A proteinaceous elicitor of the plant defense reaction known as the hypersensitive response was isolated from Erwinia amylovora, the bacterium that causes fire blight of pear, apple, and other rosaceous plants. The elicitor, named harpin, is an acidic, heat-stable, cell-envelope-associated protein with an apparent molecular weight of 44 kilodaltons. Harpin caused tobacco leaf lamina to collapse and caused an increase in the pH of bathing solutions of suspension-cultured tobacco cells. The gene encoding harpin (hrpN) was located in the 40-kilobase hrp gene cluster of E. amylovora, sequenced, and mutated with Tn5tac1. The hrpN mutants were not pathogenic to pear, did not elicit the hypersensitive response, and did not produce harpin.

SummaryAlthough soluble sugar levels affect many aspects of plant development and physiology, little is known about the mechanisms by which plants respond to sugar. Here we report the isolation of 13 sugarinsensitive (sis) mutants of Arabidopsis that, unlike wild-type plants, are able to form expanded cotyledons and true leaves when germinated on media containing high concentrations of glucose or sucrose. The sis4 and sis5 mutants are allelic to the ABA-biosynthesis mutant aba2 and the ABAinsensitive mutant abi4, respectively. In addition to being insensitive to glucose and sucrose, the sis4/ aba2 and sis5/abi4 mutants also display decreased sensitivity to the inhibitory effects of mannose on early seedling development. Mutations in the ABI5 gene, but not mutations in the ABI1, ABI2 or ABI3 genes, also lead to weak glucose-and mannose-insensitive phenotypes. Wild-type and mutant plants show similar responses to the effects of exogenous sugar on chlorophyll and anthocyanin accumulation, indicating that the mutants are not defective in all sugar responses. These results indicate that defects in ABA metabolism and some, but not all, defects in ABA response can also alter response to exogenous sugar.

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Despite extensive biochemical analyses, the biological function(s) of plant ␤-amylases remains unclear. The fact that ␤-amylases degrade starch in vitro suggests that they may play a role in starch metabolism in vivo. ␤-Amylases have also been suggested to prevent the accumulation of highly polymerized polysaccharides that might otherwise impede flux through phloem sieve pores. The identification and characterization of a mutant of Arabidopsis var. Columbia with greatly reduced levels of ␤-amylase activity is reported here. The reduced ␤-amylase 1 (ram1) mutation lies in the gene encoding the major form of ␤-amylase in Arabidopsis. Although the Arabidopsis genome contains nine known or putative ␤-amylase genes, the fact that the ram1 mutation results in almost complete loss of ␤-amylase activity in rosette leaves and inflorescences (stems) indicates that the gene affected by the ram1 mutation is responsible for most of the ␤-amylase activity present in these tissues. The leaves of ram1 plants accumulate wild-type levels of starch, soluble sugars, anthocyanin, and chlorophyll. Plants carrying the ram1 mutation also exhibit wild-type rates of phloem exudation and of overall growth. These results suggest that little to no ␤-amylase activity is required to maintain normal starch levels, rates of phloem exudation, and overall plant growth.␤-Amylases are found in vegetative tissues as well as in storage and reproductive organs, such as tubers and seeds. ␤-Amylases hydrolyze ␣-1,4-glucosidic linkages from the reducing ends of polysaccharide chains to form maltose (Beck and Ziegler, 1989). The genomes of many plant species contain several genes that encode ␤-amylases. For example, according to the database maintained by The Institute for Genomic Research (TIGR), the Arabidopsis genome includes nine genes that are identified as ␤-amylase or putative ␤-amylase genes (http://www.tigr.org/ tdb/edb2/ath1/htmls/GeneNameSearch.html). Despite the fact that they have been well characterized in vitro, the in vivo biological function(s) of ␤-amylases remains under debate (Ziegler, 1999).Several functions have been proposed for ␤-amylases in vivo. The fact that ␤-amylases hydrolyze starch in vitro (Beck and Ziegler, 1989) suggests a role for ␤-amylases in starch degradation. In addition, immunolocalization studies have shown an apparent association between ␤-amylases and starch granules (Okamoto and Akazawa, 1979;Hagenimana et al., 1992). However, these studies were conducted using polyclonal antibodies that may cross-react with starch phosphorylase. Consequently, the results of these studies should be interpreted with caution (Wang et al., 1995). In addition, starch is localized to plastids, whereas most ␤-amylases lack chloroplast transit peptides (Kreis et al., 1987;Monroe et al., 1991;Yoshida and Nakamura, 1991). Although chloroplast-localized ␤-amylases have been identified (Lao et al., 1999), the majority of ␤-amylase activity is extrachloroplastic (Beck and Ziegler, 1989;Nakamura et al., 1991). For example, 80% of the amylas...

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