Enhanced drought resistance in fructan-producing sugar beet Elizabeth A. H. Pilon-Smits a * § , Norman Terry a , Tobin Sears a , Kees van Dun b a Department of Plant and Microbial Biology, University of California, 111 Koshland Hall, Berkeley CA 94720, USA b D.J. Van der Have b.v., Van der Haveweg 2, 4410 AA Rilland, the Netherlands * Author to whom correspondence should be addressed (fax +1 970 491 0649; e-mail epsmits@lamar.colostate.edu) (Received June 15, 1998; accepted February 22, 1999) Abstract — Fructans are soluble polymers of fructose that are produced by approximately 15 % of the flowering plant species. Production of bacterial fructans in tobacco has been shown previously to lead to improved biomass production under polyethylene glycol-mediated drought stress. Here, we used the same SacB gene from Bacillus subtilis to produce bacterial fructans in sugar beet (Beta vulgaris L.). The transgenic sugar beets accumulated fructans to low levels (max. 0.5 % of dry weight) in both roots and shoots. Two independent transgenic lines of fructan-producing sugar beets showed significantly better growth under drought stress than untransformed beets. Drought stressed fructan-producing plants attained higher total dry weights (+25–35 %) than wildtype sugar beet, due to higher biomass production of leaves (+30–33 %), storage roots (+16–33 %) and fibrous roots (+37–60 %). Under well-watered conditions, no significant differences were observed between the transgenic and wildtype beets. In conclusion, the introduction of fructan biosynthesis in transgenic plants is a promising approach to improve crop productivity under drought stress. © Elsevier, Paris Beta vulgaris / drought stress / fructan / sugar beet 1. INTRODUCTION Drought is the single most limiting factor for agriculture in general [1], including sugar beet produc- tion [2]. One approach to improve drought stress resistance has been the introduction of fructan biosyn- thesis in transgenic plants [11]. Fructans are polymers of fructose, and function as the main storage carbohy- drate in approximately 15 % of the flowering plant species [5, 6, 10]. Besides being a storage carbohy- drate, fructans have had attributed to them an addi- tional role as a defense mechanism against drought stress. In addition to the solubility of fructan, this idea was based on the geographical distribution of fructan flora in drier areas, as well as the historical rise of prominent fructan-producing taxa concomitant with a climatological shift towards seasonal drought [5, 6]. In support of a possible role of fructans in drought resistance was the observation that the production of bacterial fructans in transgenic tobacco plants in- creased plant biomass production under polyethylene glycol-mediated drought stress [11]. The gene con- struct used for these studies consisted of the Bacillus subtilis SacB gene, with the CPY yeast vacuolar targeting sequence under the control of the constitutive 35S CaMV promoter [3]. Here, the same gene con- struct was used to introduce fructan biosynthesis in sugar beets. The resulting transgenic sugar beet plants were compared with their wildtype relatives with respect to growth performance under drought stress. 2. RESULTS 2.1. Production of transgenic fructan-accumulating sugar beet plants Fourteen independent transgenic lines were ob- tained when sugar beet cotelydons were transformed with the SacB gene from Bacillus subtilis. The trans- genic sugar beet plants showed fructan accumulation in both storage roots and in older leaves. Fructan accumulated with time in leaves of the transgenic plants: the oldest leaves contained fructan levels com- parable to those in the storage roots, while there was no detectable fructan in young leaves (figure 1). Simi- § Present address : Biology Department, Anatomy/Zoology Building, Colorado State University, Fort Collins, CO 80523, USA. Plant Physiol. Biochem., 1999, 37 (4), 313-317 Plant Physiol. Biochem., 0981-9428/99/4/© Elsevier, Paris