Mixing model of Phobos' bulk elemental composition for the determination of its origin: Multivariate analysis of MMX/MEGANE data

The formation process of the two Martian moons, Phobos and Deimos, is still debated with two main competing hypotheses: the capture of an asteroid or a giant impact onto Mars. In order to reveal their origin, the Martian Moons eXploration (MMX) mission by Japan Aerospace Exploration Agency (JAXA) plans to measure Phobos' elemental composition by a gamma-ray and neutron spectrometer called MEGANE. This study provides a model of Phobos' bulk elemental composition, assuming the two formation hypotheses. Using the mixing model, we established a MEGANE data analysis flow to discriminate between the formation hypotheses by multivariate analysis. The mixing model expresses the composition of Phobos in 6 key lithophile elements that will be measured by MEGANE (Fe, Si, O, Ca, Mg, and Th) as a linear mixing of two mixing components: material from Mars and material from an asteroid as represented by primitive meteorite compositions. The inversion calculation includes consideration of MEGANE's measurement errors ($E_P$) and derives the mixing ratio for a given Phobos composition, based on which the formation hypotheses are judged. For at least 65\% of the modeled compositions, MEGANE measurements will determine the origin uniquely ($E_P$ = 30\%), and this increases from 74 to 87\% as $E_P$ decreases from 20 to 10\%. Although the discrimination performance depends on $E_P$, the current operation plan for MEGANE predicts an instrument performance for $E_P$ of 20--30\%, resulting in ~70\% discrimination between the original hypotheses. MEGANE observations can also enable the determination of the asteroid type of the captured body or the impactor. The addition of other measurements, such as MEGANE's measurements of the volatile element K, as well as observations by other MMX remote sensing instruments, will also contribute to the MMX mission's goal to constrain the origin of Phobos.Comment: 34 pages, 7 figures, accepted for publication in Icaru

A novel superior factor widely controlling the rice grain quality

Synthesis of storage starch and protein accumulation is the main action of endosperm organogenesis in term of the economic importance of rice. This event is strongly disturbed by abiotic stresses such as high temperature; thus, the upcoming global warming will cause a crisis with a great impact on food production^1,2^. The enzymes for the protein storage and starch synthesis pathway should work in concert to carry out the organogenesis of rice endosperm^3-5^, but the regulatory mechanism is largely unknown. Here we show that a novel regulatory factor, named OsCEO1, acts as the conductor of endosperm organogenesis during the rice grain filling stage. The physiological properties of _floury-endosperm-2_ (_flo2_) mutants showed many similarities to symptoms of grains developed under high-temperature conditions, suggesting important roles of the responsible gene in sensitivity to high-temperature stress. Our map-based cloning identified the responsible gene for the _flo2_ mutant, _OsCEO1_, which has no homology to any genes of known function. The _OsCEO1_ belongs to a novel conserved gene family and encodes a protein composed of 1,720 amino acid residues containing a TPR (tetratricopeptide repeat) motif, which is considered to mediate a protein-protein interaction. The yeast two-hybrid analysis raised an unknown protein showing homology to a late embryogenesis abundant protein and a putative basic helix-loop-helix protein as candidates for the direct interactor for _OsCEO1_, whereas no enzyme genes for the synthesis of storage substances were detected. The _flo2_ mutant exhibited reduced expression of several genes for putative regulatory proteins as well as many enzymes involved in storage starch and proteins. These results suggest that _OsCEO1_ is a superior conductor of the novel regulatory cascade of endosperm organogenesis and may have important roles in the response to high-temperature stress

Acquired resistance to the rice blast in transgenic rice accumulating the antimicrobial peptide thanatin

Thanatin is an antimicrobial peptide with a strong and wide-ranging antimicrobial spectrum, including certain species of fungi and Gram-negative and -positive bacteria. To evaluate the application of thanatin to the generation of disease-resistant plants, we introduced a synthetic thanatin gene into rice. Several transformants that expressed the introduced gene showed significant level of antimicrobial activity. The substances showing antimicrobial activity were partially purified from these transformants and their properties were determined. The molecule with characteristics similar to those of native thanatin on the elution pattern in HPLC analysis had an identical molecular mass to that of native molecule. It should also be noted that the transformant acquired a sufficient level of resistance to the rice blast fungus, Magnaporthe oryzae, presumably due to the repressive activity of thanatin to its initial stage of infection. This result demonstrates that thanatin has antifungal activity for M. oryzae and that the introduction of the thanatin gene into rice is effective in generating a plant resistant to rice blast disease