Fructans are polyfructose molecules produced by approximately 15% of the flowering plant species. It is possible that, in addition to being a storage carbohydrate, fructans have other physiological roles. Owing to their solubility they may help plants survive periods of osmotic stress induced by drought or cold. To investigate the possible functional significance of fructans, use was made of transgenic tobacco (Nicotiana tabacum) plants that accumulate bacterial fructans and hence possess an extra sink for carbohydrate. Biomass production was analyzed during drought stress with the use of lines differing only in the presence of fructans. Fructan-producing tobacco plants performed significantly better under polyethyleneglycol-mediated drought stress than wild-type tobacco. l h e growth rate of the transgenic plants was significantly higher (+55"/0), as were fresh weight (+33%) and dry weight (+59%) yields. l h e difference in weight was observed in ai1 organs and was particularly pronounced in roots. Under unstressed control conditions the presente of fructans had no significant effect on growth rate and yield.Under all conditions the total nonstructural carbohydrate content was higher in the transgenic plants. We conclude that the introduction of fructans in this non-fructan-producing species mediates enhanced resistance to drought stress. plant fructans varies from 10 to 200 Fru units. Differences in fructan length not only result from taxonomic variation but are also subject to environmental influences: plants have been reported to respond to changing conditions by shifting the average length of their fructan pool (Pontis and De1 Campillo, 1985).In addition to plants, severa1 bacteria synthesize fructans. Bacterial fructan biosynthesis from Suc involves only one enzyme. Most bacterial fructans have a very high DP (up to 100,000) and a 2-6-linkage type with occasional2-1 branching (Dedonder, 1966).One of the goals of our studies is to elucidate why some plant taxa use fructans as the predominant storage carbohydrate instead of starch, which is ubiquitous in the plant kingdom. In other words, what is the functional significance of fructans and under which selection pressure has fructan metabolism evolved? The most obvious differences between starch and fructan are the location and solubility. Fructans are located in the vacuole and are soluble, in contrast to the insoluble plastidic starch. A possible advantage of vacuoles as storage organelles could be that the storage capacity of vacuoles may be larger than that of the plastids, since the vacuole constitutes 95% of the protoplast volume. Fructan storage capacity in plants may be further (onion, tulip), tubers (Jerusalem artichoke), or succulent stems ( A~~~~) .Fructan is indeed often accumulated to higher quantities than starch (Brocklebank and Hendry, 1989).Since fructans are soluble, they may play a role in the osmotic adjustment of natural fructan accumulators to changing environmental conditions via variation in the DP of the fructan pool. An of osmotic a...

Some inter-and intraspecific crosses may result in reduced viability or sterility in the offspring, often due to genetic incompatibilities resulting from interactions between two or more loci. Hybrid necrosis is a postzygotic genetic incompatibility that is phenotypically manifested as necrotic lesions on the plant. We observed hybrid necrosis in interspecific lettuce (Lactuca sativa and Lactuca saligna) hybrids that correlated with resistance to downy mildew. Segregation analysis revealed a specific allelic combination at two interacting loci to be responsible. The allelic interaction had two consequences: (1) a quantitative temperature-dependent autoimmunity reaction leading to necrotic lesions, lethality, and quantitative resistance to an otherwise virulent race of Bremia lactucae; and (2) a qualitative temperature-independent racespecific resistance to an avirulent race of B. lactucae. We demonstrated by transient expression and silencing experiments that one of the two interacting genes was Rin4. In Arabidopsis thaliana, RIN4 is known to interact with multiple R gene products, and their interactions result in hypersensitive resistance to Pseudomonas syringae. Site-directed mutation studies on the necrosis-eliciting allele of Rin4 in lettuce showed that three residues were critical for hybrid necrosis.

An integrated map for lettuce comprising of 2,744 markers was developed from seven intra- and inter-specific mapping populations. A total of 560 markers that segregated in two or more populations were used to align the individual maps. 2,073 AFLP, 152 RFLP, 130 SSR, and 360 RAPD as well as 29 other markers were assigned to nine chromosomal linkage groups that spanned a total of 1,505 cM and ranged from 136 to 238 cM. The maximum interval between markers in the integrated map is 43 cM and the mean interval is 0.7 cM. The majority of markers segregated close to Mendelian expectations in the intra-specific crosses. In the two L. saligna x L. sativa inter-specific crosses, a total of 155 and 116 markers in 13 regions exhibited significant segregation distortion. Data visualization tools were developed to curate, display and query the data. The integrated map provides a framework for mapping ESTs in one core mapping population relative to phenotypes that segregate in other populations. It also provides large numbers of markers for marker assisted selection, candidate gene identification, and studies of genome evolution in the Compositae.

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Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide. Breeding for resistance to this disease is a major priority for most lettuce breeding programs. Many genes and factors for resistance to B. lactucae have been reported by multiple researchers over the past *50 years. Their nomenclature has not been coordinated, resulting in duplications and gaps in nominations. We have reviewed the available information and rationalized it into 51 resistance genes and factors and 15 quantitative trait loci along with supporting documentation as well as genetic and molecular information. This involved multiple rounds of consultation with many of the original authors. This paper provides the foundation for naming additional genes for resistance to B. lactucae in the future as well as for deploying genes to provide more durable resistance. Keywords