Abstract
Key messageExposure of wheat to high temperatures during male meiosis prevents normal meiotic progression and reduces grain number. We define a temperature-sensitive period and link heat tolerance to chromosome 5D.This study assesses the effects of heat on meiotic progression and grain number in hexaploid wheat (Triticum aestivum L. var. Chinese Spring), defines a heat-sensitive stage and evaluates the role of chromosome 5D in heat tolerance. Plants were exposed to high temperatures (30 or 35 °C) in a controlled environment room for 20-h periods during meiosis and the premeiotic interphase just prior to meiosis. Examination of pollen mother cells (PMCs) from immature anthers immediately before and after heat treatment enabled precise identification of the developmental phases being exposed to heat. A temperature-sensitive period was defined, lasting from premeiotic interphase to late leptotene, during which heat can prevent PMCs from progressing through meiosis. PMCs exposed to 35 °C were less likely to progress than those exposed to 30 °C. Grain number per spike was reduced at 30 °C, and reduced even further at 35 °C. Chinese Spring nullisomic 5D-tetrasomic 5B (N5DT5B) plants, which lack chromosome 5D, were more susceptible to heat during premeiosis–leptotene than Chinese Spring plants with the normal (euploid) chromosome complement. The proportion of plants with PMCs progressing through meiosis after heat treatment was lower for N5DT5B plants than for euploids, but the difference was not significant. However, following exposure to 30 °C, in euploid plants grain number was reduced (though not significantly), whereas in N5DT5B plants the reduction was highly significant. After exposure to 35 °C, the reduction in grain number was highly significant for both genotypes. Implications of these findings for the breeding of thermotolerant wheat are discussed.
Highlights
Wheat is the most widely grown cereal crop in the world, and of the three major cereal food crops, it is the most sensitive to temperature increases (Tripathy et al 2009)
Precise staging of the pollen mother cells (PMCs) was a key part of this study, but defining the start of meiosis, and the transition points
From the staging defined by Bennett et al (1973), this would be classified as premeiotic interphase stage 3
Summary
Wheat is the most widely grown cereal crop in the world, and of the three major cereal food crops (wheat, rice and maize), it is the most sensitive to temperature increases (Tripathy et al 2009). Global temperatures are predicted to rise throughout the 21st Century (Intergovernmental Panel on Climate Change 2014), and it has been estimated that for each °C of temperature increase, global wheat production will decrease by 6% (Asseng et al 2015), and that an increase in temperature of only 4 °C could result in major risks to food security (Porter et al 2014). Wheat is more sensitive to high temperature during its reproductive phase than when it is in its vegetative phase (Fischer and Maurer 1976). The optimum temperature range for wheat growth is generally considered to be around 17–23 °C over the course of an entire growing season (Porter and Gawith 1999), and temperatures higher than this during wheat’s floral development can result in a reduction in grain yield (Fischer and Maurer 1976; Fischer 1985; Wardlaw et al 1989). Saini and Aspinall (1982), for example, observed a substantial lowering in grain yield when wheat plants were exposed to 30 °C for 1–3 days between the
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