Rice Genetics IV - IRRI books - International Rice Research Institute

duction in response to dehydration stress (Shinozaki and Yamaguchi-Shinozaki 1997)and improving plant tolerance of stress by genetic transfer of transcription factor geneshave been reported (Jaglo-Ottosen et al 1998, Kasuga et al 1999). Reviews have beenpublished on genetic analysis of osmotic adjustment in crop plants (Zhang et al 1999)and on the need to use transgenic approaches to increase grain yield (Khush 1999,Reddy et al 1999). Cushman and Bohnert (2000) have summarized genomic approachesto plant stress tolerance.Transgenic approaches offer new opportunities to improve tolerance for dehydrationstress in rice by incorporating genes involved in stress tolerance. Genes that canbe introduced into rice plants can be broadly classified into four classes. Class I includesgenes that encode enzymes synthesizing osmoprotectants, such as proline andpolyamines. Class II genes include late embryogenesis abundant (LEA) or LEA-relatedgenes. Class III genes encode enzymes that may protect rice plants from oxidationstress, such as glutathione-S-transferase, and those that synthesize mannitol. ClassIV genes encode transcription factors that regulate the expression of the many stressresponsivetarget genes.The remaining part of this chapter is devoted to describing the progress that ourlaboratory has been making with the goal of increasing dehydration-stress tolerancein rice. We also briefly discuss future prospects in this area of research to increase riceyield by minimizing loss of crops because of salt, drought, and low-temperature stress.ResultsDevelopment of a dehydration-stress-inducible promoter systemA strong and constitutive promoter is suitable for high-level expression of a gene ofinterest that is linked downstream of the promoter. However, a strong constitutivepromoter is not always desirable for plant genetic engineering because the gene productmay be toxic to the plant. Overexpression of a transgene may also compete forenergy and building blocks that are required for protein and RNA synthesis, as well asplant growth and development under normal conditions. Thus, we developed a dehydration-stress-regulatedpromoter system to confer abscisic acid (ABA)- and/or stressinduciblegene expression. An ABA-response complex (ABRC1) from the barleyHVA22 gene was fused to a minimal rice Act1 gene promoter (Act1-100P). We thenconstructed two plasmids with one or four copies of ABRC1 combined with the sameAct1-100P and HVA22(I) of barley HVA22 to drive the expression of uidA in transgenicrice plants. Three DNA-blot-positive plant lines were regenerated that showed thecorrect hybridization pattern for each construct. RNA blot analysis indicated that uidAexpression is induced by ABA, water-deficit, and NaCl treatments. GUS activity assaysin the transgenic plants confirmed that the induction of GUS activity varied from3- to 8-fold with different treatments or in different rice tissues, and that transgenicrice plants harboring four copies of ABRC1 showed 50% to 200% higher absoluteGUS activity both before and after treatments than those with one copy of ABRC1(Su et al 1998).424 Cheng et al