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). † These authors contributed equally to this work. SummaryProtecting crop yield under drought stress is a major challenge for modern agriculture. One biotechnological target for improving plant drought tolerance is the genetic manipulation of the stress response to the hormone abscisic acid (ABA). Previous genetic studies have implicated the involvement of the b-subunit of Arabidopsis farnesyltransferase (ERA1) in the regulation of ABA sensing and drought tolerance. Here we show that molecular manipulation of protein farnesylation in Arabidopsis, through downregulation of either the a-or bsubunit of farnesyltransferase enhances the plant's response to ABA and drought tolerance. To test the effectiveness of tailoring farnesylation in a crop plant, transgenic Brassica napus carrying an ERA1 antisense construct driven by a drought-inducible rd29A promoter was examined. In comparison with the nontransgenic control, transgenic canola showed enhanced ABA sensitivity, as well as significant reduction in stomatal conductance and water transpiration under drought stress conditions. The antisense downregulation of canola farnesyltransferase for drought tolerance is a conditional and reversible process, which depends on the amount of available water in the soil. Furthermore, transgenic plants were more resistant to water deficitinduced seed abortion during flowering. Results from three consecutive years of field trial studies suggest that with adequate water, transgenic canola plants produced the same amount of seed as the parental control. However, under moderate drought stress conditions at flowering, the seed yields of transgenic canola were significantly higher than the control. Using protein farnesyltransferase as an effective target, these results represent a successful demonstration of engineered drought tolerance and yield protection in a crop plant under laboratory and field conditions.

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Canola (Brassica napus L.) is one of the most important oilseed crops in the world and its seed yield and quality are significantly affected by drought stress. As an innate and adaptive response to water deficit, land plants avoid potential damage by rapid biosynthesis of the phytohormone abscisic acid (ABA), which triggers stomatal closure to reduce transpirational water loss. The ABA-mediated stomatal response is a dosage-dependent process; thus, one genetic engineering approach for achieving drought avoidance could be to sensitize the guard cell's responsiveness to this hormone. Recent genetic studies have pinpointed protein farnesyltransferase as a key negative regulator controlling ABA sensitivity in the guard cells. We have previously shown that down-regulation of the gene encoding Arabidopsis beta-subunit of farnesyltransferase (ERA1) enhances the plant's sensitivity to ABA and drought tolerance. Although the alpha-subunit of farnesyltransferase (AtFTA) is also implicated in ABA sensing, the effectiveness of using such a gene target for improving drought tolerance in a crop plant has not been validated. Here, we report the identification and characterization of the promoter of Arabidopsis hydroxypyruvate reductase (AtHPR1), which expresses specifically in the shoot and not in non-photosynthetic tissues such as root. The promoter region of AtHPR1 contains the core motif of the well characterized dehydration-responsive cis-acting element and we have confirmed that AtHPR1 expression is inducible by drought stress. Conditional and specific down-regulation of FTA in canola using the AtHPR1 promoter driving an RNAi construct resulted in yield protection against drought stress in the field. Using this molecular strategy, we have made significant progress in engineering drought tolerance in this important crop species.