光合作用除了将 CO 2 和水转化成糖和氧气以及能量以外，还生产大量的天然化学产物（ natural chemical compounds ）。但在植物进化过程中，能量生产与活性物质的合成划分成两个场所：叶绿体和内质网。前者光合作用合成 ATP 和 NADPH 以及碳水化合物。而在内质网上的 P450 单加氧酶参与生物活性物质的合成，能量来自于 NADPH 电子传递。在内质网中 NADPH 的浓度限制了次生代谢物的合成速度。 Poul Erik Jensen 等人打破进化的限制，将整条次生代谢产物合成途径转到叶绿体中，直接利用NADPH，提高了次生代谢物质的产率。该研究通过利用叶绿体中的NADPH， 直接 驱动P450快速合成天然化学物质 。 图片来源于原文 原文链接 : Redirecting Photosynthetic Reducing Power toward Bioactive Natural Product Synthesis, DOI: 10.1021/sb300128r. http://pubs.acs.org/doi/abs/10.1021/sb300128r . Abstract In addition to the products of photosynthesis, the chloroplast provides the energy and carbon building blocks required for synthesis of a wealth of bioactive natural products of which many have potential uses as pharmaceuticals. In the course of plant evolution, energy generation and biosynthetic capacities have been compartmentalized. Chloroplast photosynthesis provides ATP and NADPH as well as carbon sources for primary metabolism. Cytochrome P450 monooxygenases (P450s) in the endoplasmic reticulum (ER) synthesize a wide spectrum of bioactive natural products, powered by single electron transfers from NADPH. P450s are present in low amounts, and the reactions proceed relatively slowly due to limiting concentrations of NADPH. Here we demonstrate that it is possible to break the evolutionary compartmentalization of energy generation and P450-catalyzed biosynthesis, by relocating an entire P450-dependent pathway to the chloroplast and driving the pathway by direct use of the reducing power generated by photosystem I in a light-dependent manner. The study demonstrates the potential of transferring pathways for structurally complex high-value natural products to the chloroplast and directly tapping into the reducing power generated by photosynthesis to drive the P450s using water as the primary electron donor.
枝条皮层由于含有叶绿体，所以也能够进行光合作用。树枝或树干的光合作用在植物发育和进化 中 肯定发挥了重要作用，如同化产物减轻叶片的负担，有研究认为树干的这种光合作用对林分的木材产量发挥了显著效应。叶片进行光合作用也就算了，但我们的植物体真是“无所不用其极”，绝不浪费哪怕半次机会，于是全副武装发展生产，枝条也偷偷进行光合作用。事实上，植物的果实、苞叶、种子，甚至根都能进行光合作用，有些已经在我的博文中介绍过。 树干进行呼吸作用，而我们的枝条上的叶绿体通过光合作用重新同化还没有排除体外的二氧化碳，同时释放氧气，这可是一项伟大创举哦，这样植物就可以避免过多的能量浪费了。而释放的氧气也能提高植物的有氧呼吸，最大限度的降低无氧呼吸造成的伤害。但，枝条上的叶绿体有没有其它功能呢？最近， Schmitz 等人在对红树树种枝条光合作用的研究中发现枝条上的叶绿体能提高植物的输水能力，修复木质部栓塞，从而在植物的碳吸收和水散失的平衡中发挥作用，他们的研究论文“ Light-dependent maintenance of hydraulic function in mangrove branches: do xylary chloroplasts play a role in embolism repair? ”已经发表在国际知名期刊 New Phytologist 上。
2011-12-22更新：Nature也刊登了一篇讣告， Lynn Margulis (1938–2011) 。人都已经去世了，此文也就没有刻薄地全面介绍她的一生，而是只介绍了一些正面的。 ----------------------------------- 今天早晨，看到一则新闻，内共生假说的作者Lynn Margulis于上周去世了。 回顾Margulis的一生，应了中国那句古话，成也萧何、败也萧何。上世纪60年代，大家都在思考生物怎样通过突变+自然选择逐渐进化。Margulis另辟蹊径，提出真核生物的三种细胞器（线粒体、叶绿体/质体、基粒 basal bodies ）是曾经自由生活的细菌，进入厌氧细胞后与宿主共生而成的。据Margulis本人回忆，她的论文被退稿十五次，最后终于被 Journal of Theoretical Biology 接受（1）。这个期刊曾发表过多篇最初被看作异端邪说、但后来成为经典的论文，又如Hamilton的 Kin selection 理论。经过Margulis多年的坚持和努力，当然更离不开其他分子和细胞生物学家提供的大量证据，科学界慢慢接受了内共生假说，线粒体和叶绿体就是内共生来的。 线粒体和叶绿体作为真核生物能量代谢的主要场所，它们的起源毫无疑问是真核生物进化的重要事件。Margulis也因为这一假说闻名生物学界。最近，有些生物学家重新思考真核生物的起源，认为真核生物可能不是原先想象的，先有了细胞核再慢慢共生、进化出其他细胞器。他们认为，可能是线粒体的内共生诱发了包括细胞核的起源在内的一系列真核生物的进化事件（2-4）。如果是这样，内共生假说不仅是线粒体和叶绿体的进化理论，而是包括人类在内的所有真核生物起源的关键理论。我曾在前面的博客中评价过Margulis，我认为她对科学界的贡献远大于很多诺贝尔奖的成就。 败也萧何。思路独特是正面评价，从另一面看，就是思想偏激。内共生假说从异类到经典是成功了。但Margulis的其他异类说法和做法却为她带了很多非议， 也是我认为她很难得诺贝尔奖的原因 。这些非议和做法见维基百科中对Margulis的介绍中 Controversies 一栏。 看来呀，这个“度”还是需要把握的。 