Research
Plants have evolved mechanisms to survive in natural environments where temperature, light, water, and nutrient conditions fluctuate in complex and dynamic ways—an ability known as environmental resilience. Among these, we focus on nitrogen environmental resilience, which refers to the plant’s ability to adapt to changing nitrogen conditions. Our research aims to elucidate the molecular basis by which plants optimize nitrogen uptake and utilization under fluctuating nitrogen availability.
Nitrogen is one of the most important nutrients supporting plant growth and productivity, and modern agriculture is heavily dependent on nitrogen fertilizers. However, a large portion of applied nitrogen is not taken up by crops and instead escapes into the environment, contributing to serious problems such as greenhouse gas emissions and water pollution. To address these challenges, it is essential to scientifically understand the intrinsic mechanisms by which plants manage nitrogen use and to apply this knowledge to agriculture.
Our research group is working to uncover the molecular mechanisms underlying nitrogen environmental resilience and to build a knowledge base that contributes to the realization of sustainable agriculture.
Main Research Topics:
1. Regulatory Mechanisms Underlying Plant Responses to Fluctuating Nitrogen Availability
Plants have evolved the ability to flexibly regulate nitrogen acquisition and utilization in order to survive in natural environments where nitrogen availability is often low and spatiotemporally variable. Our research investigates the molecular mechanisms that regulate gene expression and physiological responses in accordance with nitrogen status, with a particular focus on nitrogen sensing and signaling pathways.
1-1. Mechanisms for Efficient Uptake Under Low Nitrate Availability
In aerobic soils, nitrate is the primary nitrogen source for plants. Under low-nitrogen conditions, nitrate uptake is mediated by nitrate transporters located on the plasma membranes of root cells. Plants possess multiple transporter genes, and we are investigating how these genes are differentially regulated and functionally coordinated to enable efficient nitrogen uptake.
We have shown that the high-affinity nitrate transporter genes NRT2.1, NRT2.4, and NRT2.5 are expressed in a spatially and temporally complementary manner to support efficient nitrate acquisition under low-nitrate conditions.（Kiba et al. 2012; Lezhneva et al. 2014; Kiba and Krapp 2016).
1-2. Mechanisms for Optimizing Nitrogen Responses According to Nutrient Status
In nature, nitrogen availability fluctuates continuously rather than being simply sufficient or deficient. Plants adapt to this variability by coordinating nitrogen acquisition and utilization. These adaptive responses fall broadly into two categories: the nitrogen starvation response (NSR), which promotes nitrogen use under deficiency, and the nitrate response, which enables rapid adaptation to nitrate supply. We are investigating how plants integrate these responses to optimize nitrogen use.
NIGT1s are induced by nitrate via NLP transcription factors—master regulators of nitrate-responsive gene expression—and in turn act as feedback repressors of the nitrate response. This feedback loop, along with suppression of the nitrogen starvation response (NSR), allows plants to appropriately adjust to dynamic nitrogen conditions（Kiba et al. 2018; Maeda et al. 2018).
1-3. Mechanisms for Balancing Shoot and Root Growth According to Nitrogen Status
Plants maintain whole-organism growth balance by flexibly adjusting shoot and root growth in response to nitrogen availability. This optimization requires inter-organ communication. Plants achieve long-distance signaling through vascular transport via xylem and phloem, with plant hormones playing central roles. Cytokinin, in particular, functions as a long-distance signal that reflects root nitrogen status and regulates shoot and root growth accordingly.
We identified ABCG14, an ATP-binding cassette transporter that loads root-synthesized cytokinin into the xylem for shootward transport. ABCG14 is expressed in the root vasculature, and its loss leads to decreased cytokinin levels in xylem sap and severely reduced shoot growth, demonstrating the importance of cytokinin-mediated root-to-shoot signaling in coordinating growth (Ko et al. 2014; Kiba et al. 2019).
Furthermore, we showed that cytokinins with a trans-zeatin-type side chain are transported from roots to shoots and act as long-distance growth-promoting signals, based on studies using Arabidopsis and rice（Kiba et al. 2013; Kiba et al, 2023).
2. Mechanistic Insights into the Integration of Nitrogen and Stress Responses
In natural environments, nitrogen availability fluctuates alongside other environmental factors such as temperature, light, and the availability of other nutrients. To optimize growth and survival, plants must coordinate their responses to these multiple cues. We aim to elucidate the molecular and physiological mechanisms that integrate nitrogen responses with other environmental stress responses.
2-1. Crosstalk Between Nitrogen and Phosphorus Responses
Phosphorus, like nitrogen, is one of the major macronutrients essential for plant growth and productivity. In natural soils, phosphorus is often present in poorly available forms, placing plants at constant risk of phosphate starvation. Moreover, nitrogen and phosphorus availability are both subject to complex and continuous fluctuations in natural environments, requiring plants to coordinate their responses to both.
It is known that under nitrogen-sufficient conditions, phosphate starvation responses are activated, whereas under nitrogen-deficient conditions, these responses are suppressed. Additionally, phosphate starvation has been shown to inhibit the uptake of nitrogen sources. We have identified the GARP-type transcriptional repressors NIGT1.1–1.4 (NIGT1s) as key integrators of these responses. NIGT1s are induced when nitrogen is sufficient and serve to suppress nitrogen responses while enhancing phosphate starvation responses. When nitrogen is limited, NIGT1 expression remains low, and phosphate starvation responses are not activated.
Furthermore, we demonstrated that PHR1, a central regulator of phosphate starvation responses, directly promotes NIGT1 expression. This explains how phosphate starvation can suppress nitrogen uptake at the molecular level. These findings indicate that NIGT1s function as a regulatory hub that adjusts the balance between nitrogen and phosphate responses according to the plant’s nutritional status (Kiba et al. 2018; Maeda et al. 2018; Ueda et al. 2020).
Prioritize N-starvation responses
Activate P-starvation responses
2-2. Crosstalk Between Nitrogen Responses and Other Environmental Stress Signals
We are also actively investigating how nitrogen signaling interfaces with other environmental and nutritional stress responses, including those triggered by temperature and additional nutrient imbalances.
