Information concerning the sugar status of plant cells is of great importance during a11 stages of the plant life cycle. The availability of or lack of sugars triggers many metabolic and developmental responses, and it is not surprising, therefore, that sugars profoundly affect the expression of a large number of genes (for review, see Koch, 1996;Graham, 1996). Sugar sensing occurs at the level of individual cells and the responses of such cells must be integrated at the tissue, organ, and plant level. Therefore, sugar-induced signals will interact with other sensing and signaling pathways. The mechanisms used by plant cells to sense sugars and to process this information are essentially unknown, and only recently are these questions being addressed experimentally. This lack of knowledge contrasts with the situation in yeast and bacteria, in which the molecular and physiological analysis of mutants have yielded extensive information about sugar perception (Trumbly, 1992;Ronne, 1995;Saier et al., 1995). SUGAR SENSING IN YEAST A N D ANIMALSYeast (Sacckauomyces ceuevisiae) serves as a model for investigating many basic biological questions about eukaryotes and is also an important paradigm for sugar sensing in plants. In yeast the availability of the preferred sugar substrate Glc signals the Glc repression phenomenon (for review, see Trumbly, 1992;Ronne, 1995;Thevelein and Hohmann, 1995). Glc repression dramatically alters yeast intermediary carbohydrate metabolism such that only Glc is being used as a carbon source, despite the presence of other readily accessible carbon sources. Glc is converted into Glu-6-P by HXK and is further metabolized via glycolysis. Genes involved in the metabolism of other carbon substrates are switched off, as are genes encoding key steps in gluconeogenic metabolism. A number of yeast mutants that are impaired in aspects of the Glc repression phenomenon have been isolated and their analysis has provided insight into the complexity of sugar sensing and signaling pathways. From these studies it was concluded that the Glc-phosphorylating enzyme HXK2 is a major Glc sensor responsible for sus- tained Glc repression. HXK2 activity initiates a signal transduction pathway that involves a number of different gene products (Fig. 1) and results in the repression of a large set of genes. Thus, the entry of Glc into glycolytic metabolism as mediated by HXK2 is a key step in Glc sensing.In the repression pathway the function of two protein complexes has been elucidated. These are the GLC7 type 1 protein phosphatase complex (Tu and Carlson, 1995, and refs. therein) and the SSN6/TUP1 complex, which functions as a general repressor of transcription through modulation of chromatin structure. Binding of the SSN6/TUP1 complex to specific sites is directed by the DNA-binding protein MIG1, and in this way genes that contain MIG1-binding sites are repressed. Exactly how the HXK2, GLC7, and SSNG/TUPl /MIG1 complexes are connected is unknown. For example, in the repression pathway no substrates for the REG1 ...

SummaryPlants both produce and utilize carbohydrates and have developed mechanisms to regulate their sugar status and co-ordinate carbohydrate partitioning. High sugar levels result in a feedback inhibition of photosynthesis and an induction of storage processes. We used a genetic approach to isolate components of the signalling pathway regulating the induction of starch biosynthesis. The regulatory sequences of the sugar inducible ADP-glucose pyrophosphorylase subunit ApL3 were fused to a negative selection marker. Of the four impaired sucrose induction (isi) mutants described here, two (isi1 and isi2) were speci®c to this screen. The other two mutants (isi3 and isi4) showed additional phenotypes associated with sugarsensing screens that select for seedling establishment on high-sugar media. The isi3 and isi4 mutants were found to be involved in the abscisic acid signalling pathway. isi3 is allelic to abscisic acid insensitive4 (abi4), a gene encoding an Apetala2-type transcription factor; isi4 was found to be allelic to glucose insensitive1 (gin1) previously reported to reveal cross-talk between ethylene and glucose signalling. Here we present an alternative interpretation of gin1 as an allele of the ABA-de®cient mutant aba2. Expression analysis showed that ABA is unable to induce ApL3 gene expression by itself, but greatly enhances ApL3 induction by sugar. Our data suggest a major role for ABA in relation to sugar-signalling pathways, in that it enhances the ability of tissues to respond to subsequent sugar signals.

