引言

A photoreceptor molecule in plant cells has been found to have a second job as a thermometer after dark – allowing plants to read seasonal temperature changes. Scientists say the discovery could help breed crops that are more resilient to the temperatures expected to result from climate change

A部分

An international team of scientists led by the University of Cambridge has discovered that the ‘thermometer’ molecule in plants enables them to develop according to seasonal temperature changes. Researchers have revealed that molecules called phytochromes – used by plants to detect light during the day – actually change their function in darkness to become cellular temperature gauges that measure the heat of the night.

The new findings, published in the journal , show that phytochromes control genetic switches in response to temperature as well as light to dictate plant development.

B部分

At night, these molecules change states, and the pace at which they change is ‘directly proportional to temperature’, say scientists, who compare phytochromes to mercury in a thermometer. The warmer it is, the faster the molecular change – stimulating plant growth.

C部分

Farmers and gardeners have known for hundreds of years how responsive plants are to temperature: warm winters cause many trees and flowers to bud early,  something humans have long used to predict weather and harvest times for the coming year. The latest research pinpoints for the first time a molecular mechanism in plants that reacts to temperature – often triggering the buds of spring we long to see at the end of winter.

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With weather and temperatures set to become ever more unpredictable due to climate change, researchers say the discovery that this light-sensing molecule also functions as the internal thermometer in plant cells could help us breed tougher crops. ‘It is estimated that agricultural yields will need to double by 2050, but climate change is a major threat to achieving this. Key crops such as wheat and rice are sensitive to high temperatures. Thermal stress reduces crop yields by around 10% for every one degree increase in temperature,’ says lead researcher Dr Philip Wigge from Cambridge’s Sainsbury Laboratory. ‘Discovering the molecules that allow plants to sense temperature has the potential to accelerate the breeding of crops resilient to thermal stress and climate change.’

E部分

In their active state, phytochrome molecules bind themselves to DNA to restrict plant growth. During the day, sunlight activates the molecules, slowing down growth. If a plant finds itself in shade, phytochromes are quickly inactivated enabling it to grow faster to find sunlight again. This is how plants compete to escape each other’s shade. ‘Light-driven changes to phytochrome activity occur very fast, in less than a second,’ says Wigge.

At night, however, it’s a different story. Instead of a rapid deactivation following sundown, the molecules gradually change from their active to inactive state. This is called ‘dark reversion’. ‘Just as mercury rises in a thermometer, the rate at which phytochromes revert to their inactive state during the night is a direct measure of temperature,’ says Wigge.

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‘The lower the temperature, the slower the rate at which phytochromes revert to inactivity, so the molecules spend more time in their active, growth-suppressing state. This is why plants are slower to grow in winter. Warm temperatures accelerate dark reversion, so that phytochromes rapidly reach an inactive state and detach themselves from the plant’s DNA – allowing genes to be expressed and plant growth to resume.’ Wigge believes phytochrome thermo-sensing evolved at a later stage, and co-opted the biological network already used for light-based growth during the downtime of night.

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Some plants mainly use day length as an indicator of the season. Other species, such as daffodils, have considerable temperature sensitivity, and can flower months in advance during a warm winter. In fact, the discovery of the dual role of phytochromes provides the science behind a well-known rhyme long used to predict the coming season: oak before ash we’ll have a splash, ash before oak we’re in for a soak.

Wigge explains: ‘Oak trees rely much more on temperature, likely using phytochromes as thermometers to dictate development, whereas ash trees rely on measuring day length to determine their seasonal timing. A warmer spring, and consequently a higher likeliness of a hot summer, will result in oak leafing before ash. A cold spring will see the opposite. As the British know only too well, a colder summer is likely to be a rain-soaked one.’

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The new findings are the culmination of twelve years of research involving scientists from Germany, Argentina and the US, as well as the Cambridge team. The work was done in a model system, using a mustard plant called Arabidopsis, but Wigge says the phytochrome genes necessary for temperature sensing are found in crop plants as well. ‘Recent advances in plant genetics now mean that scientists are able to rapidly identify the genes controlling these processes in crop plants, and even alter their activity using precise molecular “scalpels” ‘, adds Wigge. “Cambridge is uniquely well-positioned to do this kind of research as we have outstanding collaborators nearby who work on more applied aspects of plant biology, and can help us transfer this new knowledge into the field.

