A Ca2+-sensor switch for tolerance to elevated salt stress in Arabidopsis.

Excessive Na+ in soils inhibits plant growth. Here, we report that Na+ stress triggers primary calcium signals specifically in a cell group within the root differentiation zone, thus forming a "sodium-sensing niche" in Arabidopsis. The amplitude of this primary calcium signal and the speed of the resulting Ca2+ wave dose-dependently increase with rising Na+ concentrations, thus providing quantitative information about the stress intensity encountered. We also delineate a Ca2+-sensing mechanism that measures the stress intensity in order to mount appropriate salt detoxification responses. This is mediated by a Ca2+-sensor-switch mechanism, in which the sensors SOS3/CBL4 and CBL8 are activated by distinct Ca2+-signal amplitudes. Although the SOS3/CBL4-SOS2/CIPK24-SOS1 axis confers basal salt tolerance, the CBL8-SOS2/CIPK24-SOS1 module becomes additionally activated only in response to severe salt stress. Thus, Ca2+-mediated translation of Na+ stress intensity into SOS1 Na+/H+ antiporter activity facilitates fine tuning of the sodium extrusion capacity for optimized salt-stress tolerance.

Improving plant drought tolerance and growth under water limitation through combinatorial engineering of signalling networks.

Agriculture is by far the biggest water consumer on our planet, accounting for 70 per cent of all freshwater withdrawals. Climate change and a growing world population increase pressure on agriculture to use water more efficiently ('more crop per drop'). Water-use efficiency (WUE) and drought tolerance of crops are complex traits that are determined by many physiological processes whose interplay is not well understood. Here, we describe a combinatorial engineering approach to optimize signalling networks involved in the control of stress tolerance. Screening a large population of combinatorially transformed plant lines, we identified a combination of calcium-dependent protein kinase genes that confers enhanced drought stress tolerance and improved growth under water-limiting conditions. Targeted introduction of this gene combination into plants increased plant survival under drought and enhanced growth under water-limited conditions. Our work provides an efficient strategy for engineering complex signalling networks to improve plant performance under adverse environmental conditions, which does not depend on prior understanding of network function.

Knockout of the plastid RNase E leads to defective RNA processing and chloroplast ribosome deficiency.

Ribonuclease E (RNase E) represents a key enzyme in bacterial RNA metabolism. It plays multifarious roles in RNA processing and also initiates degradation of mRNA by endonucleolytic cleavage. Plastids (chloroplasts) are derived from formerly free-living bacteria and have largely retained eubacterial gene expression mechanisms. Here we report the functional characterization of a chloroplast RNase E that is encoded by a single-copy nuclear gene in the model plant Arabidopsis thaliana. Analysis of knockout plants revealed that, unlike in bacteria, RNase E is not essential for survival. Absence of RNase E results in multiple defects in chloroplast RNA metabolism. Most importantly, polycistronic precursor transcripts overaccumulate in the knockout plants, while several mature monocistronic mRNAs are strongly reduced, suggesting an important function of RNase E in intercistronic processing of primary transcripts from chloroplast operons. We further show that disturbed maturation of a transcript encoding essential ribosomal proteins results in plastid ribosome deficiency and, therefore, provides a molecular explanation for the observed mutant phenotype.

High-level expression of human immunodeficiency virus antigens from the tobacco and tomato plastid genomes.

Transgene expression from the plant's plastid genome represents a promising strategy in molecular farming because of the plastid's potential to accumulate foreign proteins to high levels and the increased biosafety provided by the maternal mode of organelle inheritance. In this article, we explore the potential of transplastomic plants to produce human immunodeficiency virus (HIV) antigens as potential components of an acquired immunodeficiency syndrome (AIDS) vaccine. It is shown that the HIV antigens p24 (the major target of T-cell-mediated immune responses in HIV-positive individuals) and Nef can be expressed to high levels in plastids of tobacco, a non-food crop, and tomato, a food crop with an edible fruit. Optimized p24-Nef fusion gene cassettes trigger antigen protein accumulation to up to approximately 40% of the plant's total protein, demonstrating the great potential of transgenic plastids to produce AIDS vaccine components at low cost and high yield.

Evolutionary origin of a plant mitochondrial group II intron from a reverse transcriptase/maturase-encoding ancestor.

Group II introns are widespread in plant cell organelles. In vivo, most if not all plant group II introns do not self-splice but require the assistance of proteinaceous splicing factors. In some cases, a splicing factor (also referred to as maturase) is encoded within the intronic sequence and produced by translation of the (excised) intron RNA. However, most present-day group II introns in plant organellar genomes do not contain open reading frames (ORFs) for splicing factors, and their excision may depend on proteins encoded by other organellar introns or splicing factors encoded in the nuclear genome. Whether or not the ancestors of all of these noncoding organellar introns originally contained ORFs for maturases is currently unknown. Here we show that a noncoding intron in the mitochondrial cox2 gene of seed plants is likely to be derived from an ancestral reverse transcriptase/maturase-encoding form. We detected remnants of maturase and reverse transcriptase sequences in the 2.7 kb cox2 intron of Ginkgo biloba, the only living species of an ancient gymnosperm lineage, suggesting that the intron originally harbored a splicing factor. This finding supports the earlier proposed hypothesis that the ancient group II introns that invaded organellar genomes were autonomous genetic entities in that they encoded the factor(s) required for their own excision and mobility.