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387 Genetic interactions between ABA, ethylene and sugar signaling pathways Sonia Gazzarrini and Peter McCourt* The identification of genes through mutant screens is beginning to reveal the structure of a number of signaling pathways in plants. In the past year, genes that determine the plant’s response to the hormones ethylene and abscisic acid have also been shown to be involved in sugar sensing in early seedlings. These results suggest that hormone signaling and carbon homeostasis are tightly coupled but that the architecture of these interactions is complex. Part of this complexity may be because some genetic screens on exogenous compounds produce signaling linkages that are not necessarily pertinent under normal growth conditions. Because many of the genes identified in these screens are cloned, the relevance of these interactions can now be unraveled at the molecular level. Addresses Department of Botany, 25 Willcocks Street, University of Toronto, Toronto, Ontario M5S 3B2, Canada *e-mail: [email protected] Current Opinion in Plant Biology 2001, 4:387–391 1369-5266/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations ABA abscisic acid ABI ABA INSENSITIVE ctr constitutive triple response ein ethylene insensitive era enhanced response to ABA ETR1 ETHYLENE RECEPTOR1 Introduction Most of our current notions on how plant hormones transduce a signal into a cellular response come from studies of hormone response mutants identified in Arabidopsis thaliana [1]. In certain respects, the story has been plain and simple, single mutations that alter a hormonal response are identified and placed into a signaling pathway using a combination of phenotypic analysis, genetic epistasis and gene expression. In hormone research, however, the truth is seldom plain and never simple. Genetic inferences taken from Arabidopsis mutants have never been reconciled easily with physiological studies on hormone action. The application of a single hormone often affects many different plant processes and conversely different hormones can modulate the same developmental process, suggesting a complex interaction of hormone signaling in plants. Perhaps the differences between physiological and genetic experiments are beginning to be resolved by recent studies that report the identification of known hormone response genes through screens that were not designed to find them. Genetic analysis suggests that abscisic acid (ABA) and ethylene closely interact and both function to modulate the overall carbon status during early seedling growth and development [2•–7•]. The publication of recent reviews on interactions between metabolic sensing and hormonal signaling has allowed us to focus on the genetic approaches used and possibly explain why these previously identified genes have been uncovered in new screens [8,9]. This review discusses genetic interactions between ethylene and ABA signalling, and how recent findings in the field of sugar sensing relate to these two hormones. Signaling, all the world’s a mutant Ethylene screens, having a gas Ethylene gas is involved in a wide variety of plant processes, ranging from fruit ripening and leaf senescence to abiotic and biotic stress responses. Using a combination of ethylene insensitive (ein) and constitutive triple response (ctr) mutants it appears ethylene binds to the ETHYLENE RECEPTOR 1 (ETR1) family of two-component receptor kinases and this event stops the ETR1 receptors from activating CTR1, a downstream Raf-like serine/threonine kinase [10,11]. Lack of CTR1 activation releases positive regulators, such as EIN2 and the transcriptional activator EIN3, from the negative regulation of CTR1, thereby allowing an ethylene response [12]. One component of the ethylene response pathway that functions outside of ethylene action is encoded by the EIN2 gene. Although originally discovered through ethylene insensitivity screens, new ein2 alleles have also been identified in screens involving auxin transport inhibitors [13], cytokinin [14], ABA [2•,3•] and delayed senescence [15]. Moreover, molecular dissection of this gene has shown that EIN2 responses regulated by jasmonate are separable from the EIN2 responses regulated by ethylene, indicating that this molecule can function in at least two different signaling pathways [16]. The EIN2 gene encodes a novel protein, whose amino-terminal region shows weak homology to the mammalian family of NRAMP (natural resistance-associated macrophage protein) metal