C4 photosynthesis evolved dozens of times, with a critical step being the engagement of a C4 metabolic cycle to concentrate CO2 into a bundle sheath-like compartment. While C3-C4 intermediate species show a progressive increase in the activity of a C4 metabolic cycle, the integration of the C4 and C3 biochemical cycles in enhancing photosynthetic carbon gain occurs in a punctuated manner, at an initial C4 cycle activity near 60%. Punctuated integration of the C4 cycle could result from the evolutionary acquisition of traits that coordinate the C3 and C4 biochemical cycles (for example, an enzymatic, regulatory or transport function) or from a sudden reduction in the mesophyll C3 cycle. Alternatively, a punctuated pattern could be an artifact of low numbers of C3-C4 intermediates in the evolutionary space where C4 cycle engagement occurs, due to incomplete sampling of natural diversity or evolutionary dynamics rendering such intermediates unstable. Understanding how the C4 cycle becomes integrated with the C3 cycle could reveal new avenues for engineering the C4 pathway into C3 plants. Such efforts would be facilitated by the generation of hybrids, or the discovery of additional intermediates, that span the transition from low to high C4 cycle engagement.

中文翻译：

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注意差距：C4代谢循环的进化参与支持净碳同化。

C4光合作用发展了数十次，其中关键步骤是参与C4代谢循环以将​​CO2浓缩到束状鞘状隔室中。尽管C3-C4中间物种在C4代谢循环的活动中逐渐增加，但在初始C4循环活动接近60％时，C4和C3生化循环在增强光合碳增益方面的整合以点状方式发生。C4循环的标点整合可能源于协调C3和C4生化循环（例如，酶，调节或运输功能）的性状的进化性获得或叶肉C3循环的突然减少。或者，标点符号可能是发生C4循环参与的进化空间中少量C3-C4中间产物的假象，由于对自然多样性或进化动力学的不完全采样，使得此类中间体不稳定。了解C4循环如何与C3循环整合可能会揭示将C4途径工程化为C3植物的新途径。跨越低碳到高碳4循环参与的杂种的产生或其他中间体的发现将促进这种努力。