A部分：植物细胞中的光感受器分子在黑暗中被发现有第二个作用，它能够作为温度计来读取季节性的温度变化。科学家表示，这一发现可以帮助培育出更能抵御气候变化导致的温度的作物。

B部分：由剑桥大学领导的国际科学家团队发现，植物中的“温度计”分子使其能够根据季节性的温度变化发展。研究人员揭示，被植物用来在白天检测光的光感受器分子，实际上在黑暗中改变其功能，成为测量夜晚热量的细胞温度计。

C部分：几百年来，农民和园丁们一直知道植物对温度的反应有多么敏感：温暖的冬天会导致许多树木和花朵提早发芽，这是人们长期以来用来预测来年天气和收获时间的方法。最新研究首次准确定位了植物中对温度反应的分子机制，这通常会触发我们在冬末期盼看到的春天的花蕾。

D部分：随着气候变化导致的天气和温度变得越来越不可预测，研究人员表示，这种光感受分子也能作为植物细胞内部温度计的发现，可能有助于培育出更能抵御热压和气候变化的作物。剑桥大学森伯里实验室的首席研究员菲利普·威格博士表示：“据估计，到2050年农业产量需要翻一番，但气候变化是实现一目标重大威胁。小麦和稻米等主要作物对高温敏感。温度应激每升高一度，作物产量就会下降约10%。”发现植物感知温度的分子有助于加快培育出能够抵御热压和气候变化的作物。

E部分：在其活跃状态下，光感受器分子与DNA结合以限制植物的生长。白天，阳光激活这些分子，减缓生长速度。如果一棵植物处于阴影中，光感受器分子会迅速失活，使其能够更快地生长以寻找阳光。这就是植物竞争逃离彼此阴影的方式。威格表示：“光驱动的光感受器活动的变化非常快，不到一秒钟。”然而，在夜晚情况就不同了。光感受器分子不再迅速失活，而是逐渐从活跃状态转变为非活跃状态。这被称为“暗反转”。威格说：“就像水银在温度计中上升一样，光感受器分子在夜间恢复到非活跃状态的速率是温度的直接衡量。”。

F部分：威格解释道：“温度越低，光感受器分子恢复到非活跃状态的速率越慢，因此分子在其活跃的、抑制生长的状态下停留的时间更长。这就是为什么植物在冬天生长较慢的原因。温度升高会加速暗反转，使光感受器分子迅速达到非活跃状态并从植物的DNA上解离出来，从而使基因表达和植物生长恢复。”威格认为，光感受器分子的温度感知是在后期进化的，并利用了夜晚休息期间已经用于光驱动生长的生物网络。

G部分：有些植物主要使用白天长度作为季节的指示器。其他一些物种，如水仙花，对温度非常敏感，在温暖的冬天可能会提前几个月开花。事实上，发现光感受器分子的双重作用为一个长期以来用于预测季节的众所周知的韵文提供了科学依据：“橡树胜过白蜡树，我们将会有雨水；白蜡树胜过橡树，我们将会有大雨。”。

威格解释道：“橡树更加依赖于温度，很可能利用光感受器分子作为温度计来决定发育，而白蜡树则依赖于测量白天长度来确定其季节性。一个更温暖的春季，因此更可能有一个炎热的夏季，将导致橡树在白蜡树之前长出叶子。一个寒冷的春季将出现相反的情况。正如英国人非常了解的那样，一个更冷的夏天很可能是一个多雨的夏天。”

H部分：这项新发现是由德国、阿根廷和美国的科学家剑桥团队共同进行了十二年的研究的结晶。这项工作是在一个名为拟南芥的模型系统中进行的，但威格表示，用于感知温度的光感受器基因也存在于作物植物中。“植物遗传学的最新进展使科学家们能够迅速识别控制这些过程的基因，并使用精确的分子“手术刀”来改变它们的活性，”威格补充道。“剑桥在这方面具有独特的研究条件，因为我们附近有一些研究更应用方面植物生物学的杰出合作伙伴，他们可以帮助我们将这些新的知识转化到实际应用中。”