transporters. Interestingly, the cytoplasmic carboxyl terminus, which is absent in the NRAMP family but is necessary and sufficient to activate downstream ethylene components of the pathway, has some overall structural topology to the yeast glucose sensor Snf3 [16,17]. If EIN2 functions as a sensor in non-ethylene mediated processes, it is also possible that the ethylene response pathway itself functions in the absence of ethylene. Possibly, in the absence of the gas, the ETR1 pathway is positively activated to regulate cell growth. Inhibition of ETR1 by ethylene leads to inactivation of the pathway, which in turn inhibits cell expansion. In this scenario, it is easy to see how a variety of positive growth signals could integrate with ethylene through the ETR1 pathway to modulate cell growth. 388 Cell signalling and gene regulation Table 1 Sugar insensitive Arabidopsis mutants that are affected in hormone action. Mutant Gene affected Reference gin (glucose insensitive) gin1 gin5 gin6 ABA2 Unknown ABI4 [7•] [4•] [4•] sis (sugar insensitive) sis1 sis4 sis5 CTR1 ABA2 ABI4 [7•] [5•] [5•] sun (sucrose-uncoupled) sun6 ABI4 [6•] ABA screens, spreading your seed ABA plays a major role in late seed development and adaptation to environmental stresses. To date, mutations that decrease sensitivity of Arabidopsis seed to ABA have identified five ABA-insensitive genes (ABI1–ABI5), but only mutations in ABI1 and ABI2 affect both vegetative and seed ABA responsiveness (reviewed in [18,19]). The ABI1 and ABI2 genes encode homologous type 2C protein phosphatases, and reduction-of-function mutations in either of these genes suggest that dephosphorylation negatively regulates ABA signaling [20,21]. The ABI3, ABI4 and ABI5 genes encode transcription factors of the B3 domain, APETALA2 (AP2) domain and bZIP factor classes, respectively, but mutations at these loci only influence seed ABA responsiveness [22,23,24•]. Analysis of these three genes using a combination of overexpression and loss-of-function alleles suggests that these transcription factors work in combination to regulate seed ABA response and late embryogenesis [25]. mutations in components that interact with ABA rather than for genes that are directly involved in the ABA signaling pathway. Perhaps screens involving another aspect of ABA signaling, such as water stress or ABA-specific gene expression, may be more insightful. Genetic screens that are based on ABA-reporter genes have been developed [33], and screens involving aberrant gene expression under osmotic stress find mutants that show increased sensitivity of gene expression to ABA [34]. Interestingly, these mutants are altered only in a subset of ABA responses, suggesting that ABA signaling is complex. ABA–ethylene interactions, the chicken and the egg In the past year, two independent screens designed to identify mutants involved in perturbing ABA responsiveness identified previously characterized ethylene signaling mutants [2•,3•]. era3 mutants, which were originally identified as ABA hypersensitive, were found to be allelic to ein2. Furthermore, ctr1 and ein2 mutants were identified as enhancers and suppressors of abi1 mutants, respectively. Other ethylene insensitive mutants also showed increased ABA responsiveness leading to the conclusion that ethylene is a negative regulator of ABA signaling in Arabidopsis seeds [2•,3•]. Unexpectedly, the same ethylene insensitive mutants that showed increased seed ABA responsiveness exhibited reduced root ABA responsiveness [2•,3•]. More perplexing, it was found that increased ethylene synthesis conferred a similar ABA-insensitive root phenotype, although exogenous ABA did not induce ethylene synthesis. How a reduction of ethylene response or an increase in ethylene synthesis can cause the same ABA response phenotype in the root is unclear, but once again it raises the possibility that the ETR1 response pathway does regulate some ethylene-independent signal in Arabidopsis. Sugar screens, the sweet smell of success Mutants with enhanced response to ABA at the level of germination (i.e. era mutants) have identified a protein farnesyl transferase (ERA1) as an attenuator of both seed and vegetative ABA sensitivity [26,27]. However, the pleiotrophy of era1 and the large number of potential farnesylated proteins in the Arabidopsis genome have made it difficult to identify which farnesylated target(s) is involved in ABA signaling [28–31]. Although ABA response mutants exist, a simple ABA signaling pathway has not been defined. This may