To find evidence for a connection between heat stress response, oxidative stress, and common stress tolerance, we studied the effects of elevated growth temperatures and heat stress on the activity and expression of ascorbate peroxidase (APX). We compared wild-type Arabidopsis with transgenic plants overexpressing heat shock transcription factor 3 (HSF3), which synthesize heat shock proteins and are improved in basal thermotolerance. Following heat stress, APX activity was positively affected in transgenic plants and correlated with a new thermostable isoform, APX^S. This enzyme was present in addition to thermolabile cytosolic APX1, the prevalent isoform in unstressed cells. In HSF3-transgenic plants, APX^S activity was detectable at normal temperature and persisted after severe heat stress at 44°C. In nontransgenic plants, APX^S was undetectable at normal temperature, but could be induced by moderate heat stress. The mRNA expression profiles of known and three new Apx genes were determined using real-time PCR. Apx1 and Apx2 genes encoding cytosolic APX were heat stress and HSF dependently expressed, but only the representations of Apx2 mRNA met the criteria that suggest identity between APX^S and APX2: not expressed at normal temperature in wild type, strong induction by heat stress, and HSF3-dependent expression in transgenic plants. Our data suggest thatApx2 is a novel heat shock gene and that the enzymatic activity of APX2/APX^S is required to compensate heat stress-dependent decline of APX1 activity in the cytosol. The functional roles of modulations of APX expression and the interdependence of heat stress and oxidative stress response and signaling mechanisms are discussed.

Origin and rearrangement of ribosomal DNA repeats in natural allotetraploid Nicotiana tabacum are described. Comparative sequence analysis of the intergenic spacer (IGS) regions of Nicotiana tomentosiformis (the paternal diploid progenitor) and Nicotiana sylvestris (the maternal diploid progenitor) showed species-specific molecular features. These markers allowed us to trace the molecular evolution of parental rDNA in the allopolyploid genome of N. tabacum; at least the majority of tobacco rDNA repeats originated from N. tomentosiformis, which endured reconstruction of subrepeated regions in the IGS. We infer that after hybridization of the parental diploid species, rDNA with a longer IGS, donated by N. tomentosiformis, dominated over the rDNA with a shorter IGS from N. sylvestris; the latter was then eliminated from the allopolyploid genome. Thus, repeated sequences in allopolyploid genomes are targets for molecular rearrangement, demonstrating the dynamic nature of allopolyploid genomes.

The potential of different house‐keeping genes for their use as internal standards of gene expression under changing environmental conditions and in different organs of plants was assessed. Using real‐time PCR mRNA levels were precisely quantified for preselected actin and ribosomal protein genes in Arabidopsis thaliana (L.) Heinh. and Nicotiana tabacum L. grown at normal temperature and following heat stress. In tobacco leaves the mRNA levels of the constitutively expressed ribosomal protein gene Nt‐L25 and the actin genes Nt‐ACT9 and At‐ACT66 were strongly reduced (to approximately 10%) during heat stress. Heat stress applied at the temperature optimum (37 °C) for elicitation of a heat stress response to Arabidopsis leaves resulted in a strong induction (several thousand‐fold) of the mRNA heat shock protein genes, At‐HSP17.6 and At‐HSP18.2. Concomitantly, the mRNA levels of constitutively expressed actin 2 (At‐ACT2) and ribosomal protein L23 (At‐L23a) genes were reduced to approximately 50% of the levels in leaves incubated at room temperature. Conversely, under severe heat stress conditions (44 °C), the induction of At‐HSP17.6 and At‐HSP18.2 mRNAs was insignificant, the mRNA levels of At‐ACT2 remained at approximately the same levels as in leaves incubated at room temperature, whereas the mRNA level of At‐L23 declined. The mRNA levels of At‐ACT2 and At‐L23a examined in stem, flower and siliques of Arabidopsis plants grown under non‐stress condition showed differential alterations; the mRNA level of ribosomal protein L23 correlates with the metabolic activity of tissues. The potential use of house‐keeping gene expression as standards in expression profiling and the mechanisms modulating the mRNA levels are discussed.

Ribosomal RNA genes (rDNA) represent a useful tool to study reticulate evolution. In allopolyploid plant species biparental or uniparental inheritance of 35S rDNA was demonstrated. Uniparental inheritance; as a result of differential elimination of one of the parental rDNA, can be accompanied by structural rearrangements of rDNA ofthe other parent and by formation of new rDNA variants. Particularly, homo‐genization of rDNA in allopolyploids appears to be an example of fast concerted evolution of repeated sequences. At the functional level, interaction between parental 35S rDNA loci in hybrids/allopolyploids leads to differential transcription/silencing, known as nucleolar dominance (ND). Depending on the combination of parental species, ND may be strong and stable, or weak, unstable and even reversible. Differential transcription of parental rDNA is regulated via cytosine methylation of DNA, histone modifications, and chromatin remodelling factors, but mechanisms providing discrimination of parental rDNA remain poorly understood. Probably, interplay between several factors such as local structural features of rDNA (especially subrepeated elements in the intergenic spacer region), unlinked loci, and the chro‐mosomal/genomic context determine ND. The ND and rDNA rearrangements reflect the dynamic nature and evolutionary plasticity of the genome in allopolyploids.