Transgenically Enhanced Sorbitol Synthesis Facilitates Phloem Boron Transport and Increases Tolerance of Tobacco to Boron Deficiency1

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Abstract
The mobility of elements within plants contributes to a plant species' tolerance of nutrient deficiencies in the soil. The genetic manipulation of within-plant nutrient movement may therefore provide a means to enhance plant growth under conditions of variable soil nutrient availability. In these experiments tobacco (Nicotiana tabacum) was engineered to synthesize sorbitol, and the resultant effect on phloem mobility of boron (B) was determined. In contrast to wild-type tobacco, transgenic tobacco plants containing sorbitol exhibit a marked increase in within-plant B mobility and a resultant increase in plant growth and yield when grown with limited or interrupted soil B supply. Growth of transgenic tobacco could be maintained by reutilization of B present in mature tissues or from B supplied as a foliar application to mature leaves. In contrast, B present in mature leaves of control tobacco lines could not be used to provide the B requirements for new plant growth. 10B-labeling experiments verified that B is phloem mobile in transgenic tobacco but is immobile in control lines. These results demonstrate that the transgenic enhancement of within-plant nutrient mobility is a viable approach to improve plant tolerance of nutrient stress.