SUMMARYCyanogenic glucosides are amino acid-derived defence compounds found in a large number of vascular plants. Their hydrolysis by specific b-glucosidases following tissue damage results in the release of hydrogen cyanide. The cyanogenesis deficient1 (cyd1) mutant of Lotus japonicus carries a partial deletion of the CYP79D3 gene, which encodes a cytochrome P450 enzyme that is responsible for the first step in cyanogenic glucoside biosynthesis. The genomic region surrounding CYP79D3 contains genes encoding the CYP736A2 protein and the UDP-glycosyltransferase UGT85K3. In combination with CYP79D3, these genes encode the enzymes that constitute the entire pathway for cyanogenic glucoside biosynthesis. The biosynthetic genes for cyanogenic glucoside biosynthesis are also co-localized in cassava (Manihot esculenta) and sorghum (Sorghum bicolor), but the three gene clusters show no other similarities. Although the individual enzymes encoded by the biosynthetic genes in these three plant species are related, they are not necessarily orthologous. The independent evolution of cyanogenic glucoside biosynthesis in several higher plant lineages by the repeated recruitment of members from similar gene families, such as the CYP79s, is a likely scenario.

SummaryThe Arabidopsis bZIP transcription factor gene ATB2 has been shown previously to be expressed in a light-regulated and tissue-specific way. Here we describe the precise localization of ATB2 expression, using transgenic lines containing an ATB2 promoter-GUS reporter gene construct. The observed expression pattern suggests a role for ATB2 in the control of processes associated with the transport or utilization of metabolites. Remarkably, expression of the ATB2-GUS reporter gene construct was specifically repressed by sucrose. Other sugars, such as glucose and fructose, alone or in combination, were ineffective. Repression was observed at external sucrose concentrations exceeding 25 mM. Transcript levels of both the endogenous ATB2 gene and the ATB2-GUS reporter gene were not repressed by sucrose, suggesting that sucrose affects mRNA translation. This translational regulation involves the ATB2 leader sequence because deletion of the leader resulted in loss of sucrose repression. Our results provide evidence for a sucrose-specific sugar sensing and signalling system in plants.

ABSTRACT

Insect herbivory is often restricted by glucosylated plant chemical defence compounds that are activated by plant β-glucosidases to release toxic aglucones upon plant tissue damage. Such two-component plant defences are widespread in the plant kingdom and examples of these classes of compounds are alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides as well as glucosinolates and salicinoids. Conversely, many insects have evolved a diversity of counteradaptations to overcome this type of constitutive chemical defence. Here we discuss that such counter-adaptations occur at different time points, before and during feeding as well as during digestion, and at several levels such as the insects’ feeding behaviour, physiology and metabolism. Insect adaptations frequently circumvent or counteract the activity of the plant β-glucosidases, bioactivating enzymes that are a key element in the plant’s two-component chemical defence. These adaptations include host plant choice, non-disruptive feeding guilds and various physiological adaptations as well as metabolic enzymatic strategies of the insect’s digestive system. Furthermore, insect adaptations often act in combination, may exist in both generalists and specialists, and can act on different classes of defence compounds. We discuss how generalist and specialist insects appear to differ in their ability to use these different types of adaptations: in generalists, adaptations are often inducible, whereas in specialists they are often constitutive. Future studies are suggested to investigate in detail how insect adaptations act in combination to overcome plant chemical defences and to allow ecologically relevant conclusions.

Cyanogenesis, the release of hydrogen cyanide from damaged plant tissues, involves the enzymatic degradation of amino acid-derived cyanogenic glucosides (a-hydroxynitrile glucosides) by specific b-glucosidases. Release of cyanide functions as a defense mechanism against generalist herbivores. We developed a high-throughput screening method and used it to identify cyanogenesis deficient (cyd) mutants in the model legume Lotus japonicus. Mutants in both biosynthesis and catabolism of cyanogenic glucosides were isolated and classified following metabolic profiling of cyanogenic glucoside content. L. japonicus produces two cyanogenic glucosides: linamarin (derived from Val) and lotaustralin (derived from Ile). Their biosynthesis may involve the same set of enzymes for both amino acid precursors. However, in one class of mutants, accumulation of lotaustralin and linamarin was uncoupled. Catabolic mutants could be placed in two complementation groups, one of which, cyd2, encoded the b-glucosidase BGD2. Despite the identification of nine independent cyd2 alleles, no mutants involving the gene encoding a closely related b-glucosidase, BGD4, were identified. This indicated that BGD4 plays no role in cyanogenesis in L. japonicus in vivo. Biochemical analysis confirmed that BGD4 cannot hydrolyze linamarin or lotaustralin and in L. japonicus is specific for breakdown of related hydroxynitrile glucosides, such as rhodiocyanoside A. By contrast, BGD2 can hydrolyze both cyanogenic glucosides and rhodiocyanosides. Our genetic analysis demonstrated specificity in the catabolic pathways for hydroxynitrile glucosides and implied specificity in their biosynthetic pathways as well. In addition, it has provided important tools for elucidating and potentially modifying cyanogenesis pathways in plants.

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