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The maintenance of Mg2+ homeostasis in the plant is essential for viability. This review describes Mg2+ functions and balancing in plants, with special focus on the existing knowledge of the involved transport mechanisms. Mg2+ is essential for the function of many cellular enzymes and for the aggregation of ribosomes. Mg2+ concentrations also modulate ionic currents across the chloroplast and the vacuolar membranes, and might thus regulate ion balance in the cell and stomatal opening. The significance of Mg2+ homeostasis has been particularly established with regard to Mg2+'s role in photosynthesis. Mg2+ is the central atom of the chlorophyll molecule, and fluctuations in its levels in the chloroplast regulate the activity of key photosynthetic enzymes. Relatively little is known of the proteins mediating Mg2+ uptake and transport in plants. The plant vacuole seem to play a key role in Mg2+ homeostasis in plant cells. Physiological and molecular evidence indicate that Mg2+ entry to the vacuole is mediated by Mg2+/H+ exchangers. The Arabidopsis vacuolar Mg2+/H+ exchanger, AtMHX, is highly transcribed at the vascular tissue, apparently most abundantly at the xylem parenchyma. Inclusion of Mg2+ ions into the vacuoles of this tissue may determine their partitioning between the various plant organs. Impacts of Mg2+ imbalance are described with respect for both plant physiology and for its nutritional value to animal and human.

Cyclins are cell cycle regulators whose proteins oscillate dramatically during the cell cycle. Cyclin steady-state mRNA levels also fluctuate, and there are indications that both their rate of transcription and mRNA stability are under cell cycle control. Here, we demonstrate the transcriptional regulation of higher eukaryote cyclins throughout the whole cell cycle with a high temporal resolution. The promoters of two Arabidopsis cyclins, cyc3aAt and cyclAt, mediated transcriptional oscillation of the 8-glucuronidase (gus) reporter gene in stably transformed tobacco BY-2 cell lines. The rate of transcription driven by the cyc3aAt promoter was very low during G1, slowly increased during the S phase, peaked at the G2 phase and G2-to-M transition, and was down-regulated before early metaphase. In contrast, the rate of the cyclAt-related transcription increased upon exit of the S phase, peaked at the G2-to-M transition and during mitosis, and decreased upon exit from the M phase. This study indicates that transcription mechanisms that seem to be conserved among species play a significant role in regulating the mRNA abundance of the plant cyclins. Furthermore, the transcription patterns of cyc3aAt and cyclAt were coherent with their slightly higher sequence similarity to the A and B groups of animal cyclins, respectively, suggesting that they may fulfill comparable roles during the cell cycle.Cyclins are activators of specific serine/threonine protein kinases, termed CDKs, which drive progression of the eukaryotic cell cycle (reviewed in ref. 1). Based upon sequence analyses, most plant cyclins identified so far can be divided into those showing slightly higher sequence similarity to either the A or B groups of animal cyclins, but these similarities are not sufficient to exclusively assign them to either group (2, 3). Animal A-and B-type cyclins have distinct patterns of expression and fulfill different roles throughout the cell cycle (reviewed in ref. 4). As the roles of cyclins are far more understood in animals than in plants, further affiliation of a plant cyclin to a certain group of animal cyclins based on a similar expression pattern may give a clue to its function. The Arabidopsis cyclAt and cyc3aAt cyclin genes represent plant cyclins with slightly higher homology to the B and A groups of animal cyclins, respectively (2). Whole-mount in situ hybridization of Arabidopsis root tips treated with cell cycle blockers indicated that steady-state mRNA levels of cyclAt are high at early metaphase and low at early S phase, while the opposite was true for cyc3aAt (2). These data indicated that mRNA levels of cyc3aAt are increased in advance to that of cyclAt, but were not sufficient to give a complete picture of the expression pattern and transcriptional regulation of these cyclins throughout the whole cell cycle.Fluctuation in the steady-state mRNA level of a cell cycle gene may be regulated by a change in the rate of transcription, or in mRNA stability, or both. Transcriptional regulation of cell cycle genes...

SummaryA major nutritional drawback of many crop plants is their low content of several essential amino acids, particularly lysine. The biosynthesis of lysine in plants is regulated by several feedback loops. Dihydrodipicolinate synthase (DHPS) from Escherichia coli, a key enzyme in lysine biosynthesis, which is considerably less sensitive to lysine accumulation than the endogenous plant enzyme has been expressed in chloroplasts of tobacco leaves. Expression of the bacterial enzyme was accompanied by a significant increase in the level of free lysine. No increase in protein-bound lysine was evident. Free lysine accumulation was positively correlated with the level of DHPS activity in various transgenic plants. Compartmentalization of DHPS in the chloroplast was essential for its participation in lysine biosynthesis as no lysine overproduction was obtained in transgenic plants that expressed the bacterial enzyme in the cytoplasm. The elevated level of free lysine in the transgenic plants was sufficient to inhibit, in vivo, a second key enzyme in lysine biosynthesis, namely, aspartate kinase, with no apparent influence on lysine accumulation. The present report not only provides a better understanding of the regulation of lysine biosynthesis in higher plants but also offers a new strategy to improve the production of this essential amino acid.