Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK.
Department of Plant Biology, Carnegie Institution for Science, 260 Panama Street, Stanford, CA 94305, USA.
J Exp Bot. 2017 Jun 1;68(14):3903-3913. doi: 10.1093/jxb/erx197.
The pyrenoid of the unicellular green alga Chlamydomonas reinhardtii is a microcompartment situated in the centre of the cup-shaped chloroplast, containing up to 90% of cellular Rubisco. Traversed by a network of dense, knotted thylakoid tubules, the pyrenoid has been proposed to influence thylakoid biogenesis and ultrastructure. Mutants that are unable to assemble a pyrenoid matrix, due to expressing a vascular plant version of the Rubisco small subunit, exhibit severe growth and photosynthetic defects and have an ineffective carbon-concentrating mechanism (CCM). The present study set out to determine the cause of photosynthetic limitation in these pyrenoid-less lines. We tested whether electron transport and light use were compromised as a direct structural consequence of pyrenoid loss or as a metabolic effect downstream of lower CCM activity and resulting CO2 limitation. Thylakoid organization was unchanged in the mutants, including the retention of intrapyrenoid-type thylakoid tubules, and photosynthetic limitations associated with the absence of the pyrenoid were rescued by exposing cells to elevated CO2 levels. These results demonstrate that Rubisco aggregation in the pyrenoid functions as an essential element for CO2 delivery as part of the CCM, and does not play other roles in maintenance of photosynthetic membrane energetics.
单细胞绿藻衣藻的淀粉核是位于杯状叶绿体中心的一个微区室，其中包含多达 90%的细胞 Rubisco。淀粉核被密集的、打结的类囊体小管网络穿过，据推测其影响类囊体的生物发生和超微结构。由于表达了一种来自维管束植物的 Rubisco 小亚基，无法组装淀粉核基质的突变体表现出严重的生长和光合作用缺陷，并且其碳浓缩机制（CCM）无效。本研究旨在确定这些无淀粉核突变体光合作用限制的原因。我们测试了电子传递和光利用是否因淀粉核缺失的直接结构后果而受损，或者是否因较低的 CCM 活性和由此产生的 CO2 限制导致的代谢效应而受损。在突变体中，类囊体组织没有改变，包括保留类囊体内型类囊体小管，并且通过将细胞暴露在高 CO2 水平下，可挽救与淀粉核缺失相关的光合作用限制。这些结果表明，淀粉核中 Rubisco 的聚集作为 CCM 的一部分输送 CO2 的必需元素起作用，而在维持光合作用膜能量方面不起其他作用。