reflect a more complex signaling topology or perhaps indicates that not enough response genes have been molecularly identified. It is also possible that the assay for identifying ABA-responsive mutants is not focused enough on ABA action. Unlike the ‘triple response’, which appears to be ethylene specific, germination efficiency in the presence of exogenous ABA can be influenced by a myriad of factors; for example, both gibberellin and ethylene response mutants have altered seed ABA responsiveness [2•,3•,32]. Hence, screens involving germination may enrich more for Externally supplied sugar has different effects on various stages of early growth in Arabidopsis. Whereas low concentrations can stimulate wild-type germination, higher concentrations repress both cotyledon, early seedling development and photosynthetic gene expression. By taking advantage of the effects of high sugar concentrations on Arabidopsis seedling growth, a number of groups have identified mutants that have been classified as defective in sugar response (reviewed in [35–37]). Further analysis on many of these lines, however, showed they were defective in ABA biosynthesis, ABA sensitivity or ethylene response (Table 1; [4•–7•]). Numerous experiments have now suggested a close interaction between the germination and growth of young seedlings on exogenous sugar and ABA/ethylene action. Young seedlings deficient in ABA biosynthesis (i.e. aba1, aba2 and aba3) or ABA sensitivity (i.e. abi4 and abi5) are insensitive to high levels of sugar and, unlike wild-type plants, these mutants do not show sugar-dependent repression of photosynthetic gene expression. In addition, Genetic interactions between ABA, ethylene and sugar signaling pathways Gazzarrini and McCourt germination on high sugar increases ABA levels in wildtype plants [4•]. Conversely, low concentrations of exogenous sugar relieve the inhibitory effects of ABA on wild-type seed germination, although these seedlings fail to green or develop true leaves [38,39]. Thus, low sugar levels interfere with the inhibitory effects of ABA on germination, whereas inhibition of seedling development post-germination by high sugar concentrations is dependent on ABA synthesis. The inhibitory effect of high sugar levels on early seedling growth is confined to approximately two days post-germination; hence, there is a post-germination, sugar-sensitive developmental window that affects the seedlings response to ABA [7•]. Once a seedling is photosynthetically competent, sensitivity to ABA may decrease, resulting in a plantlet that is relatively insensitive to sugar-induced ABA synthesis after this point (Figure 1). Interestingly, ABI5 expression studies suggest that this transcription factor functions in a short post-germination developmental window [40•]. This time frame may define the same sensitive stage used to screen for sugar mutants. High glucose-induced repression of cotyledon and shoot development can also be overcome by the addition of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC), suggesting that ethylene can antagonize glucose inhibition of early seedling growth [41]. Moreover, ethylene overproducing (eto1) and ethylene constitutive signaling (ctr1) mutants are insensitive to high glucose levels, whereas ethylene insensitive mutants (etr1) are hypersensitive [7•,41]. These experiments suggest that increasing ethylene concentrations work to decrease the sensitivity of early seedlings to glucose. As glucose inhibition functions at least partially through ABA, ethylene may decrease ABA biosynthesis or sensitivity (Figure 1). Consistent with this idea, epistatic analysis between etr1 (sugar hypersensitive) and aba2 (sugar insensitive), indicates that ABA functions at or downstream of the ETR1 signaling pathway during early seedling development on high glucose [7•,41]. Mutations that reduce ethylene signaling do increase ABA concentrations in Arabidopsis [2•], thus it is possible that high glucose levels could signal through the ethylene pathway to regulate ABA biosynthesis. Alternatively, sugarinduced ABA synthesis or sensitivity may be antagonized by ethylene, as is seen in the earlier ABA–ethylene germination interaction [2•,3•]. Conclusions Genetic analysis indicates that hormone signaling pathways functionally intersect with each other and with possibly many other signaling pathways. However, questions still remain. The lack of sugar-related phenotypes for abi1, abi2 and abi3 suggests that reducing seed ABA sensitivity alone is not enough to confer