SummaryThe regulation of flavonol biosynthesis was studied in anthers and pistils of Solanum tuberosum.

Post-transcriptional gene silencing of beta-1,3 glucanase genes in the transgenic tobacco line T17 is characterised by an increased turnover and, as a consequence, reduced levels of gn1 transgene and endogenous beta-1,3 glucanase mRNAs. Here, additional gn1 RNAs, both larger and smaller than the full-length messenger, are shown to accumulate in silenced plants of the transgenic tobacco line T17. The longer-than-full-length gn1 RNAs are the result of cryptic processing of the gn1 messenger. The small gn1 RNAs in silenced plants correspond to distal and proximal parts of the mature gn1 messenger. The proximal RNA products are intact at their 5' extremity, but terminate at different positions at the 3'-end. The distal RNA products contain a poly(A) tail and are truncated to various positions at the 5'-end. These observations indicate that degradation of the mature gn1 transcript does not start at the 5'- or 3'-end, but rather are consistent with degradation of the gn1 transcript starting with an endonucleolytic cleavage followed by internal exonuclease digestion. Importantly, the truncated products are more abundant in silenced plants than in expressing plants. This suggests, together with the previously reported silencing-related increased gn1 mRNA turnover and the similar rates of gn1 transcription in silenced and expressing T17 plants, that the predominant decay route for the gn1 transcripts differs between silenced and expressing conditions.

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In a program aimed at studying genes ex-ly few data have been obtained that shed light on the mopressed in pistils, the cDNA clone STS 15 was isolated lecular basis of a successful pollen-pistil interaction, from a cDNA library of pollinated pistils of Solanum tu-Such an interaction is characterized by germination and berosum and was found to be expressed only in pistils. During development of the pistil, the accumulation of STS 15 transcripts, which are 0.7 kb long, reached a max imum just before anthesis and declined in fully open flowers. Southern blot analysis revealed that stslS was present as a small gene family in dihaploid potato. In situ hybridization experiments indicated that STS15 was strongly expressed in the cortex of the style and at a low level in the stigma. No hybridization signal was observed in the transmitting tissue. The temporal and spatial ex pression patterns of STS 15 indicate that the gene prod ucts of the stslS gene might be involved in the function of the stylar cortex or in making the pistil competent for pollination.

A gene, sts14, coding for a highly expressed mRNA in pistils of Solanum tuberosum, was isolated. Northern blot and in situ analyses demonstrated that the gene was expressed throughout pistil development in both the stylar cortex and the stigma. The deduced STS14 protein displays similarity to the pathogenesis-related PR-1 proteins. A possible function for protection or guidance of the pollen tubes through the pistil is discussed.

Mature Brassica oleracea pollen grains are covered with a lipophilic pollen coat containing a variety of proteins. Screening of an anther cDNA expression library for the coding sequences of such proteins resulted in the isolation of a number of cDNA clones encoding glycine-rich oleosins. The proteins were shown to be attached to the lipophilic coat material only and to be absent elsewhere in the plant. Within the coat, several forms of the pollen coat oleosin with dif ferent molecular weights were detected. The forms are encoded by different transcripts that originate from a single gene. Expression of this gene is restricted to the tapetum and is quantitatively regulated by the water content of the an ther. Similar oleosins were found in the pollen coat of B. aibogtobra and B. napus.

SummaryIn the transgenic tobacco line T17, plants homozygous for the gn1 transgene display developmentally regulated post-transcriptional silencing of basic b-1,3-glucanase genes. Previously, it has been shown that silencing involves a markedly increased turnover of silencingtarget glucanase mRNAs. Using a two-component viral reporter system facilitated a comparison, in a quantitative manner, of the relative silencing ef®ciencies of various sequences derived from the gn1 transgene. The results show that target sites for the silencing mechanism are present throughout the coding region of the gn1 mRNA. Similar-sized coding region sequences along the entire gn1 mRNA display a similar susceptibility to the silencing mechanism. The susceptibility to silencing increases as the coding region elements increase in size. Relative to internal sequences, the 5¢ and 3¢ terminal regions of the gn1 mRNA are inef®cient targets for the silencing machinery. Importantly, sequences of the gn1 transgene that are not part of the mature gn1 mRNA are not recognized by the silencing machinery when expressed in chimeric viral RNAs. These results show that the glucanase silencing mechanism in T17 plants is primarily directed against gn1 mRNA-internal sequences and that terminal sequences of the gn1 mRNA are relatively unaffected by the silencing mechanism.