The phytohormone cytokinin has been shown to affect many aspects of plant development ranging from the regulation of the shoot apical meristem to leaf senescence. However, some studies have reported contradictory effects of cytokinin on leaf physiology. Therefore cytokinin treatments cause both chlorosis and increased greening and both lead to decrease or increase in cell size. To elucidate this multifaceted role of cytokinin in leaf development, we have employed a system of temporal controls over the cytokinin pool and investigated the consequences of modulated cytokinin levels in the third leaf of Arabidopsis. We show that, at the cell proliferation phase, cytokinin is needed to maintain cell proliferation by blocking the transition to cell expansion and the onset of photosynthesis. Transcriptome profiling revealed regulation by cytokinin of a gene suite previously shown to affect cell proliferation and expansion and thereby a molecular mechanism by which cytokinin modulates a molecular network underlying the cellular responses. During the cell expansion phase, cytokinin stimulates cell expansion and differentiation. Consequently, a cytokinin excess at the cell expansion phase results in an increased leaf and rosette size fueled by higher cell expansion rate, yielding higher shoot biomass. Proteome profiling revealed the stimulation of primary metabolism by cytokinin, in line with an increased sugar content that is expected to increase turgor pressure, representing the driving force of cell expansion. Therefore, the developmental timing of cytokinin content fluctuations, together with a tight control of primary metabolism, is a key factor mediating transitions from cell proliferation to cell expansion in leaves.
Cadmium (Cd) is classified as a serious pollutant due to its high toxicity, high carcinogenicity, and widespread presence in the environment. Phytoremediation represents an effective low-cost approach for removing pollutants from contaminated soils, and a crop with significant phytoremediation potential is flax. However, significant differences in Cd accumulation and tolerance were previously found among commercial flax cultivars. Notably, cv. Jitka showed substantially higher tolerance to elevated Cd levels in soil and plant tissues than cv. Tábor. Here, significant changes in the expression of 14 proteins (related to disease/defense, metabolism, protein destination and storage, signal transduction, energy and cell structure) were detected by image and mass spectrometric analysis of two-dimensionally separated proteins extracted from Cd-treated cell suspension cultures derived from these contrasting cultivars. Further, two proteins, ferritin and glutamine synthetase (a key enzyme in glutathione biosynthesis), were only up-regulated by Cd in cv. Jitka, indicating that Cd tolerance mechanisms in this cultivar may include maintenance of low Cd levels at sensitive sites by ferritin and low-molecular weight thiol peptides binding Cd. The identified changes could facilitate marker-assisted breeding for Cd tolerance and the development of transgenic flax lines with enhanced Cd tolerance and accumulation capacities for phytoremediating Cd-contaminated soils.
Histidine-containing phosphotransmitters (AHPs) transfer a phosphoryl group from membrane receptors to effectors in the nucleus. Five AHP genes have been identified in Arabidopsis. Real-time reverse transcription polymerase chain reaction (RT-PCR) was employed to quantify contents of individual transcripts in Arabidopsis leaves, roots, stems, flowers and siliques. High organ specificity of gene expression was found in AHP1, AHP2, AHP4 while expression of AHP3 and AHP5 appears more ubiquitous. We detected two AHP5 specific PCR products (AHP5 and AHP5L). Out of the five AHP5 introns, the second one was retained unspliced in the longer product (AHP5L) while AHP5 corresponded to a completely spliced AHP5 mRNA. The ratio between alternately and completely spliced AHP5 mRNAs was highest in flowers and lowest in siliques and stems. No evidence for alternative splicing in the remaining AHP transcripts was found. Two open reading frames were identified in AHP5L. The conserved part of the phosphotransfer domain remains unaffected in the longer one as the translation initiation at an intron ATG would result in a polypeptide coded by exons 3 to 6 and a short extension encoded by the intron part of the alternative open reading frame. However, the isoelectric point would shift by about 3 units towards neutral.
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