a sugar-insensitive phenotype. This paradox has led to the placement of ABI4 and ABI5 in a separate ABA signaling pathway. However, this placement is confounded by the observation that abi5 mutants lose 389 Figure 1 High sugar Ethylene ABA State 1 Lipid breakdown State 2 Photosynthesis Sugar Current Opinion in Plant Biology Hypothetical model of the effects of sugar and hormones on early seedling growth in Arabidopsis. The carbon required for early seedling growth is dependent on the catabolism of lipids and proteins stored in the embryo (State 1). When seedlings are in State 1 they are highly sensitive to ABA. As the seedlings become photosynthetically competent (State 2), the sensitivity to ABA decreases. Exogenous application of non-physiological concentrations of sugar during various stages of seedling growth results in an increase in the endogenous levels of ABA. However, depending on the developmental state, ABA causes an inhibition of seedling growth in State 1 but has little effect during State 2. Sensitivity of early seedling growth towards high sugar concentrations is negatively regulated by ethylene, which, as shown by epistatic studies, acts at or upstream of the ABA signaling pathway. their glucose insensitivity in the presence of exogenous ABA [4•]. The recent finding that ABI5 is post-translationally modified in an ABA-dependent manner does imply that some downstream factors may require ABA to function, and may explain some of the ABA dependence of sugar sensing [40•]. However, signaling complexities also raise concerns about how well the genetic interactions observed through these screens reflect the in vivo situation. As with hormone response screens, the timing of application and the concentrations of sugar used in many experiments are artificial. Sugars can both stimulate and inhibit processes in a development-dependent manner; hence, caution must be used in the interpretation of experiments that involve continuous sugar application. Both ethylene and ABA are stress-induced hormones, so perhaps the connection of these compounds to high sugar concentrations exists only under developmentally irrelevant stress conditions. For example, the production of high ABA due to the exogenous application of glucose could encourage a germinating embryo that is younger than two days to re-enter late embryogenesis, a developmental 390 Cell signalling and gene regulation decision that ordinarily does not occur. If this is true, a signaling interaction is created between ABA and sugar that is not developmentally appropriate. Both the ABA and ethylene mutants identified in these sugar screens are also resistant to high osmolytes and new abi4 alleles have been identified in salt-stress screens [5•,7•,42]. Although the osmo-tolerant phenotype of sugar-insensitive mutants could be the result of insensitivity to high sugar, it is also possible that intracellular accumulation of sugar could protect them against osmotic stress. Thus, screens using high exogenous sugars might select for osmo-tolerant mutants. The correct anatomy of a signaling network is essential as its structure always affects its function. The diagrams defined by genetics not only tell us how information is transferred throughout the organism but also indicate the robustness and stability of the transmission. With the advent of complete sets of mutant knockout lines and global transcript profiling, we are entering a stage in which all the elements involved in these pathways will be known. It is, therefore, essential that the mutants identified in genetic screens give us a physiologically and developmentally relevant blueprint of wild-type signaling. abrogates the sugar-induced upregulation of ABI4. This study also demonstrates that sensitivity to high exogenous sugar in Arabidopsis requires ABA and that high sugar in the medium induces ABA synthesis. 5. • Laby RJ, Kincaid MS, Kim D, Gibson SI: The Arabidopsis sugarinsensitive mutants sis4 and sis5 are defective in abscisic acid synthesis and response. Plant J 2000, 23:587-596. The sugar-insensitive mutants sis4 and sis5, isolated by their ability to germinate and grow on 0.3 M sucrose, were found to be allelic to aba2 and abi4, respectively. However, other ABA-responsive mutants (abi1, abi2, abi3 and abi5) showed wild-type response to high sugar during germination, indicating that not all ABA-response mutants are sugar defective. 