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We all know how hard it is to shift an economy from stagnation to growth, but in nature, a transition of this kind occurs on a routine basis – whenever plant seeds shift from a dormant state to that of germination.
This transition calls for sudden, far-ranging changes in all the plant’s systems: The plant’s entire genome undergoes reprogramming that affects the expression of genes and the manufacture of proteins. Hundreds of proteins – those that help the seed develop and block premature germination – stop being produced, whereas the production of hundreds of other proteins – those needed for germination – is set in motion.
“The germination revolution” plays a crucial role both in nature and in agriculture because it has an enormous impact on the harvest. Generally, the greater the proportion of germinating seeds and the faster their germination, the bigger the harvest. In fact, farmers use various methods to stimulate germination artificially – for example, by soaking dry seeds for a short while in a germination-promoting solution. Understanding germination on the molecular and metabolic levels is, therefore, vitally important: It can lead to sophisticated ways of improving the rate and speed of germination, thereby increasing crop harvest. Moreover, it can help control the precise timing: When the seeds germinate too early, while still attached to the parent plant and before they reach the soil, they are unlikely to develop into new plants.
Prof. Gad Galili of the Weizmann Institute’s Plant Sciences Department has conducted pioneering studies on the “germination revolution.” He has recently discovered that a molecular process called selective autophagy, which is central to the plant’s life, also takes part in controlling the germination.
In general, autophagy, which means “self-eating,” involves the dismantling of proteins, and it occurs in numerous organisms; originally discovered in yeast, it exists also in animals, including human beings. Autophagy has been the subject of extensive studies – among other reasons because in humans, defects in selective autophagy are implicated in Alzheimer’s, Parkinson’s and other common diseases. In the plant world, the main function of autophagy is to save the plant in times of trouble. Since plants, unlike animals, cannot run away from danger, their survival depends on flexible mechanisms that help them cope with mortal threats by other means. Large-scale autophagy, for instance, can prevent plant death when nutrients are in short supply.
Lately, however, it has become clear that selective autophagy, in which individual proteins or other cellular components are dismantled without connection to distress or danger, takes place in plants at certain times, including the preparation for germination. Galili has discovered about ten genes that might be involved in selective autophagy in plants. In studies published this year in The Plant Cell and other journals, he has focused on two of these genes, ATI1 and ATI2, which are unique to plants – that is, they do not exist in animals.
In experiments with the model plant Arabidopsis, Galili and his team have shown that enhancing the expression of ATI1 stimulates germination. When this gene’s activity increased, so did the proportion of germinating seeds; moreover, these seeds germinated faster. The scientists hypothesize that ATI1 and ATI2 promote germination by dismantling proteins that block this process. When the scientists blocked the activity of the two genes, ATI1 and ATI2, germination was reduced. In addition to clarifying the roles of these genes, the scientists also obtained further details about their functioning – for example, where exactly in the plant cell their activity takes place. Prof. Galili’s team included postdoctoral fellow Dr. Arik Honig and graduate student Tamar Avin-Wittenberg.
The study of selective autophagy in plants is still in its infancy, but unraveling it in greater detail should enable plant researchers to improve plant growth and crop yields by enhancing seed germination on demand.
Prof. Gad Galili’s research is supported by the Lerner Family Plant Science Research Endowment Fund; and the Adelis Foundation. Prof. Galili is the incumbent of the Bronfman Professorial Chair of Plant Science.
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