Mitigation of heat-induced spikelet sterility at flowering by transferring a QTL for the early-morning flowering trait, <i>qEMF3</i>, into the genetic background of <i>japonica</i> cultivars

<p>High temperatures over 35℃ at anthesis induces spikelet sterility. There is a concern that high-temperature-induced spikelet sterility will decrease rice yield due to global warming. We have developed near-isogenic lines (NILs) carrying the QTL (qEMF3) for avoiding high-temperature-induced sterility at anthesis by flowering at a cooler temperature in the early morning in the indica genetic background. We demonstrated that high-temperature-induced sterility can be reduced by using NILs. However, it has not been proved whether qEMF3 induces early-morning flowering (EMF) in the genetic background of japonica rice, and whether it produces the effect of avoiding high-temperature-induced sterility.</p><p>In this study, qEMF3 was introduced into the genetic background of four japonica rice cultivars, “Hitomebore”, “Hinohikari”, “Nikomaru”, and “Toyomeki”, by backcrossing. By using the NILs, early-morning flowering and the avoidance of high-temperature-induced sterility were verified. In a pot test of high-temperature treatment at only one flowering day in growth chambers in which the temperature was raised from 6:00 a.m., each qEMF3 NIL flowered 2–4 hours earlier than the recurrent parent cultivar under all genetic backgrounds. qEMF3 NILs mostly finished flowering by 10:30 a.m., when the temperature reached 35°C.</p><p>As a result, each qEMF3 NIL did not encounter high-temperature stress conditions at anthesis and could maintain a high level of fertility. On the other hand, in an experiment conducted on a concrete-framed paddy field in a greenhouse in Kagoshima Prefecture in Japan, where continuous high-temperature treatment at around 35℃ was performed during the heading stage, each qEMF3 NIL showed a higher seed fertility rate than the recurrent parent cultivar. From this, it was concluded that the EMF QTL (qEMF3) is an effective QTL for mitigating high-temperature-induced sterility even in the genetic background of japonica rice.</p>