Why seedlings always grow toward the light

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Scientists from the Carnegie Institution at Stanford University and Wageningen University, part of Wageningen UR, have discovered how cells in the stems of seedlings use blue light to grow toward the light. During earlier research it was discovered that the cells in the seedling stems responded to blue light used in the microscope. The scientists have now discovered how the blue light causes small structures in the cell, the microtubules, to grow perpendicular to their normal orientation. As a result, the growth direction of seedlings also changes, making them head towards the light. The scientists discovered which protein is responsible for making the microtubules grow perpendicular to the normal direction. The results were published in Science.

Most people know that seedlings grow toward the light. But how do they do that? It is a question that many scientists have wrestled with over the years. In earlier studies, scientists were looking into the cells of seedlings. Using special microscopes with blue laser light, they visualized the structures that are important for the growth direction of plant cells. Something surprising happened: within 10 minutes, the arrangement of the microtubules changed completely under the microscope.

Microtubules and fototropine

Through smart use of image analysis techniques, the scientists were able to show that - within a few minutes - blue light results in generation of large numbers of new microtubules which are all perpendicular to the pre-existing microtubules. They also discovered how the cells in seedlings detect the light in these conditions, namely through phototropin, one of the proteins that allow plants to detect light.

New microtubules are perpendicular to the pre-existing

In addition, it was determined that the generation of large numbers of microtubules that are perpendicular to pre-existing microtubules is caused by the protein katanin. Seedlings that do not contain the katanin protein cannot make their microtubules change direction and are unable to grow toward the light. Katanin is activated by phototropin and can sever microtubules in half where they intersect other microtubules. Severing results in two microtubules each of which grow in the same orientation. Microtubules that are perpendicular to the pre-existing microtubules make many intersections. Every time new microtubules intersect with other microtubules positioned in the original direction, katanin can sever the new microtubules to create new branches. Within a few minutes, this results in ‘trees’ of microtubules within the cells that are perpendicular to the original direction.

Growth and development depend strongly on quality and quantity

The growth and development of plants depends strongly on the quality and quantity of light they detect. Plants depend on light for photosynthesis but are unable to move themselves. Instead, they grow towards the light. This study shows that the reorganization of microtubules plays an essential role in this process.

The research was supported by an NSF award, the Carnegie Institution for Science, the Foundation for Fundamental Research on Matter (FOM), funded by the Netherlands Organisation for Scientific Research (NWO), the EU-NEST programme, a CASPIC award, the TOYOBO BIOFOUNDATION and the Human Frontier Science Programme.

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