New ways for old genes

Abstract

We already know from classical physiological studies that plant signals interact with each other to generate downstream developmental responses. Arabidopsis genetics is now about to identify the molecular links between signaling pathways in plants. Recently, five groups have identified mutations in genes that had been previously implicated in other pathways, revealing interactions between abscisic acid (ABA) and ethylene signal transduction as well as between ABA and sugar signaling.Recently, Majid Ghassemian et al. 1xRegulation of abscisic acid signaling by the ethylene response pathway in Arabidopsis. Ghassemian, M. et al. Plant Cell. 2000; 12: 1117–1126PubMedSee all References1 and Nathalie Beaudoin et al. 2xInteractions between abscisic acid and ethylene signaling cascades. Beaudoin, N. et al. Plant Cell. 2000; 12: 1103–1115PubMedSee all References2 reported that they recovered alleles of ein2 in genetic screens for mutants with enhanced responses to ABA at seed germination. The EIN2 locus had previously been shown to determine sensitivity to another plant hormone, ethylene. In parallel, Francisco Arenas-Huertero et al. 3xAnalysis of Arabidopsis glucose insensitive mutants, gin5 and gin6, reveals a central role of the plant hormone ABA in the regulation of plant vegetative development by sugar. Arenas-Huertero, F. et al. Genes Dev. 2000; 14: 2085–2096PubMedSee all References3, Ron Laby et al. 4xThe Arabidopsis sugar-insensitive mutants sis4 and sis5 are defective in abscisic acid synthesis and response. Laby, R.J. et al. Plant J. 2000; 23: 587–596Crossref | PubMedSee all References4 and Casper Huijser et al. 5xThe Arabidopsis SUCROSE UNCOUPLED-6 gene is identical to ABSCISIC ACID INSENSITIVE-4: involvement of abscisic acid in sugar responses. Huijser, C. et al. Plant J. 2000; 23: 577–585Crossref | PubMedSee all References5 reported that they isolated alleles of abi4 in mutant screens for sugar insensitivity during seedling development. A mutation in ABI4 was originally shown to confer resistance to ABA at seed germination. Altogether, these findings show that ABA and ethylene, along with sugar and ABA transduction pathways, share common genetic determinants. However, the connections between these two pairs of signaling cascades are not restricted to one genetically defined step in the cascade. Mutations in other elements of the ethylene transduction pathway, such as ctr1, which confers constitutive response to ethylene, or etr1, which is a member of the ethylene receptor family, also modulate seed sensitivity to ABA during germination. In fact, Beaudoin et al. also recovered ctr1 in a screen for decreased sensitivity to ABA. This suggests that the ethylene transduction pathway impinges on the ABA signaling cascade to regulate seed germination.In the case of the interplay between sugar sensing and ABA signaling, ABI4 plays a prominent role whereas mutations in other ABI genes have little or no influence on the sugar sensitivity of seedling development. However, mutants impaired in ABA biosynthesis are also insensitive to sugar, suggesting that ABA production is required for sugar sensing. These interesting findings point to an extensive cross talk between ABA and another stress signal molecule, ethylene, and between ABA and the sugar nutrient-sensing pathways.However, these are probably just flashes of light on parts of the transduction networks that allow plants to integrate diverse signals such as environmental signals, nutrient status and developmental signals to produce adaptive responses. Further studies show that ethylene and ABA signaling pathways also interact to regulate root development but, in this organ, mutations that confer ethylene insensitivity also decrease ABA-mediated inhibition of root growth1,2. Hopefully, new tools such as DNA microarrays, which enable us to monitor changes in gene expression triggered by a signal or a mutation on a genome wide scale, should help us to uncover the molecular basis for signal cross talk in plants in more detail.