In response to environmental changes, plants coordinate growth balance and metabolisms at a whole plant level including optimization of growth balance of the aerial part and root system and coordination of metabolism and morphogenesis. In order to realize the coordinated environmental responses, inter-cellular and inter-organ communication is necessary. In this communication, various signaling molecules, such as phytohormones, play important roles. However, mechanisms underlying the signal communication is not well understood yet. We are conducting research projects focusing on the role of plant hormones and other signaling molecules with focusing on nutritional responses. In particular, cytokinin, which promote plant growth and development, is an important signaling molecule in aiming improvement of plant productivity, so it is one of the main research subjects of our laboratory.
Arabidopsis thaliana, Cytokinin, Nitrogen signaling, Oryza sativa, Phytohormones
Main Research Topics:
- Elucidation of cytokinin biosynthesis and transport, and their regulatory systems
- Elucidation of mechanisms underlying organ-to-organ communication of nutritional status
- Identification of novel cytokinins and its function produced by phytopathogens
- Identification of key genes for low-nitrogen input/stable-output agriculture
Elucidation of cytokinin biosynthesis and transport, and their regulatory systems
Cytokinins are a class of phytohormones, which enhances cytokinesis of plant cells in the presence of auxin. Over the past half-century, cytokinins have been found to be involved in various aspects of the developmental processes, including senescence, apical dominance, root proliferation, phyllotaxis, and reproductive competence. To regulate the processes, cytokinin metabolism must be tightly controlled. However, the whole picture of the cytokinin metabolic pathway has not yet been completely elucidated. The aim of our research is to fully understand cytokinin functions by elucidating cytokinin metabolism. We have identified key genes for its biosynthesis and transport (for a review, Osugi and Sakakibara 2015), and now studies regulatory mechanisms in response to environmental cues.
（fig. 1）Cytokinin biosynthesis pathway and transporters
Elucidation of mechanisms underlying organ-to-organ communication of nutritional status
Plants photosynthesize and produce all biomolecules from inorganic nutrients, but supply of inorganic nutrients from the soil is not always enough. In response to internal and external nutritional status, plants coordinately control metabolic systems and morphogenesis, and also optimize growth of the aerial part and root system at a whole plant level. In order to realize the coordinated environmental response, inter-cellular and inter-organ communication is necessary. In this communication, various signaling molecules, such as phytohormones, play important roles. We study organ-to-organ communication mechanisms in nutritional response focusing on the role of signal molecule such as phytohormones. Cytokinin is known to play an important role as an inter-organ communication molecule that is transported over long distance via xylem and phloem. We study the mechanism of long-range transport and action of cytokinins(Kiba et al. 2013; Osugi et al. 2017).
Recent studies indicate that organ-to-organ communication regulates expression of genes involved in iron absorption in response to fluctuated iron availability. We also conduct study of this molecular mechanism.
（fig. 2）Growth optimization by cytokinin via long distance transport
Identification of novel cytokinins and its function produced by phytopathogens
Cytokinin is also synthesized by phytopathogenic bacteria and a key factor in some plant diseases. For instance, A. tumefaciens infects plants and induces the formation of tumors by integrating the T-DNA region of the Ti-plasmid into the plant nuclear genome. Tumors are formed because the T-DNA encodes enzymes that modify the synthesis of two plant growth hormones, auxin and cytokinin. We had demonstrated that a cytokinin biosynthesis enzyme, Tmr, which is encoded by the Agrobacterium T-DNA region, is targeted to plastids of infected plant cells, and increases cytokinin production by modifying the biosynthetic pathway in the host plant（Sakakibara et al. 2005; Ueda et al. 2012).
As another example, Rhodococcus fascians infection causes unique leafy gall symptoms reminiscent of cytokinin over-effect. The fasciation (fas) locus, an operon encoding several genes homologous to cytokinin biosynthesis and metabolism. However, real function of these genes has not been fully elucidated. We are tackling research to identify novel cytokinin species and its function.
（fig. 3）Agrobacterium increases cytokinin production in the host plant.
(fig. 4) Rhodococcus fascians infection causes unique leafy gall symptoms reminiscent of cytokinin over-effect.
Identification of key genes for low-nitrogen input/stable-output agriculture
Nitrogen (N) is one of the most important nutrients for plants; its availability often limits plant growth and productivity. In agricultural systems, the use of nitrogen fertilizers has increased crop yield over the past 50 years. However, excess nitrogen fertilizer use has created unintended human and environmental problems, including surface and groundwater pollution, biogenic greenhouse gas emissions, and an increase in production cost. To solve these problems, nitrogen fertilizer use must be curved. On the other hand, plants frequently face N-limited growth conditions in natural systems because excess N is easily leached by rainwater and/or consumed by microorganisms. Thus, plants have evolved elaborate responses (N-limitation responses) including changes in morphology and metabolism, and an increase in nitrogen uptake to survive N-limiting conditions. A comprehensive understanding of the molecular network underlying these responses would lead to generating a crop with improved N use efficiency or N-limitation tolerance.
We have been focusing on the mechanism plants employ to absorb nitrogen nutrient (nitrate) efficiently in N-limited conditions. Nitrate uptake in N-limited conditions is carried out by a family of nitrate transporters NITRATE TRANPORTER (NRT). We have revealed that spatial and temporal cooperation between multiple NRTs are important to efficiently absorb nitrate under N limitation (Kiba et al. 2012; Kiba and Krapp 2016; Lezhneva et al. 2014). Recently we identified a family of transcription factors involved in the regulation of the cooperation by governing N-limitation response-related genes including NRTs in response to N availability . We are continuing our research to get a full molecular picture underlying the N-limitation responses.
（fig. 5）Multiple knockout mutants of NRT