Role of phloem-mobile sucrose and auxin in the development of the root system

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Plant growth relies on the activity of stem cells clustered at shoot tips, leaf margins, and root tips, and also stem cell clusters that are quiescent and await activation. The activity of these so-called meristems determines whether plants are tall with few branches, short and bushy, etc. Similarly, the architecture of the root system is determined by the activity of meristems at the tips of primary and lateral roots, and the initial activation of these meristems in root primordia. It is essential that the activity of the plant's many meristems is coordinated to produce a plant whose morphology is optimized for its habitat. Indeed, root and shoot system growth is known to be coordinated, but how this coordination occurs is poorly understood. Previous studies clearly demonstrated that artificially increasing sucrose within a physiological range in the leaves dramatically stimulates lateral root formation, suggesting that the sucrose normally produced by leaves through photosynthesis is regulating root system development. This implicates a long distance mechanism that communicates the sucrose status of the aerial tissues to the root system. Sucrose moves from shoot to root system via the phloem, where it is unloaded at root meristems based on the steep concentration gradient. Interestingly, the plant hormone auxin is also produced in the leaves and moved to the root system via the phloem, This project will investigate how photosynthate from aerial tissues drives phloem allocation to root meristems, delivering auxin and thereby regulating their activity. This project will use transgenic plants that alter the levels of sucrose and auxin translocated in the phloem, together with fluorescent dyes that allow direct visualization of phloem movement and delivery, to test this model. If successful, the results from this project will 1) demonstrate that phloem acts to integrate plant organ development at different locations; and 2) determine the molecular signal that is carried in the phloem. This would be one of very few examples in which coordinated development in the whole-plant is understood mechanistically at the physiological and molecular levels. The broader impacts of this project include training of students at all levels and scientific outreach to the community.