SummarySix-Carbon (C 6 -) volatiles, including the aldehydes trans-2-hexenal, hexanal and cis-3-hexenal, as well as their corresponding alcohols, are produced from damaged or wounded plant tissue as a product of the enzymatic activity of hydroperoxide lyase (HPL), a component of the lipoxygenase (LOX) pathway. Aerial treatment of Arabidopsis seedlings with 10 µM concentrations of trans-2-hexenal induces several genes known to be involved in the plant's defense response, including phenylpropanoidrelated genes as well as genes of the LOX pathway. Genes encoding the pathogenesis-related proteins PR-1 or PR-2, however, were not induced. Trans-2-hexenal induction thus closely mimics the group of genes induced by methyl jasmonate (MeJA), also a LOX-derived volatile. However, unlike MeJA, trans-2-hexenal did not induce hydroxymethylglutaryl-coenzyme A reductase (HMGR) or thionin2-1. The inductive effect seemed to be limited to C 6 -related volatiles, as C 8 -, C 9 -and other related volatiles did not induce LOX mRNA levels. As has been demonstrated for MeJA, trans-2-hexenal quantitatively reduced wild-type seed germination. Trans-2-hexenal also reduced the germination frequency of the MeJA resistant Arabidopsis mutant, jar1-1, supporting the notion that trans-2-hexenal and MeJA are recognized via different mechanisms. In addition, trans-2-hexenal had a moderate inhibitory effect on root length relative to similar concentrations of MeJA and was approximately 10-fold less effective than MeJA at inducing anthocyanin accumulation in Arabidopsis seedlings. These results suggest that C 6 -volatiles of the LOX pathway act as a wound signal in plants, but result in a moderate plant response relative to MeJA at both the physiological and molecular level.

Significant progress has been made in elucidating the mechanism of abscisic acid (ABA)-regulated gene expression, including the characterization of an ABA-responsive element (ABRE), which is regulated by basic domain/Leu zipper transcription factors. In addition to the ABRE, a coupling element (CE1) has been demonstrated to be involved in ABAinduced expression. However, a trans factor that interacts with CE1 has yet to be characterized. We report the isolation of a seed-specific maize ABI4 homolog and demonstrate, using a PCR-based in vitro selection procedure, that the maize ABI4 protein binds to the CE-1 like sequence CACCG. Using electrophoretic mobility shift assays, we demonstrate that recombinant ZmABI4 protein binds to the CE1 element in a number of ABA-related genes. ZmABI4 also binds to the promoter of the sugar-responsive ADH1 gene, demonstrating the ability of this protein to regulate both ABA-and sugar-regulated pathways. ZmABI4 complements Arabidopsis ABI4 function, because abi4 mutant plants transformed with the ZmABI4 gene have an ABA-and sugar-sensitive phenotype. Identification of the maize ABI4 ortholog and the demonstration of its binding to a known ABA response element provide a link between ABA-mediated kernel development and the regulation of ABA response genes.

Phenylalanine ammonia-lyase (PAL) catalyzes the first step in phenylpropanold synthesis. The role of PAL In pathway regulation was investigated by measurement of product accumulation as a function of enzyme activity in a coilection of near-isogenic transgenic tobacco plants exhibiting a range of PAL levels from wild type to 0.2% of wild type. In leaf tissue, PAL level is the dominant factor regulating accumulation ofthe major product chlorogenic acid and overall flux into the pathway. In stems, PAL at wild-type levels contributes, together with downstream steps, in the regulation of lguin deposition and becomes the dominant, rate-determining step at levels 3-to 4-fold below wild type. The metabolic impact of elevated PAL levels was investigated in transgenic leaf callus that overexpressed PAL. Accumulation of the flavonoid rutin, the major product in wild-type callus, was not increased, but several other products accumulated to similarly high levels. These data indicate that PAL is a key step in the regulation of overall flux into the pathway and, hence, accumulation of major phenylpropanoid products, with the regulatory architecture of the pathway poised so that downstream steps control partitioning into different branch pathways.

The phenylpropanoid pathway is responsible for the synthesis of a large range of natural products in plants, including flavonoids (pigments and UV protectants), the structural polymer lignin, and antimicrobial furanocoumarin and isoflavonoid phytoalexins (Hahlbrock and Scheel, 1989; Dixon and Paiva, 1995). Salicylic acid, which is involved in the establishment of both local and systemic plant defense responses, is also a product of this pathway (Klessig and Malamy, 1994). Although the importance of phenylpropanoid natural products makes the pathway an obvious target for plant improvement by metabolic engineering, little is known about the control of flux into the various branches of the pathway. Many phenylpropanoid

It has been proposed that natural products synthesized by plants contribute to their resistance to pests and pathogens. We show here that transgenic tobacco plants with suppressed levels of the phenylpropanoid biosynthetic enzyme phenylalanine ammonia-lyase (L-phenylalanine ammonialyase, EC 4.3