Manipulation of in vivo sorbitol production alters boron uptake and transport in tobacco

Abstract

Recent evidence that some species can retranslocate boron as complexes with sugar alcohols in the phloem suggests a possible mechanism for enhancing boron efficiency. We investigated the relationship between sugar alcohol (sorbitol) content, boron uptake and distribution, and translocation of foliar-applied, isotopically enriched 10B in three lines of tobacco (Nicotiana tabacum) plants differing in sorbitol production. In tobacco line S11, transformed with sorbitol-6-phosphate dehydrogenase, the production of sorbitol was accompanied by an increase in the concentration of boron in plant tissues and an increased uptake of boron compared with either tobacco line A4, transformed with antisense orientation of sorbitol-6-phosphate dehydrogenase, or wild-type tobacco (line SR1, zero-sorbitol producer). Foliar application of 10B to mature leaves was translocated to the meristematic tissues only in line S11. These results demonstrate that the concentration of the boron-complexing sugar alcohol in the plant tissue has a significant effect on boron uptake and distribution in plants, whereas the translocation of the foliar-applied 10B from the mature leaves to the meristematic tissues verifies that boron is mobile in sorbitol-producing plants (S11) as we reported previously. This suggests that selection or transgenic generation of cultivars with an increased sugar alcohol content can result in increased boron uptake, with no apparent negative effects on short-term growth.

Figure 1

¹⁰B concentration in mature leaves, meristematic tissues, stems, and roots of tobacco plants. Transgenic S11, sense orientation (A–D); transgenic A4, antisense orientation (E–H); and wild-type SR1 (I–L) plants were grown in 0.1 (•), 1.0 (○), and 10 mg L⁻¹ (▾) ¹⁰B for a period of 3 weeks. Bars represent means ± se of four replicates.

Figure 2

Net uptake of ¹⁰B (after ¹⁰B treatment) per organ in S11 (A), A4 (B), SR1 (C), and I_M (D) of tobacco plants grown in 0.1, 1.0, and 10 mg L^{−1 10}B for a period of 3 weeks. Bars represent means of four replicates ± se.

Figure 3

Total dry weight in S11, A4, and SR1. Plants were grown in 0.1, 1.0, and 10 mg L^{−1 10}B for a period of 3 weeks. Bars represent means ± se of four replicates.

Figure 4

Correlation between sorbitol content and I_M of ¹⁰B (A) and ¹⁰B content (B) in S11. Plants were grown in 0.1 (•), 1.0 (▪), and 10 mg L⁻¹ (▴) ¹⁰B for 3 weeks. ^**, P < 0.01.

Figure 5

Changes in ¹⁰B concentration in mature leaves in untreated (A) and treated (B) plants, and meristematic tissues in untreated (C) and treated (D) plants over a period of 250 h. ¹⁰B was applied to the mature leaves of S11 (•), A4 (▪), and SR1 (▴) at a concentration of 700 mg L⁻¹.