Overexpression of IAA1 with domain II mutation impairs cell elongation and cell division in inflorescences and leaves of Arabidopsis.

The auxin/indoleacetic acid (Aux/IAA) proteins are negative regulators of the auxin response factors (ARFs) that regulate expression of auxin-responsive genes. The Aux/IAA proteins have four conserved domains. Domain II is responsible for the rapid degradation of these proteins. Degradation of the Aux/IAA proteins, mediated by a SCF(TIR1) E3 ubiquitin protein ligase complex, is critical for auxin-regulated gene expression. Using a steroid-hormone-inducible system, we had previously shown that a protein-stability-enhancing mutation in domain II of IAA1 (iaa1) impaired diverse auxin responses. Inhibition of hypocotyl elongation, leaf expansion, and stem elongation by overexpression of iaa1 suggested that cell enlargement and/or cell division might be affected. We here examined the effects of the domain II mutation on cellular anatomy using light microscopy. Our results show that overexpression of iaa1 in Arabidopsis significantly reduced cell length and cell number and affected cell shape in inflorescences and leaves in a dexamethasone (DEX)-dependent manner. These results suggest that IAA1 might be involved in cell elongation as well as in cell division in the aerial parts of Arabidopsis plants. In addition, the formation of both phloem and xylem in leaves and stems was also impaired in a DEX-dependent manner, indicating a potential involvement of IAA1 in vascular development.

A synthetic de-greening gene circuit provides a reporting system that is remotely detectable and has a re-set capacity.

Plants have evolved elegant mechanisms to continuously sense and respond to their environment, suggesting that these properties can be adapted to make inexpensive and widely used biological monitors, or sentinels, for human threats. For a plant to be a sentinel, a reporting system is needed for large areas and widespread monitoring. The reporter or readout mechanism must be easily detectable, allow remote monitoring and provide a re-set capacity; all current gene reporting technologies fall short of these requirements. Chlorophyll is one of the best-recognized plant pigments with an already well-developed remote imaging technology. However, chlorophyll is very abundant, with levels regulated by both genetic and environmental factors. We designed a synthetic de-greening circuit that produced rapid chlorophyll loss on perception of a specific input. With induction of the de-greening circuit, changes were remotely detected within 2 h. Analyses of multiple de-greening circuits suggested that the de-greening circuit functioned, in part, via light-dependent damage to photosystem cores and the production of reactive oxygen species. Within 24-48 h of induction, an easily recognized white phenotype resulted. Microarray analysis showed that the synthetic de-greening initiated a process largely distinct from normal chlorophyll loss in senescence. Remarkably, synthetically de-greened white plants re-greened after removal of the inducer, providing the first easily re-settable reporter system for plants and the capacity to make re-settable biosensors. Our results showed that the de-greening circuit allowed chlorophyll to be employed as a simple but powerful reporter system useful for widespread areas.

Either soluble or plastidic expression of recombinant protoporphyrinogen oxidase modulates tetrapyrrole biosynthesis and photosynthetic efficiency in transgenic rice.

Protoporphyrinogen oxidase (Protox) is the last shared enzyme of the porphyrin pathway. As a continuation of our previous work in which the transgenic rice plants expressing the Bacillus subtilis Protox in the cytoplasm or the plastid showed resistance to diphenyl ether herbicide, this study was undertaken to identify the effects of tertapyrrole biosynthesis in these transgenic rice plants. The transgenic plants either targeted into plastids or expressed in cytoplasm showed higher Protox activity than wild-type plants did. Photosynthetic activity, measured as a quantum yield of photosystem II, was slightly higher in transgenic plants than in wild-type plants, but chlorophyll contents were not significantly different between transgenic and wild-type plants. As for porphyrin biosynthesis, both cytoplasm-expressed and plastid-targeted transgenic plants showed increased synthesis of aminolevulinic acid, Mg-Proto IX, and protoheme in comparison to wild-type plants whereas synthesis of protoporphyrin IX was similar for wild-type and transgenic plants. These results indicate that either cytoplasm or plastid expression of B. subtilis Protox in rice can upregulate the porphyrin pathway leading to increase in photosynthetic efficiency in plants.

Molecular characterization of the gene encoding rice allene oxide synthase and its expression.

The gene encoding rice allene oxide synthase, OsAOS, was intronless and had nucleotide sequences with the high GC content of 67%. Deduced amino acid sequences had very high similarity with other AOS proteins, in particular 74% similarity to barley, characterized by the conserved motifs of P450 cytochrome of the CYP74A family. Purified recombinant rice AOS protein expressed in Escherichia coli converted 13-hydroperoxylinolenic acid to allene oxide. Several restriction enzyme digestions and Southern analysis showed that OsAOS was likely to have two copies in its genome. The basal level of OsAOS expression was detected in various tissues and the transcription level was increased by jasmonate treatment.