Poster: Mineral nutrition

Abs # 202: Physiological effect of magnesium deficiency on sugar partitioning in Beta vulgaris

Presenter:	Hermans, Christian R M, chermans@lamar.colostate.edu
Authors	Hermans, Christian R M (A) (B)  Bush, Daniel R (A)   Strasser, Reto R (C)   Delrot, Serge  (D)   Verbruggen, Nathalie  (A)
Affiliations:	(A): Colorado State University (B): Université Libre de Bruxelles (C): Université de Genève (D): Université de Poitiers
Web Site:	http://www.ulb.ac.be/sciences/lpgmp

Accumulation of sucrose in source leaves is believed to be a major growth constraint of Mg deficient plants. Sucrose phloem loading may be the most sensitive step because it is mediated by a H⁺/suc symporter. Low Mg level could interfere with H⁺-pumping or, alternatively, it may have a direct impact on symporter activity. Hydroponic culture of sugar beets permitted to induce Mg deficiency. The shoot biomass, but not the root biomass, was lower at d 20 after the withdrawal of Mg. A clear inverse relationship between Mg and sugars contents in leaves was found. Mg deficiency promoted a marked increase in sucrose and starch content (up to a factor 5) in the most recently developed leaves, which had the lowest Mg content. ¹⁴[C]-sucrose labelling studies showed that sucrose export from these leaves was inhibited. Meanwhile, sucrose export from the oldest leaves, which were close to the roots did not seem to be restricted. In response to sucrose accumulation, possibly in the apoplasm of the mesophyll, BvSUT1 gene (coding for a H⁺/suc symporter) was induced. However increased expression was not matched with enhanced sucrose phloem loading. These results are in contrast to the data by Chiou & Bush (1998), which state lower BvSUT1 transcripts levels in leaves fed with sucrose via the transpiration stream. However, Ransom-Hodgkins et al. (2003) hypothesized that high sucrose accumulation in the phloem resulted in down regulation of BvSUT1 expression, with concomitant decrease in message abundance. Here, sucrose appears to accumulate outside the CCSE complex, as reflected in decreased sucrose export. Therefore, the site of sucrose accumulation in leaves must be determined to understand better the impact of low Mg on phloem loading.