SUMMARY
Leaf senescence is a hiphly-controlled sequence of events comprising the final stage of development. Cells remain viable during the process and new gene expression is required. There is some similarity between senescence in plants and programmed cell death in animals. In this review, different classes of senescence-related genes are defined and progress towards isolating such genes is reported. A range of internal and external factors which appear to cause leaf senescence is considered and various models for the mechanism of senescence- initiation are described. The current understanding of senescence at the wrganelle and molecular levels is presented. Finally, same ideas are mooted as to why senescence occurs and why it should be studied further.

Gene expression during leaf senescence

Two endochitinases (EC 3.2.1.14) of Mr values of ≈34,000 and ≈32,000 have been purified from cultured tissues of Nicotiana tabacum cv. Havana 425. The chitinase content of cloned tobacco pith tissues subcultured on hormone-free medium increases by ≈5-fold to 8% of the soluble protein over a 7-day period. This induction is inhibited >90% by addition of combinations of the plant hormones auxin and cytokinin to the culture medium. Chitinase is also developmentally regulated in the intact plant. Not detectable in leaves near the top of the plant, it is 1-4% of the soluble protein in roots and lower leaves. A cDNA clone of tobacco chitinase was isolated containing a single, large open reading frame of 310 amino acids that includes the complete amino acid sequence of the mature enzyme. Chitinase and chitinase mRNA measured by RNA blot analysis show similar patterns of regulation indicating that chitinase accumulation is controlled, at least in part, at the mRNA level. The patterns were also similar to those obtained with glucan endo-1,3-β-glucosidase (EC 3.2.1.39) suggesting that the two enzymes are coordinately regulated.

Regulation of a plant pathogenesis-related enzyme: Inhibition of chitinase and chitinase mRNA accumulation in cultured tobacco tissues by auxin and cytokinin

When grown in the absence of light, Arabidopsis thaliana deetiolated (det) mutants develop many of the characteristics of light-grown plants, including the development of leaves and chloroplasts, the inhibition of hypocotyl growth elongation, and elevated expression levels of light-regulated genes. We show here that dark-grown wild-type seedlings exhibit similar phenotypic traits if any one of a variety of cytokinins are present in the growth medium. We further show that the striking phenotype of det mutants is unlikely to be caused by different levels of cytokinins in these mutants. The three major Arabidopsis cytokinins, zeatin, zeatin riboside, and isopentenyladenosine, accumulate to similar levels in wild-type seedlings grown in either the light or the dark. There is no consistently different pattern for the levels of these cytokinins in wild-type versus det1 or det2 mutants. However, det1 and det2 have an altered response to cytokinin in a detached leaf senescence assay and in tissue culture experiments. A model is proposed in which light and cytokinins act independently or sequentially through common signal transduction intermediates such as DET1 and DET2 to control the downstream light-regulated responses.

http://www.plantphysiol.org/content/plantphysiol/104/2/339.full.pdf

A Role for Cytokinins in De-Etiolation in Arabidopsis (det Mutants Have an Altered Response to Cytokinins)

Arabidopsis thaliana plants treated with exogenous cytokinins accumulate anthocyanin pigments. We have characterized this response because it is potentially useful as a genetic marker for cytokinin responsiveness. Levels of mRNAs for four genes of the anthocyanin biosynthesis pathway, phenylalanine ammonia lyase 1 (PAL1), chalcone synthase (CHS), chalcone isomerase (CHI), and dihydroflavonol reductase (DFR) were shown to increase coordinately in response to benzyladenine (BA). However, nuclear run-on transcription experiments suggested that although CHS and DFR are controlled by BA at the transcriptional level, PAL1 and CHI are controlled by BA posttranscriptionally. CHS mRNA levels increased within 2 h of BA spray application, and peaked by 3 h. Levels of PAL1 mRNA did not increase within 6 h of BA spray. We also showed that PAL1, CHS, CHI, and DFR mRNA levels fluctuate during a 24-h period and appear to be controlled by a circadian clock. The relation between cytokinin regulation and light regulation of CHS gene transcription is discussed.

Induction of Anthocyanin Accumulation by Cytokinins in Arabidopsis thaliana.

Plant gene expression can be markedly altered in response to cytokinins, leading to an increase or decrease in the abundance of specific transcripts. Cytokinins control gene expression at the transcriptional level as well as posttranscriptionally. Their influence on transcript abundance can be rapid but also includes long-term changes. The observed alterations are for the most part quantitative, sometimes with only subtle differences and rarely include qualitative changes such as a complete repression of gene expression. Relatively small differences in the endogenous cytokinin concentration can change the steady-state mRNA levels of genes. Cytokinin control of gene expression can be cell-type specific. Protein phosphorylation plays a role in the transmission of the cytokinin signal. Cytokinin specific cis-elements and trans-acting factors have not yet been described. Cytokinin target genes are frequently regulated by additional stimuli, such as light or auxin. The products of the regulated genes play a role in diverse biological processes, such as cell division, photosynthesis, chloroplast development, disease resistance and nutrient metabolism. This spread reflects the variety of cytokinin responses in plants. The isolated genes present valuable tools to dissect cytokinin signal transduction pathways and to understand the functional mechanism of this hormone. Rapid progress in this field will be instrumental for a deeper insight in the molecular mechanisms of cytokinin activity.

Cytokinins as regulators of gene expression

Tobacco cell suspension cultures responded to cytokinins (for instance kinetin) by full chloroplast differentiation. The hormone had the effect of stimulating the appearance of a few prominent plastid proteins. Synthesis of the light-harvesting chlorophyl a/b-binding protein (LHCP) in response to kinetin was noteworthy (Axelos M. et al.: Plant Sci Lett 33:201–212, 1984). Poly(A)+RNAs were prepared from cells grown in the presence of or without added kinetin. Poly(A)+RNA recovery and translation activity were not quantitatively altered by the hormone treatment. In vitro translation of polyadenylated mRNA into precursor polypeptides of LHCP (pLHCP) was quantified by immunoprecipitation and SDS-PAGE fractionation of pLHCP immunoprecipitates: pLHCP-mRNA translating activity was found to be stimulated in parallel to mature LHCP accumulation by kinetin-induced cells. Dot-blot and northern-blot hybridizations of poly(A)+RNA were carried out, using as a probe a pea LHCP-cDNA clone (Broglie R. et al.: Proc Natl Acad Sci USA 78: 7304–7308, 1981). A ten-fold increase of the level of pLHCP-encoding sequences was observed in poly(A)+RNA prepared from 9-d kinetin-stimulated cells, compared to control cells. Oligo(dT)-cellulose-excluded RNA fractions exhibited very low hybridization levels, in the same ratios as those obtained with poly(A)+RNA. Thus, the expression of LHCP-gene activity, in response to kinetin addition to tobacco cell suspension cultures, is regulated by the level of pLHCP-encoding mRNA rather than by translational or post-translational controls. re]19850218 rv]19850605 ac]19850613

Péaud-Lenoël C

Papers

Cytokinins modulate the expression of genes encoding the protein of the light-harvesting chlorophyll a/b complex.