Hiroaki Ueda

Strigolactone Regulates Leaf Senescence in Concert with Ethylene in Arabidopsis

Prolonged dark treatment induces ethylene synthesis and consequent induction of strigolactone synthesis in the leaf to promote leaf senescence. Leaf senescence is not a passive degenerative process; it represents a process of nutrient relocation, in which materials are salvaged for growth at a later stage or to produce the next generation. Leaf senescence is regulated by various factors, such as darkness, stress, aging, and phytohormones. Strigolactone is a recently identified phytohormone, and it has multiple functions in plant development, including repression of branching. Although strigolactone is implicated in the regulation of leaf senescence, little is known about its molecular mechanism of action. In this study, strigolactone biosynthesis mutant strains of Arabidopsis (Arabidopsis thaliana) showed a delayed senescence phenotype during dark incubation. The strigolactone biosynthesis genes MORE AXIALLY GROWTH3 (MAX3) and MAX4 were drastically induced during dark incubation and treatment with the senescence-promoting phytohormone ethylene, suggesting that strigolactone is synthesized in the leaf during leaf senescence. This hypothesis was confirmed by a grafting experiment using max4 as the stock and Columbia-0 as the scion, in which the leaves from the Columbia-0 scion senesced earlier than max4 stock leaves. Dark incubation induced the synthesis of ethylene independent of strigolactone. Strigolactone biosynthesis mutants showed a delayed senescence phenotype during ethylene treatment in the light. Furthermore, leaf senescence was strongly accelerated by the application of strigolactone in the presence of ethylene and not by strigolactone alone. These observations suggest that strigolactone promotes leaf senescence by enhancing the action of ethylene. Thus, dark-induced senescence is regulated by a two-step mechanism: induction of ethylene synthesis and consequent induction of strigolactone synthesis in the leaf.

Variation in age and size among breeding populations at different altitudes in the Japanese newts, Cynops pyrrhogaster

The age distribution of Cynops pyrrhogaster was studied by skeletochronology on 12 breeding populations inhabiting altitudes ranging from 120 m to 1140 m on Shikoku Island, Japan. In populations inhabiting altitudes 500 m or less, the mean SVL were smaller than in those that lived at higher altitudes. In populations inhabiting altitudes less than 500 m, minimum age at maturation was three years. In populations inhabiting altitudes of 500 m or more, the minimum age at maturation was four to seven years. The number of testes lobes was influenced by age and body size and was variable among populations.

Reproductive isolating mechanisms and molecular phylogenetic relationships among Palearctic and Oriental brown frogs.

Abstract Crossing experiments were made among various brown frog species and populations collected from Japan, China, Russia and Taiwan. The main purpose of these experiments was to confirm the existence of reproductive isolating mechanisms among Rana pirica from Japan, R. chensinensis from China and R. chensinensis from Russia, and between these three taxa and the other brown frogs distributed in the Palearctic and Oriental regions. It was found that there was no or a slight gametic isolation among the three taxa. While there was a nearly equal number of male and female offspring in the control groups, the hybrid frogs were all males, and completely sterile upon attaining sexual maturity. Thus, each of the Japanese R. pirica and the Russian R. chensinensis is a valid species, distinct from the Chinese R. chensinensis. The phylogenetic tree based on nucleotide sequence data from the mitochondrial 12S and 16S rRNA genes of the Palearctic and Oriental brown frogs showed that the three taxa are included in a cluster together with the other species with 2n=24 chromosomes. The present crossing experiments and molecular data support the hypothesis that each of them is a separate but closely related species.

Fluctuation in abundance and age structure of a breeding population of the Japanese brown frog, Rana japonica Günther (Amphibia, Anura)

Abstract A breeding population of Rana japonica was studied at a marsh on the campus of Hiroshima University in Higashi-Hiroshima during the five years 1995-1999. The mark-recapture study showed that the size of the breeding population varied from year to year, and increased more than twofold in 1999 in comparison with the preceding years. The sex ratio of the breeding population (male/female) was from nearly 1.0 to 1.6. Frogs of both sexes were estimated to breed for the first time at the age of one or two years, and their maximum age was four years according to skeletochronology using phalanges and mark-recapture. Modes of the estimated ages were one year for males during the study years except 1997, but one or two years for females. Two thirds of breeding frogs, irrespective of their sex, were estimated to breed only once throughout their lives.

A Green-Cotyledon/Stay-Green Mutant Exemplifies the Ancient Whole-Genome Duplications in Soybean

The recent whole-genome sequencing of soybean (Glycine max) revealed that soybean experienced whole-genome duplications 59 million and 13 million years ago, and it has an octoploid-like genome in spite of its diploid nature. We analyzed a natural green-cotyledon mutant line, Tenshin-daiseitou. The physiological analysis revealed that Tenshin-daiseitou shows a non-functional stay-green phenotype in senescent leaves, which is similar to that of the mutant of Mendels green-cotyledon gene I, the ortholog of SGR in pea. The identification of gene mutations and genetic segregation analysis suggested that defects in GmSGR1 and GmSGR2 were responsible for the green-cotyledon/stay-green phenotype of Tenshin-daiseitou, which was confirmed by RNA interference (RNAi) transgenic soybean experiments using GmSGR genes. The characterized green-cotyledon double mutant d1d2 was found to have the same mutations, suggesting that GmSGR1 and GmSGR2 are D1 and D2. Among the examined d1d2 strains, the d1d2 strain K144a showed a lower Chl a/b ratio in mature seeds than other strains but not in senescent leaves, suggesting a seed-specific genetic factor of the Chl composition in K144a. Analysis of the soybean genome sequence revealed four genomic regions with microsynteny to the Arabidopsis SGR1 region, which included the GmSGR1 and GmSGR2 regions. The other two regions contained GmSGR3a/GmSGR3b and GmSGR4, respectively, which might be pseudogenes or genes with a function that is unrelated to Chl degradation during seed maturation and leaf senescence. These GmSGR genes were thought to be produced by the two whole-genome duplications, and they provide a good example of such whole-genome duplication events in the evolution of the soybean genome.

Geographical Differentiation in a Japanese Stream-Breeding Frog, Buergeria buergeri, Elucidated by Morphometric Analyses and Crossing Experiments

Abstract Hiroshima and Aomori populations of Buergeria buergeri (hereinafter abbreviated as HIROSHIMA and AOMORI, respectively) were morphologically differentiated in both sexually matured frogs and tadpoles. The mean snout-vent lengths of females were 67.4 mm in HIROSHIMA and 50.4 mm in AOMORI, and those of males were 42.9 mm in HIROSHIMA and 37.2 mm in AOMORI. The mean body weights of females of HIROSHIMA and AOMORI just after spawning were 18.0 g and 7.2 g, and those of males of HIROSHIMA and AOMORI were 4.9 g and 3.8 g, respectively. AOMORI tadpoles were rather stocky and their appearance seemed to be more adaptive to lentic water than HIROSHIMA tadpoles. The lower lip of the tadpoles at stage XIII consisted of 2 continuous and 1 broken teeth rows in HIROSHIMA, whereas 3 continuous and 1 broken ones in AOMORI. The growth rate of the embryos was higher in AOMORI than in HIROSHIMA. The embryos of AOMORI were more tolerant to high temperature and less tolerant to low temperature than those of HIROSHIMA. Hybrids between these two populations showed considerably reduced viability in either combination of reciprocal crosses.