Kaori Kikuchi

Roles and regulation of cytokinins in tomato fruit development

Cytokinins (CKs) are thought to play important roles in fruit development, especially cell division. However, the mechanisms and regulation of CK activity have not been well investigated. This study analysed CK concentrations and expression of genes involved in CK metabolism in developing tomato (Solanum lycopersicum) ovaries. The concentrations of CK ribosides and isopentenyladenine and the transcript levels of the CK biosynthetic genes SlIPT3, SlIPT4, SlLOG6, and SlLOG8 were high at anthesis and decreased immediately afterward. In contrast, trans-zeatin concentration and the transcript levels of the CK biosynthetic genes SlIPT1, SlIPT2, SlCYP735A1, SlCYP735A2, and SlLOG2 increased after anthesis. The expression of type-A response regulator genes was high in tomato ovaries from pre-anthesis to early post-anthesis stages. These results suggest that the CK signal transduction pathway is active in the cell division phase of fruit development. This study also investigated the effect of CK application on fruit set and development. Application of a synthetic CK, N-(2-chloro-pyridin-4-yl)-N’-phenylurea (CPPU), to unpollinated tomato ovaries induced parthenocarpic fruit development. The CPPU-induced parthenocarpic fruits were smaller than pollinated fruits, because of reduction of pericarp cell size rather than reduced cell number. Thus, CPPU-induced parthenocarpy was attributable to the promotion of cell division, not cell expansion. Overall, the results provide evidence that CKs are involved in cell division during development of tomato fruit.

Isolation and functional characterization of the FLOWERING LOCUS T homolog, the LsFT gene, in lettuce.

High temperature-induced bolting of lettuce is undesirable agriculturally, making it important to find the mechanism governing the transition from vegetative to reproductive growth. FLOWERING LOCUS T (FT) genes play important roles in the induction of flowering in several plant species. To clarify floral induction in lettuce, we isolated the FT gene (LsFT) from lettuce. Sequence analysis and phylogenetic relationships of LsFT revealed considerable homology to FT genes of Arabidopsis, tomato, and other species. LsFT induced early flowering in transgenic Arabidopsis, but was not completely effective compared to AtFT. LsFT mRNA was abundant in the largest leaves under flowering-inducible conditions (higher temperatures). Gene expression was correlated with flower differentiation of the shoot apical meristem. Our results suggest that LsFT is a putative FT homolog in lettuce that regulates flower transition, similar to its homolog in Arabidopsis. This is the first information on the lettuce floral gene for elucidating regulation of the flowering transition in lettuce.

The endogenous level of GA1 is upregulated by high temperature during stem elongation in lettuce through LsGA3ox1 expression.

Bolting of lettuce is promoted by high temperatures. Gibberellins (GAs) play an important role in the bolting of several plant species, and it has been reported that exogenous GAs induce bolting and early flowering in lettuce. To clarify the role of GAs in this process, we examined the expression of genes involved in GA metabolism (LsGA20ox-1 and -2, LsGA3ox-1 and -2, and LsGA2ox-1 and -2) and endogenous GAs in lettuce stems. Quantitative reverse-transcription PCR indicated that the expression of a GA 3-oxidase gene, LsGA3ox1, is significantly upregulated by high (35/25 degrees C) temperature compared to low (25/15 degrees C) temperature, whereas transcription of the GA 20-oxidase gene, which is upregulated in long-day conditions in arabidopsis and spinach during bolting, is not clearly affected. Quantification of GA by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) showed that high temperature also upregulates the content of GA(1), a bioactive GA in lettuce. Our results suggest that LsGA3ox1 is a candidate for the gene responsible for the increase in GA(1) during lettuce bolting at high temperatures.

Heavy-ion-induced mutants in sweet pepper isolated by M1 plant selection

We examined the effects of heavy-ion bombardment on mutagenesis in sweet pepper (Capsicum annuum L.). Dose–response studies indicated that 10 Gy irradiation of 12C or 20Ne ions on dry seeds is suitable for inducing mutations in plants. From 20Ne-irradiated M1 plants, putative mutants included two dwarf plants and one plant whose pericarp was yellow were isolated. Phenotypes of their M2 progeny were similar to those of the M1 plants and did not segregate. F1 plants resulting from reciprocal crosses between the mutants and wild-type plants showed the wild-type phenotype, but phenotypes of F2 and BC1F1 segregated at 1:3 (mutant:wild) and 1:1, respectively. These crossing experiments indicate that the three mutants have monogenic recessive mutations in nuclear genes. In light of these data, we discuss the effectiveness of using heavy-ion bombardment to mutate sweet peppers.

Imaging of Carbon Translocation to Fruit Using Carbon-11-Labeled Carbon Dioxide and Positron Emission Tomography

Carbon kinetics into the fruit is an agricultural issue on the growth and development of the sink organs to be harvested. Particularly, photoassimilate translocation and distribution are important topics for understanding the mechanism. In the present work, carbon-11 (11C) labeled photoassimilate translocation into fruits of tomato has been imaged using carbon-11-labeled carbon dioxide and the positron emission tomography (PET). Dynamice PET data of gradual increasing of 11C activity and its distribution is acquired quantitatively in intact plant body. This indicates that the 3-D photoassimilate translocation into the fruits is imaged successfully and carbon kinetics is analyzable to understand the plant physiology and nutrition.

Imaging for carbon translocation to a fruit of tomato with carbon-11-labeled carbon dioxide and positron emission tomography

Carbon kinetics into the fruit is an agricultural issue on the growth and development of the organ to be harvested. Particularly, photoassimilate translocation and distribution are important topics for understanding the mechanism. In the present work, carbon-11 (11C) labeled photoassimilate translocation into fruits of tomato has been imaged using carbon-11-labeled carbon dioxide and the positron emission tomography (PET). Dynamice PET data of gradual increasing of 11C activity and its distribution is acquired quantitatively in intact plant body. This indicates that the three dimensional photoassimilate translocation into the fruits is imaged successfully and carbon kinetics is analyzed to understand the plant physiology and nutrition.

Molecular imaging for plant physiology: Imaging of carbon translocation to sink organs

Radionuclide-based imaging technologies have provided exciting new opportunities to life science researchers. However, success in the field of plant science is limited because most methods for studying plants are invasive and require statistical analysis and a large number of test plants. To clarify the mechanism of the growth and development of the agricultural produces to be harvested, we performed imaging experiments of sugar translocation to the sink organ of fruit. A leaf near the target fruits (eggplant and tomato) was exposed to carbon-11-labeled carbon dioxide, and the translocations of carbon-11-labeled photoassimilate into fruits were assessed by images obtained using the positron-emitting tracer imaging system and small animal positron emission tomography system. Serial images thus obtained showed gradually increasing 11C activity and its nonuniform distribution in the fruit. These experimental methods will be useful in not only investigating plant physiology, such as mechanisms underlying fruit growth, but also solving certain environmental and food problems.