Phytohormones as Regulators of Mitochondrial Gene Expression in Arabidopsis thaliana.

The coordination of activities between nuclei and organelles in plant cells involves information exchange, in which phytohormones may play essential roles. Therefore, the dissection of the mechanisms of hormone-related integration between phytohormones and mitochondria is an important and challenging task. Here, we found that inputs from multiple hormones may cause changes in the transcript accumulation of mitochondrial-encoded genes and nuclear genes encoding mitochondrial (mt) proteins. In particular, treatments with exogenous hormones induced changes in the GUS expression in the reporter line possessing a 5'-deletion fragment of the RPOTmp promoter. These changes corresponded in part to the up- or downregulation of RPOTmp in wild-type plants, which affects the transcription of mt-encoded genes, implying that the promoter fragment of the RPOTmp gene is functionally involved in the responses to IAA (indole-3-acetic acid), ACC (1-aminocyclopropane-1-carboxylic acid), and ABA (abscisic acid). Hormone-dependent modulations in the expression of mt-encoded genes can also be mediated through mitochondrial transcription termination factors 15, 17, and 18 of the mTERF family and genes for tetratricopeptide repeat proteins that are coexpressed with mTERF genes, in addition to SWIB5 encoding a mitochondrial SWI/SNF (nucleosome remodeling) complex B protein. These genes specifically respond to hormone treatment, displaying both negative and positive regulation in a context-dependent manner. According to bioinformatic resources, their promoter region possesses putative cis-acting elements involved in responses to phytohormones. Alternatively, the hormone-related transcriptional activity of these genes may be modulated indirectly, which is especially relevant for brassinosteroids (BS). In general, the results of this study indicate that hormones are essential mediators that are able to cause alterations in the transcript accumulation of mt-related nuclear genes, which, in turn, trigger the expression of mt genes.

Opposite roles of the Arabidopsis cytokinin receptors AHK2 and AHK3 in the expression of plastid genes and genes for the plastid transcriptional machinery during senescence.

KEY MESSAGE: Cytokinin membrane receptors of the Arabidopsis thaliana AHK2 and AHK3 play opposite roles in the expression of plastid genes and genes for the plastid transcriptional machinery during leaf senescence Loss-of-function mutants of Arabidopsis thaliana were used to study the role of cytokinin receptors in the expression of chloroplast genes during leaf senescence. Accumulation of transcripts of several plastid-encoded genes is dependent on the АНК2/АНК3 receptor combination. АНК2 is particularly important at the final stage of plant development and, unlike АНК3, a positive regulator of leaf senescence. Cytokinin-dependent up-regulation of the nuclear encoded genes for chloroplast RNA polymerases RPOTp and RPOTmp suggests that the hormone controls plastid gene expression, at least in part, via the expression of nuclear genes for the plastid transcription machinery. This is further supported by cytokinin dependent regulation of genes for the nuclear encoded plastid σ-factors, SIG1-6, which code for components of the transcriptional apparatus in chloroplasts.

Hypericin, a potential new BH3 mimetic.

Many types of cancer such as prostate cancer, myeloid leukemia, breast cancer, glioblastoma display strong chemo resistance, which is supported by enhanced expression of multiple anti-apoptotic Bcl-2, Bcl-XL and Mcl-1 proteins. The viable anti-cancer strategies are based on developing anti-apoptotic Bcl-2 proteins inhibitors, BH3 mimetics. Our focus in past years has been on the investigating a new potential BH3 mimetic, Hypericin (Hyp). Hyp is a naturally occurring photosensitive compound used in photodynamic therapy and diagnosis. We have demonstrated that Hyp can cause substantial effects in cellular ultrastructure, mitochondria function and metabolism, and distribution of Bcl2 proteins in malignant and non-malignant cells. One of the possible mechanisms of Hyp action could be the direct interactions between Bcl-2 proteins and Hyp. We investigated this assumption by in silico computer modelling and in vitro fluorescent spectroscopy experiments with the small Bcl2 peptide segments designed to correspond to Bcl2 BH3 and BH1 domains. We show here that Hyp interacts with BH3 and BH1 peptides in concentration dependent manner, and shows the stronger interactions than known BH3 mimetics, Gossypol (Goss) and ABT-263. In addition, interactions of Hyp, Goss and ABT263, with whole purified proteins Bcl-2 and Mcl-1 by fluorescence spectroscopy show that Hyp interacts stronger with the Bcl-2 and less with Mcl-1 protein than Goss or ABT-263. This suggest that Hyp is comparable to other BH3 mimetics and could be explore as such. Hyp cytotoxicity was low in human U87 MG glioma, similar to that of ABT263, where Goss exerted sufficient cytotoxicity, suggesting that Hyp acts primarily on Bcl-2, but not on Mcl-1 protein. In combination therapy, low doses of Hyp with Goss effectively decreased U87 MG viability, suggesting a possible synergy effect. Overall, we can conclude that Hyp as BH3 mimetic acts primarily on Bcl-2 protein and can be explored to target cells with Bcl-2 over-expression, or in combination with other BH3 mimetics, that target Mcl-1 or Bcl-XL proteins, in dual therapy.

Differential impact of heat stress on the expression of chloroplast-encoded genes.

Heat shock is one of the major abiotic factors that causes severe retardation in plant growth and development. To dissect the principal effects of hyperthermia on chloroplast gene expression, we studied the temporal dynamics of transcript accumulation for chloroplast-encoded genes in Arabidopsis thaliana and genes for the chloroplast transcription machinery against a background of changes in physiological parameters. A marked reduction in the transcript amounts of the majority of the genes at the early phases of heat shock (HS) was followed by a return to the baseline levels of rbcL and the housekeeping genes clpP, accD, rps14 and rrn16. The decline in the mRNA levels of trnE (for tRNAglu) and the PSI genes psaA and psaB was opposed by the transient increase in the transcript accumulation of ndhF and the PSII genes psbA, psbD, and psbN and their subsequent reduction with the development of stress. However, the up-regulation of PSII genes in response to elevated temperature was absent in the heat stress-sensitive mutants abi1 and abi2 with the impaired degradation of D2 protein. The expression of rpoA and rpoB, which encode subunits of PEP, was strongly down-regulated throughout the duration of the heat treatment. In addition, heat stress-induced PEP deficiency caused the compensatory up-regulation of the genes for the nuclear-encoded RNA polymerases RPOTp and RPOTmp, the PEP-associated proteins PAP6 and PAP8, the Ser/Thr protein kinase cPCK2, and the stress-inducible sigma factor gene SIG5. Thus, heat stress differentially modulates the transcript accumulation of plastid-encoded genes in A. thaliana at least in part via the expression of HS-responsive nuclear genes for the plastid transcription machinery.