Breakthrough Discovery at University of Tsukuba

Researchers at the University of Tsukuba have made a significant advance in plant science by identifying the SlIAA9 gene as a critical factor in enabling tomato seeds to germinate and grow under high-temperature conditions. This finding, detailed in a recent study published in Plant Physiology and Biochemistry, highlights how mutations in SlIAA9 allow tomato mutants to maintain robust germination rates even when exposed to prolonged heat stress, a challenge increasingly faced by farmers worldwide due to climate change.

The study, led by Associate Professor Seung Won Kang from the Institute of Life and Environmental Sciences, utilized two independent loss-of-function mutant lines of the tomato variety Micro-Tom. While wild-type tomatoes showed drastically reduced germination rates, shortened shoots and roots, and abnormal seedling development under heat stress simulating temperatures above 32 degrees Celsius, the SlIAA9 mutants thrived. This resilience positions the gene as a promising target for developing climate-resilient crop varieties.

University of Tsukuba, renowned for its contributions to agricultural sciences—ranking among Japan's top institutions in this field—continues to lead innovations through its Tsukuba Plant Innovation Research Center (T-PIRC). T-PIRC focuses on creating next-generation high-value plants, and this research exemplifies their commitment to sustainable agriculture.

The Growing Threat of Heat Stress to Tomato Production

Tomatoes, a staple in Japanese cuisine and a major crop with annual production exceeding 1.8 million tons, are highly sensitive to temperature fluctuations. High temperatures during the germination phase induce thermo-dormancy or thermo-inhibition, where seeds fail to sprout even after conditions improve. In Japan, record heatwaves like the one in 2025, which raised average summer temperatures by 2.36 degrees Celsius, have led to sunburn on fruits, declined pollination rates, and reduced yields for tomatoes and other vegetables.

Climate projections indicate more frequent extreme weather, exacerbating these issues. For instance, heat stress can reduce tomato yields by up to 75 percent in severe cases globally, and similar patterns are emerging in Japan, where summer heat disrupts supply chains to major markets like Tokyo. Seed germination failure is particularly detrimental, as it affects stand establishment and overall crop productivity.

This vulnerability underscores the urgency for genetic solutions. Traditional breeding has limitations, but insights into genes like SlIAA9 offer a molecular pathway to engineer tolerance without compromising fruit quality or yield.

Unraveling the Role of SlIAA9 in Auxin Signaling

SlIAA9, or Solanum lycopersicum Indole-3-Acetic Acid Inducible 9, is a transcriptional repressor in the auxin signaling pathway. Auxin, a key plant hormone, regulates seed germination, growth, and stress responses. Normally, SlIAA9 suppresses auxin-responsive genes, but its absence in mutants derepresses these pathways, enhancing adaptability.

Prior research linked SlIAA9 mutations to parthenocarpy—seedless fruit development—but the Tsukuba team focused on germination under heat. The mutants not only germinated effectively but also showed vigorous post-germination growth, with normal seedling morphology despite exposure to 40-45 degrees Celsius for weeks.

Experimental Design and Molecular Insights

The Tsukuba researchers exposed seeds to controlled high-temperature regimes mimicking field conditions. Wild-type seeds exhibited high reactive oxygen species (ROS) accumulation, leading to cellular damage, while mutants upregulated antioxidant enzymes like superoxide dismutase and catalase, mitigating oxidative stress.

Heat shock protein 70 (HSP70), which chaperones proteins against denaturation, was strongly induced in mutants. Hormonally, abscisic acid (ABA) sensitivity—promoting dormancy—was reduced, while ethylene biosynthesis genes, aiding germination and recovery, were hyperactivated. These changes collectively bypass heat-induced inhibition.

The full study, accessible via DOI: 10.1016/j.plaphy.2026.111103, provides detailed transcriptomic data supporting SlIAA9 as a negative regulator of heat resilience. Funded by SATREPS (JST-JICA), it bridges Japanese and international agricultural research.

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Implications for Japan's Agricultural Sector

Japan's tomato industry, valued at billions of yen, faces mounting pressure from warming climates. Greenhouse cultivation dominates, but summer heatwaves still cause quality drops and yield losses of 20-30 percent in affected regions. Integrating SlIAA9 modifications via CRISPR or marker-assisted breeding could stabilize germination, ensuring reliable seedling establishment even in overheated nurseries.

For prefectures like Hokkaido and Tochigi, major producers, this means extended growing seasons and reduced reliance on imports. The Ministry of Agriculture, Forestry and Fisheries (MAFF) emphasizes climate adaptation, and Tsukuba's work aligns with national goals for resilient horticulture.

More details on Japan's tomato challenges can be found in MAFF reports, highlighting a 0.15 percent annual production dip projected through 2027 without interventions.

Tsukuba Plant Innovation Research Center's Pioneering Role

T-PIRC at University of Tsukuba drives innovation in high-function plants, fostering industry-academia collaborations. Assoc. Prof. Kang's team builds on prior SlIAA9 studies for parthenocarpy, expanding to abiotic stress tolerance. The center's ecosystem supports startups commercializing traits like heat tolerance.

University of Tsukuba ranks highly in QS Agriculture & Forestry (top 100 Asia) and US News Agricultural Sciences (top 340 globally), making it a hub for such breakthroughs. Collaborations with JIRCAS enhance tropical crop applications, relevant for Japan's overseas aid.

Pathways to Breeding Heat-Resilient Tomatoes

Targeted SlIAA9 editing promises varieties with stable germination above 35 degrees Celsius. Step-by-step: Identify alleles via sequencing, use CRISPR-Cas9 for precise knockouts, screen for thermos-tolerance, and field-test in Japanese greenhouses. Multi-trait stacking with disease resistance could yield superior cultivars.

Challenges include regulatory approval for gene-edited crops—Japan's fast-track system aids this—and consumer acceptance. Success stories like non-browning mushrooms demonstrate feasibility.

This approach extends to other Solanaceae like peppers and eggplants, diversifying Tsukuba's impact.

Global Relevance and Japan's Leadership

While Japan imports much of its tomatoes, domestic innovation boosts food security. Globally, heat stress threatens 20-40 percent yield losses by 2050; SlIAA9 insights aid breeding in India, Southeast Asia. SATREPS funding underscores Japan's role in sustainable development.

Stakeholders: Farmers gain reliable crops; policymakers advance carbon-neutral ag; researchers open new auxin-heat crosstalk avenues.

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Future Directions and Ongoing Research

Next: Validate in elite varieties, assess fruit yield under field heat, explore interactions with drought. Kang's lab pursues SATREPS projects for tropical veggies. Tsukuba eyes AI-accelerated breeding.

Actionable insights: Seed companies prioritize SlIAA9 markers; universities expand genomics courses. This discovery exemplifies higher education's vital role in addressing climate-agriculture nexus.