Senescence-induced ectopic expression of the A. tumefaciens ipt gene in wheat delays leaf senescence, increases cytokinin content, nitrate influx, and nitrate reductase activity, but does not affect grain yield.

The manipulation of cytokinin levels by senescence-regulated expression of the Agrobacterium tumefaciens ipt gene through its control by the Arabidopsis SAG12 (senescence-associated gene 12) promoter is an efficient tool for the prolongation of leaf photosynthetic activity which potentially can affect plant productivity. In the present study, the efficiency of this approach was tested on wheat (Triticum aestivum L.)-a monocarpic plant characterized by a fast switch from vegetative to reproductive growth, and rapid translocation of metabolites from leaves to developing grains after anthesis. When compared with the wild-type (WT) control plants, the SAG12::ipt wheat plants exhibited delayed chlorophyll degradation only when grown under limited nitrogen (N) supply. Ten days after anthesis the content of chlorophyll and bioactive cytokinins of the first (flag) leaf of the transgenic plants was 32% and 65% higher, respectively, than that of the control. There was a progressive increase in nitrate influx and nitrate reductase activity. However, the SAG12::ipt and the WT plants did not show differences in yield-related parameters including number of grains and grain weight. These results suggest that the delay of leaf senescence in wheat also delays the translocation of metabolites from leaves to developing grains, as indicated by higher accumulation of ((15)N-labelled) N in spikes of control compared with transgenic plants prior to anthesis. This delay interferes with the wheat reproductive strategy that is based on a fast programmed translocation of metabolites from the senescing leaves to the reproductive sinks shortly after anthesis.

Folbos, a new foldback element in rice.

A new class I foldback element, Folbos, has been discovered in O. sativa L. Its long terminal inverted repeats (IVRs) are 303 and 331 bp long and the left one encodes a short open reading frame of 76 codons. The IVRs consist of inner and outer domains, the latter built up of 6 tandem repeats of about 30 bp each. The central region is represented by 90 bp conservative stretch adjacent to a variable length (19-33 bp) A-tail, which in most cases includes the sequence 5'-TGACTT-3'. Folbos targets AT-rich regions and the insertion results in 7 bp target site duplications. Half of the copies found in annotated sequences of O. sativa japonica cv. Nipponbare are positioned in close proximity to (< 1kb) or within the transcribed regions, thus they have the potential to contribute to plant genome evolution.

Induction of cell division-related genes in quiescent (G0 ) sugar beet cells.

Sugar beet cells maintained in the stationary phase of the batch culture cycle for 2 or more days have been shown to exhibit many of the characteristics of quiescent (G0 ) cells. When such cells were subcultured into fresh medium they progressed through a period of DNA synthesis to a highly synchronised first division, 6 days after subculture. The onset of DNA synthesis and cell division were each delayed by 2 days relative to the timing of the events when the cells were subcultured immediately before entry into the stationary phase. The regulation of gene expression during this extended transition from the G0 phase back to the cell division cycle was investigated. The cell division cycle-related genes Bvcdc2, Betvu;CycA2, Arath;CycB1;1 histone H4 and Bvcrk1 (a novel cdc2-like gene) showed widely differing patterns of expression. Bvcdc2 transcripts were present at low levels in quiescent cells whereas crk1, cyclin and histone transcripts were not detectable. Expression of both Bvcrk1 and Betvu; CycA2 was induced within 1 h after subculture into fresh medium, whereas histone H4 gene expression was not detectable for 24 h and showed a marked increase between 24 and 48 h. B-type cyclin transcripts were not detectable until more than 48 h after subculture. The addition of either sucrose or MS macronutrients to quiescent sugar beet cells was not sufficient to re-initiate cell division but both medium components were able to stimulate the expression of the two 'early' genes (Betvu;CycA2 and Bvcrk1) within 6 h. Furthermore, although the sugar beet cells were habituated, i.e. they were routinely grown without added plant growth regulators, treatment of quiescent cells with IAA and kinetin also induced expression of Betvu;CycA2 and Bvcrk1 without subsequent cell division.