1. Lynn S: On the origin of mitosing cells. J Theor Biol 1967, 14: 225-IN226. 2. Martin W, Koonin EV: Introns and the origin of nucleus-cytosol compartmentalization. Nature 2006, 440: 41-45. 3. Lane N, Martin W: The energetics of genome complexity. Nature 2010, 467: 929-934. 4. Koonin EV: The origin of introns and their role in eukaryogenesis: A compromise solution to the introns-early versus introns-late debate? Biol Direct 2006, 1: 22. 注：因美国人习惯姓氏随夫，Lynn Margulis的早期论文署名Lynn Sagan。 本博文还参考了The scientist上的新闻报道： Evolutionary Pioneer Dies at 73 ，转载如下文： Evolutionary Pioneer Dies at 73 Lynn Margulis, an innovative thinker who proposed symbiosis as a major mechanism for speciation, passed away last week. By Edyta Zielinska | November 28, 2011 Lynn Margulis Wikimedia commons Evolutionary biologist Lynn Margulis died last week (November 22) at the age of 73. She was best known for proposing the theory of endosymbiosis, which states that rather than evolving via genetic mutation, new species were more likely to have come about via parasitic or symbiotic relationships that became permanently inter-dependent over time. “She was always stimulating; she always had a new idea, some new connection she had seen and she couldn’t wait to tell you about,” Steve Goodwin, Dean of the College of Natural Resources and the Environment told MassLive.com . Margulis showed early aptitude in science, enrolling at the University of Chicago and earning her bachelor’s degree in zoology by the age of 18. Shortly thereafter she married her first husband, the astronomer Carl Sagan. The marriage ended by the time she got her doctorate in genetics from the University of California, Berkeley, in 1965. She developed her ideas on symbiosis in the late 1960s, and tried to publish her ideas in 15 journals before finally being accepted by the Journal of Theoretical Biology , according to The New York Times . Though it was highly controversial at the time, serial symbiosis is widely accepted among evolutionary scientists today. In the 1970s, she became a supporter of James Lovelock’s Gaia hypothesis, which proposed that the earth could be thought of as a complex system whose atmospheric and mineral components existed in symbiosis with living organisms, allowing biota as a whole to self-perpetuate. She taught evolutionary biology for nearly 40 years, first at the Boston University and then at the University of Massachusetts, where I had the opportunity to experience her carefully crafted course. I came to the class expecting Margulis to expound on the theories that she had championed. Instead, she exposed our small seminar class to the experiments of many researchers whose work provided evidence for her ideas, and invited us to make own conclusions. “If science doesn’t fit in with the cultural milieu, people dismiss science—they never reject their cultural milieu!” said Margulis in the book The Third Culture: Beyond the Scientific Revolution . In the same chapter, Richard Dawkins wrote: “ I greatly admire Lynn Margulis’s sheer courage and stamina in sticking by the endosymbiosis theory, and carrying it through from being an unorthodoxy to an orthodoxy.” According to The New York Times , Margulis died from a stroke . She is survived by a daughter Jennifer Margulis and three sons Dorion Sagan, Jeremy Sagan, Zachary Margulis-Ohnuma.
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