6. • Huijser C, Kortstee A, Pego J, Weisbeek P, Wisman E, Smeekens S: The Arabidopsis sucrose uncoupled-6 gene is identical to abscisic acid insensitive-4: involvement of abscisic acid in sugar responses. Plant J 2000, 23:577-585. Using a gene reporter screen rather than seedling growth, sucrose-uncoupled mutants (sun) were identified. The sun6 mutant, which showed reduced sugar-mediated repression of photosynthetic gene expression, was a new ABI4 allele. As in the work described in [5•], other ABA-responsive mutants (abi1, abi2, abi3 and abi5) did not display sugar-insensitive phenotypes. 7. • Gibson SI, Laby RJ, Kim D: The sugar-insensitive1 (sis1) mutant of Arabidopsis is allelic to ctr1. Biochem Biophys Res Commun 2001, 280:196-203. The isolation of mutants resistant to 0.3 M sucrose during germination identified sis1, a new allele of ctr1. Whereas constitutive ethylene response mutants are insensitive to the inhibitory effect of high sugar on early seedling growth, ethylene-insensitive mutants showed a sugar-hypersensitive phenotype. This paper linked ethylene and sugar responses. 8. Gibson SI: Plant sugar response pathways. Part of a complex regulatory web. Plant Physiol 2000, 124:1532-1539. 9. Coruzzi G, Zhou L: Carbon and nitrogen sensing and signaling in plants: emerging ‘matrix effect’. Curr Opin Plant Biol 2001, 4:247-253. Update Recently, Rook et al. [43•] have shown that although ABA is unable to induce expression of the ADP-glucose pyrophosphorylase Apl3 gene it enhances the induction of this gene by sugar. The authors suggest that sugar-induced ABA synthesis makes the seedling more sensitive to ABAdependent sugar signals, a hypothesis that is consistent with the model proposed in Figure 1. Acknowledgement 10. Hua J, Meyerowitz EM: Ethylene responses are negatively regulated by a receptor gene family in Arabidopsis thaliana. Cell 1998, 94:261-271. 11. Gamble R, Coonfield, Schaller GE: Histidine kinase activity of the ETR1 ethylene receptor from Arabidopsis. Proc Natl Acad Sci USA 1998, 95:7825-7829. 12. Kieber JJ, Rothenberg M, Roman G, Feldmann KA, Ecker J: CTR1, a negative regulator of the ethylene response pathway in Arabidopsis, encodes a member of the RAF family of protein kinases. Cell 1993, 72:427-441. This work was supported by a grant from the Natural Sciences and Engineering Research Council (NSERC). 13. Fujita H, Syono K: Genetic analysis of the effects of polar auxin inhibitors on root growth in Arabidopsis thaliana. Plant Cell Physiol 1996, 37:1094-1101. References and recommended reading 14. Su W, Howell SH: A single genetic locus, ckr1, defines Arabidopsis mutants in which root growth is resistant to low concentrations of cytokinin. Plant Physiol 1992, 99:1569-1574. Papers of particular interest, published within the annual period of review, have been highlighted as: • of special interest •• of outstanding interest 1. McCourt P: Genetic analysis of hormone signaling. Annu Rev Plant Physiol Plant Mol Biol 1999, 50:219-243. 2. • Ghassemian M, Nambara E, Cutler S, Kawaide H, Kamiya Y, McCourt P: Regulation of abscisic acid signaling by the ethylene response pathway in Arabidopsis. Plant Cell 2000, 12:1117-2000. This study and work by Beaudoin et al. [3•] demonstrate that many ethylene response mutants also have altered ABA responses, depending on the tissue that is assayed. Ethylene appears to be a negative regulator of ABA action in the seed but positively regulates some aspects of ABA action in the root. These studies suggest that correct signaling in a variety of ABA modulated pathways in plants requires a functional ETR1 response pathway. 15. Oh SA, Park JH, Lee GI, Paek KH, Park SK, Nam HG: Identification of three genetic loci controlling leaf senescence in Arabidopsis thaliana. Plant J 1997, 12:527-535. 16. Alonso JM, Hirayama T, Roman G, Nourizadeh S, Ecker J: EIN2, a bifunctional transducer of ethylene and stress responses in Arabidopsis. Science 1999, 284:2148-2152. 17. Özcan S, Dover J, Johnston M: Glucose sensing and signaling by two glucose receptors in the yeast Saccharomyces cerevisiae. EMBO J 1998, 17:2566-2573. 18. Leung J, Giraudat J: Abscisic acid signal transduction. Annu Rev Plant Physiol Plant Mol Biol 1998, 49:199-222. 19. Bonetta D, McCourt P: Genetic analysis of ABA signal transduction pathways. Trends Plant Sci 1998, 3:231-235. 3. • 20. Gosti F, Beaudoin N, Serizet C, Webb AAR, Vartanian N, Giraudat J: ABI1 protein phosphatase 2C is a negative regulator of abscisic acid signaling. Plant Cell 1999, 11:1897-1909. 