Heat stress can detrimentally affect the reproductive capacity of many plants. The effect of a 7 or 14 d heat stress on flowering, seed set, pollen viability and germinability of flax ( Linum usistatissimum L.) was assessed under growth chamber conditions. An incremental (2 ∞ ∞ ∞ ∞ C/h), cyclical (daytime high 40 ∞ ∞ ∞ ∞ C and night-time low 18 ∞ ∞ ∞ ∞ C) heat stress was applied 12 d after the initiation of flowering. Although flower formation in flax was not affected by heat stress, boll formation and seed set were reduced with onset of the heat stress. On removal of heat stress the stressed plants showed a compensatory response, flowering and producing bolls at a greater rate than the control plants. Heat stress significantly prolonged flowering by 17 d. Boll weight and seed weight were reduced with heat stress and the number of malformed, sterile seed increased three-fold after 14 d of heat stress. Pollen viability and appearance were negatively affected after 6 and 10 d of heat stress, respectively. Pollen germinability decreased by the sixth day of heat stress, with no pollen germinating by the tenth day. Effects of heat stress on pollen viability and germinability alone, which did not occur until after the sixth day of the stress, could not account for the decreased boll formation due to heat stress in flax. These observations suggest that a combined effect of heat stress on both pollen and ovules contributes to decreased boll formation and seed set in flax.

Transcriptional activity of a 573-bp fragment of HSP101 (At1g74310) incorporated into a Mutator-like element (MULE) transposon was investigated in Arabidopsis thaliana Columbia. Sequence identity between the HSP101-MULE arrangement and a continuous segment of the original HSP101 promoter, 5' UTR exon, and open reading frame (ORF) was high (87%) but lower in the 5' UTR intron (69%). Collectively, the HSP101 ORF, the MULE 5' terminal inverted repeat (TIR), and the 1.3 kb immediately upstream of the TIR is located on chromosome IV, and we refer to it as HSP101B. Located within the HSP101B promoter, upstream of 2 heat shock elements (HSEs), are 4 COR15a-like low-temperature response elements (LTREs). The HSP101B ORF was transcribed in the leaves and influorescences of high-temperature stress (HTS) treated Arabidopsis thaliana but not in low-temperature stress (LTS) and control plants. Transiently transformed Arabidopsis seedlings, as well as stable transformed lines of Linum usitatissimum (flax) and Brassica napus (canola) containing a HSP101B promoter:GUS construct, showed either LTS-, or LTS- and HTS-, induced beta-glucuronidase expression. Results from PCR amplifications of HpaII- and MspI-digested Arabidopsis genomic DNA suggest that endogenous expression of HSP101B may be downregulated by partial methylation of the HSP101B sequence between the TIRs of the associated MULE.

To investigate the functional conservation of cis regulatory elements controlling AGAMOUS (AG) expression, we placed the promoter region of AG from Arabidopsis thaliana into a close relative, Brassica napus, and a distantly related species, Linum usitatissimum, and analyzed the subsequent expression patterns in each species. Spatially, the expression patterns in all three species were marginally similar, in that expression was confined primarily to the reproductive organs and nectarium. Within organs however, tissue-specific expression patterns were not conserved between species. Unlike Arabidopsis, the transgenic AG cis elements did not express in the ovules of B. napus and L. usitatissimum. Temporally, the pattern of AG cis-element expression in B. napus was identical to that of Arabidopsis during early development; however, the AG cis elements did not express at all during early flower development in L. usitatissimum. These results suggest that although regulatory factors controlling the generalized local expression of AG have been conserved between these species, those controlling temporal and tissue-specific expression have not.Key words: AGAMOUS, cis elements, regulation, Arabidopsis, Brassica napus, Linum usitatissimum.

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