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Plants’ loss of shoot activates photosynthesis in roots

Posted April 21, 2017

Researchers at the University of Tokyo and their collaborators revealed a mechanism in plants that increases their roots’ photosynthetic capacity via the plant hormone cytokinin when the plants lose their shoots—the aboveground key organs for photosynthesis. The findings hold promise of shedding light on how plants enhance their photosynthetic capabilities under varying conditions.

Visualization of photosynthetic activity in Arabidopsis roots. These images of photosynthetic activity were produced using a chlorophyll fluorescence imager. Researchers grew Arabidopsis root samples for seven days, with those marked “intact” belonging to plants with their shoots unimpaired, while the ones marked “detached” are missing shoots. In normal (wild-type) Arabidopsis, photosynthetic activity increased in roots of plants that had their shoots removed, as shown by increased blue-to-purple regions in the image. Furthermore, roots of plants artificially induced to overexpress GNL (GNLox) showed increased photosynthetic activity, even with the shoots intact. Image credit: Koichi Kobayashi.

Plastids are organelles, or structural units within a cell that carry out specific functions, that are found only in plants; they serve various purposes in different cells, supporting the functions of a variety cells. Among plastids, chloroplasts are responsible for photosynthesis and thereby essential for plant growth. However, the mechanisms of how plants control chloroplast development to accommodate the functions of different tissues and cells had eluded scientists.

In the current study, the research group led by Assistant Professor Koichi Kobayashi, Professor Tatsuru Masuda, and Professor Hajime Wada at the Graduate School of Arts and Sciences, the University of Tokyo, demonstrated using Arabidopsis thaliana—a small flowering plant commonly used in plant experiments—that when plants lose their shoots (leaves and stems), they increase the photosynthetic capacity of their roots through enhanced sensitivity to cytokinin via a protein that reacts to wounding in plants. The group further revealed that another protein engaged in chloroplast development (transcription factor GNL) is deeply involved in this response. Plant roots generally rely on aboveground photosynthesis as their energy source; however, the current results suggest that when plants lose their aboveground organs, namely the shoots, they induce chloroplast development and enhance photosynthetic activity in the roots, as well as repair damaged tissues, by altering their hormonal balance—thereby increasing their likelihood for survival.

The current findings will contribute greatly to help elucidate the mechanisms by which plants maintain and expand photosynthetic productivity by flexibly adjusting to varying growth and environmental conditions.

“Most studies on plant photosynthesis until now were conducted using only the leaves. In this study, however, we focused on the greening phenomena of roots that normally do not photosynthesize, and gained new insights into the regulatory mechanisms of chloroplast development in plants,” says Kobayashi. He continues, “If you find a shoot regenerating from the root of a weed a few days after you pulled it out from the ground, the mechanism we unearthed in our current study, in which photosynthesis in the roots becomes enhanced, may be at work.”

The research outcome is the result of a collaboration with postdoctoral research fellow Akira Iwase and team leader Keiko Sugimoto at RIKEN Center for Sustainable Resource Science.

Source: University of Tokyo

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