动、环境污染和全球环境变化有关的土壤功能丧失程度的需要而确定的。目前认为最处于危险的土壤生物区系是那些大型动物群的有机物破碎者、土壤生物扰动者、某些专门的细菌(如硝化细菌和固氮菌)以及形成真菌的菌根中贫乏物种的功能类群。在开展这些优先性研究时,需要使用长期的和大尺度的田间试验,以及地理统计学和地理信息系统等现代方法进行试验研究。

Diurnal Dependence of Growth Responses to Shade in Arabidopsis： Role of Hormone, Clock, and Light Signaling

We investigated the diurnal dependence of the hypocotyl-growth responses to shade under sunlight-night cycles in Arabidopsis thaliana. Afternoon shade events promoted hypocotyl growth, while morning shade was ineffective. The Ihy-D, elf3, lux, pif4 pifS, tocl, and quadruple della mutants retained the response to afternoon shade and the lack of response to morning shade while the Ihyccal mutant responded to both morning and afternoon shade. ThephyB mutant, plants overexpressing the multidrug resistance-like membrane protein ABCB19, and the iaa17/axr3 loss-of-function mutant failed to respond to shade. Transient exposure of sunlight-grown seedlings to synthetic auxin in the afternoon caused a stronger promotion of hypocotyl growth than morning treatments. The promotion of hypocotyl growth by afternoon shade or afternoon auxin required light perceived by phytochrome A or cryptochromes during the previous hours of the photoperiod. Although the ELF4-ELF3-LUX complex, PIF4, PIF5, and DELLA are key players in the generation of diurnal hypocotyl-growth patterns, they exert a minor role in the control of the diurnal pattern of growth responses to shade. We conclude that the strong diurnal dependency of hypocotyl-growth responses to shade relates to the balance between the antagonistic actions of LHY-CCA1 and a light-derived signal.

Abscisic Acid, High-Light, and Oxidative Stress Down-Regulate a Photosynthetic Gene via a Promoter Motif Not Involved in Phytochrome- Mediated Transcriptional Regulation

In etiolated seedlings, light perceived by phytochrome promotes the expression of light-harvesting chlorophyll a/b protein of photosystem Ⅱ （Lhcb） genes. However, excess of photosynthetically active radiation can reduce Lhcb expression. Here, we investigate the convergence and divergence of phytochrome, high-light stress and abscisic acid （ABA） signaling, which could connect these processes. Etiolated Arabidopsis thaliana seedlings bearing an Lhcb promoter fused to a reporter were exposed to continuous far-red light to activate phytochrome and not photosynthesis, and treated with ABA. We identified a cis-acting region of the promoter required for down-regulation by ABA. This region contains a CCAC sequence recently found to be necessary for ABI4-binding to an Lhcb promoter. However, we did not find a G-box-binding core motif often associated with the ABI4-binding site in genes promoted by light and repressed by ABI4. Mutations involving this motif also impaired the responses to reduced water potential, the response to high photosynthetic light and the response to methyl viologen but not the response to low temperature or to Norflurazon. We propose a model based on current and previous findings, in which hydrogen peroxide produced in the chloroplasts under high light conditions inter- acts with the ABA signaling network to regulate Lhcb expression. Since the mutation that affects high-light and methyl viologen responses does not affect phytochrome-mediated responses, the regulation by retrograde and phytochrome signaling can finally be separated at the target promoter level.