Transgenically enhanced sorbitol synthesis facilitates phloem boron transport and increases tolerance of tobacco to boron deficiency

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.

Figure 1

Seed yield of tobacco lines (S11, SR1, and A4) grown for 28 d with adequate B and then transferred to 0 μg mL⁻¹ B, 0.05 μg mL⁻¹ B supplied to the roots, or 100 μg mL⁻¹ B supplied to three mature leaves. Seed yield was determined 56 d after transfer to treatment solutions. Values represent means ± se of six replicates.

Figure 2

B deficiency symptoms in the control tobacco line SR1 (left) and transgenic tobacco (S11), supplied with B as a foliar application to three mature leaves. Symptoms exhibited in SR1 include flower bud abortion, deformation, reduced elongation, and chlorosis of young leaves. The control tobacco line A4 exhibited symptoms similar to those observed in SR1 (not shown). No symptoms of B deficiency were observed in growing tissues of transgenic tobacco line S11.