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Having originated in tropical regions, rice is more sensitive to cold stress than other grain crops, such as rye, wheat, and barley, which originated in temperate regions. Despite the limited knowledge about the exact mechanisms of cold tolerance, breeders have generated new rice cultivars that can grow in Hokkaido, Japan’s northern limit of rice cultivation, for more than a hundred years. However, greater cold tolerance in rice would enable more stable production and elevate productivity. Because cold tolerance is a complex quantitative trait, it is difficult to pyramid the genes that could improve plant tolerance to this abiotic stress. Therefore, it is important to develop effective strategies for improving cold tolerance in rice. In this review, the use of a ‘cross-tolerance’ strategy is proposed, and the current status of such strategy introduced. Discipline: Biotechnology, Crop production Additional key words: abiotic stress, reactive oxygen species, heat shock factor *Corresponding author: e-mail hyasuda@affrc.go.jp Received 28 February 2016; accepted 10 June 2016. Introduction The expansion of areas for rice production is important for world food security in the face of significant population growth, particularly in Asia and Africa. Hokkaido, in northern Japan, represents the northern limit for the economical production of rice. Rice breeders in Hokkaido must contend with cool summers and relatively short durations of sunshine during rice heading. However, rice growers in the area still face such problems as ensuring the cropping season and avoiding chilling damage. There are two major critical periods for the cultivation of rice in cold regions. These include the stage from germination through seedling growth and the booting stage. Low temperatures adversely affect germination and early growth, as rice seedlings are sensitive to cold. Therefore, farmers grow rice seedlings in greenhouse nurseries to ensure germination and adequate early development of the seedlings. Later, cold stress at the booting stage inhibits pollen development, which leads to reduced fertility and yield. Cold tolerance at the booting stage is a more important issue as it directly affects farmers’ incomes. Over the past few decades, conventional breeding has generated rice cultivars with cold tolerance at the booting stage, thereby gradually improving cold tolerance. The adverse effects of cold stress have thus been avoided at early stages by a special cultivation technique and at the booting stage by the effective use of genetic resources. Global warming can certainly increase the incidence of extreme weather patterns and events, such as hot and cool summers or localized torrential rains. Even now, plant cold tolerance is one of the most important agricultural problems to be addressed in the Hokkaido region. Due to their immobility, plants have evolved several molecular mechanisms to withstand abiotic stresses in the environment. Therefore, the mechanisms of tolerance to abiotic stresses due to the changeable environment should be investigated in detail, with the resulting information being used to create more stable and higher-yielding crops. We have been conducting experiments designed to control the cold tolerance of rice at two critical periods: from germination through the seedling stage, and at the booting stage. The results of these experiments suggest that cold tolerance at both stages might be enhanced by the overexpression of reactive oxygen species (ROS)-scavenging enzymes and activation of the heat shock response under cold conditions. In this review, we discuss the relationship between cold tolerance, ROS, and the heat shock response,

Cross-tolerance to Thermal Stresses and its Application to the Development of Cold Tolerant Rice