Regulation of Em Gene Expression in Rice

Abstract

Expression of the Em gene was characterized in rice (Oryza sativa L.) suspension cultures following exposure of the cultures to various combinations of abscisic acid (ABA) and salt. Response-saturating concentrations of either ABA (50 micromolar) or NaCI (0.4 molar) rapidly induced (by 60 minutes) the accumulation of Em mRNA, with a maximum accumulation occurring 12 to 24 hours after treatment. NaCI-induced Em expression was accompanied by a doubling of endogenous ABA levels as determined by immunoassay. Inhibition of ABA biosynthesis by fluridone during NaCI treatment reduced the levels of endogenous ABA by fourfold and Em expression by 50%. Desiccation of the cultures to 12 to 15% of their initial fresh weight increased endogenous ABA more than twofold and was accompanied by an increase in Em mRNA levels. Exposure of the cultures to heat shock temperatures, chilling, or ultraviolet light neither increased endogenous ABA levels nor induced Em expression. When a subthreshold or saturating level of NaCI was added in combination with increasing levels of ABA, Em transcripts were detected at ABA concentrations that alone did not induce expression of Em. Treatment with saturating levels of both NaCI and ABA resulted in a doubling of Em transcript levels over the maximum signal for each treatment alone. Hence, our data suggested that salt interacted synergistically with ABA, in part because of the increased sensitivity of rice cells to ABA. The effect of salt stress on Em gene expression in rice suspension cells appeared to operate through two pathways: one is mediated through increases in the level of ABA; the other is via a unique salt response pathway that includes an intermediate that is common to both the salt and ABA response chains.Physiological and molecular responses to ABA are controlled by the levels of ABA as well as by the sensitivity of tissues that are competent to respond (16,17,35). One factor that appears to alter the response of plant cells to ABA is V.The interaction between reduced v and ABA has been experimentally well illustrated in rapeseed germination (29) germination (26). Schopfer and Plachy (29) showed that reduced I imposed by osmotic stress and ABA can quantitatively substitute for each other to inhibit rapeseed germination. For example, 50% inhibition of rapeseed germination is achieved with either 22 lsM ABA, 10 gM ABA + 6 bars osmoticum, or 11 bars osmoticum. This two-factor interaction results in a shift in the concentration-response curve for ABA inhibition of germination toward lower ABA concentrations in the presence of an osmoticum, i.e. an osmotically induced change in the sensitivity of the tissue to ABA.The capacity of exogenous ABA or osmoticum to quantitatively substitute for each other suggests a common intermediate in the osmotic stress and ABA response pathways. It is unclear, however, whether osmotic stress operates solely through changes in ABA levels to effect the response. Evidence for osmoticum operating through ABA has been suggested by studies in which e...

Interaction of Osmotic Stress, Temperature, and Abscisic Acid in the Regulation of Gene Expression in Arabidopsis

“…Similar results were reported by Bostock and Quatrano (1992), who found that osmotic stress and ABA act synergistically in inducing maize Em gene expression. We isolated a group of Arabidopsis single-gene mutations that confer enhanced responses to both osmotic stress and ABA (Ishitani et al, 1997).…”

Convergent Induction of Osmotic Stress-Responses

“…Combinations of ABA, Cytokinin, and Salt NaCl, 6-BAP, and ABA were applied together to determine their combined effects on PEPCase induction and proline accumulation. When ABA was added together with either 6-BAP (Fig.…”

“…Thus, Em expression is not exclusively governed by alterations in endogenous ABA levels. In rice suspension cells, NaCl and exogenous ABA act synergistically, inducing the rice Em homolog in an additive fashion (6). It has been suggested that more than one mechanism (and molecule) may be responsible for Em induction and other 'ABA-responsive' genes in vivo during environmental challenges (34).…”

The Arabidopsis Group 1 LATE EMBRYOGENESIS ABUNDANT Protein ATEM6 Is Required for Normal Seed Development

“…This correlation suggests that these proteins are an integral part of one or more tolerance mechanisms. Many LEA genes can also be induced by the application of abscisic acid and by various kinds of water stress in both reproductive and vegetative tissues (Berge et al, 1989;Bostock and Quatrano, 1992;Wilhelm and Thomashow, 1993;Bies et al, 1998). In addition, there is at least one example of a constitutively expressed LEA gene (Welin et al, 1994).…”