4. • 21. Merlot S, Gosti F, Guerrier D, Vavasseur A, Giraudat J: The ABI1 and ABI2 protein phosphatase 2C act in a negative feedback regulatory loop of the abscisic acid signaling pathway. Plant J 2001, 25:295-303. Beaudoin N, Serizet C, Gosti F, Giraudat J: Interaction between abscisic acid and ethylene signaling cascades. Plant Cell 2000, 12:1103-1115. See annotation [2•]. 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Plant J 1999, 19:569-578. 24. Finkelstein RR, Lynch TJ: The Arabidopsis abscisic acid response • gene ABI5 encodes a basic leucine zipper transcription factor. Plant Cell 2000, 12:599-609. The molecular identification of ABI5, along with previous studies showing that ABI3 and ABI4 are also transcription factors [22,23], shows the importance of transcriptional regulation during embryogenesis in determining ABA responsiveness to the seed. Furthermore, the homology of ABI5 to a bZIP domain is intriguing as TRAB1, a maize bZIP transcription factor, has been shown to interact with maize VP1, the ABI3 ortholog. 35. Smeekens S, Rook F: Sugar sensing and sugar-mediated signal transduction in plants. Plant Physiol 1997, 115:7-13. 25. Söderman EM, Brocard IM, Lynch TJ, Finkelstein RR: Regulation and function of the Arabidopsis ABA-insensitive4 gene in seed and abscisic acid response signaling networks. Plant Physiol 2000, 124:1752-1765. 26. Cutler S, Ghassemian M, Bonetta D, Cooney S, McCourt P: A protein farnesyl transferase involved abscisic acid signal transduction in Arabidopsis. Science 1996, 273:1239-1241. 27. Pei ZM, Ghassemian M, Kwak CM, McCourt P, Schroeder JI: Role of farnesyltransferase in ABA regulation of guard cell anion channels and plant water loss. Science 1998, 282:287-290. 28. Bonetta D, Bayliss P, Sun S, Sage T, McCourt P: Farnesylation is involved in meristem organization in Arabidopsis. Planta 2000, 211:182-190. 29. Yalovsky S, Rodriguez-Concepcion M, Bracha K, Toledo-Ortiz G, Gruissen W: Prenylation of the floral transcription factor APETALA1 modulates its function. Plant Cell 2000, 12:1257-1266. 30. Ziegelhoffer EC, Medrano LJ, Meyerowitz EM: Cloning of the Arabidopsis WIGGUM gene identifies a role for farnesylation in meristem development. Proc Natl Acad Sci USA 2000, 97:7633-7638. 31. Nambara E, McCourt P: Protein farnesylation in plants: a greasy tale. Curr Opin Plant Biol 1999, 2:388-392. 32. Steber CM, Cooney SE, McCourt P: Isolation of the GA-response mutant sly1 as a suppressor of ABI1-1 in Arabidopsis thaliana. Genetics 1998, 149:509-521. 33. Foster R, Chua NH: An Arabidopsis mutant with deregulated ABA gene expression: implication for negative regulator function. Plant J 1999, 17:363-372. 36. Smeekens S: Sugar regulation of gene expression in plants. Curr Opin Plant Biol 1998, 1:230-234. 37. Sheen J, Zhou L, Jang JC: Sugar as signaling molecules. Curr Opin Plant Biol 1999, 2:410-418. 38. Garraciubo A, Legaria JP, Covarrubias AA: Abscisic acid inhibits germination of mature Arabidopsis seeds by limiting the availability of energy and nutrients. Planta 1997, 203:182-187. 39. Finkelstein RR, Linch TJ: Abscisic acid inhibition of radicle emergence but not seedling growth is suppressed by sugars. Plant Physiol 2000, 122:1179-1186. 40. Lopez-Molina L, Mongrand S, Chua NH: A postgermination • developmental arrest checkpoint is mediated by abscisic acid and requires the ABI5 transcription factor in Arabidopsis. Proc Natl Acad Sci USA 2001, 98:4782-4787. The authors of this paper demonstrate that ABI5 works in a short developmental window post-germination to regulate the ABA responsiveness of the seedling. Furthermore, ABI5 is post-translationally modified in an ABAdependent manner, demonstrating, for the first time, a molecular mechanism linking an ABI transcription factor with ABA signaling. 41. Zhou L, Jang JC, Jones TL, Sheen J: Glucose and ethylene signal transduction crosstalk revealed by an Arabidopsis glucose-insensitive mutant. Proc Natl Acad Sci USA 1998, 95:10294-10299. 42. Quesada V, Ponce MR, Micol J: Genetic analysis of salt-tolerant mutants in Arabidopsis thaliana. Genetics 2000, 154:421-436. 43. Rook F, Corke F, Card R, Munz G, Smith C, Bevan MW: Impaired • sucrose-induction mutants reveal the modulation of sugarinduced starch biosynthetic gene expression by abscisic acid signalling. Plant J 2001, 26:421-433. The isi3 and isi4 mutants, impaired in the sugar induction of the starch biosynthetic Apl3 gene, were found to be allelic to abi4 and aba2, respectively. Interestingly, ABA alone is unable to induce Apl3 expression, but strongly enhances its induction by sugar, suggesting that ABA influences how tissues respond to subsequent sugar signals.