Abstract
BackgroundThe harvest index (HI) is a measure of the biological success of forming harvestable products. However, our understanding of the genetic basis of HI in rice (Oryza sativa L.) is limited, because it is a complex trait consisting of various yield-related traits and physiological attributes. YTH183 is a high-yielding line with large panicles and high HI derived from a cross between the Indica Group variety IR 64 and the NPT line IR 69093-41-2-3-2 (YP5).ResultsHere, we detected two novel QTLs for HI, designated qHI5.1 on chromosome 5 and qHI8.1 on chromosome 8, by using 155 recombinant inbred lines (RILs) derived from the cross between IR 64 and YTH183. The YTH183 allele at qHI5.1 contributed to a wide grain, resulting in heavy grain weight and panicle weight, and was consistently effective under the different environmental conditions of subtropical (Ishigaki) and temperate (Tsukuba) regions. Genetic polymorphism revealed that qHI5.1 was identical to GSE5/GW5, which is known to control the grain weight. On the other hand, although qHI8.1 functioned additively with qHI5.1 for higher HI, it did not show any significant effect on grain or panicle weight. In addition, its effects on HI were shown only in the first seasons at Ishigaki but not at Tsukuba or in the second season at Ishigaki.ConclusionOur results indicate that qHI5.1 controls the grain size, regardless of whether environmental conditions are of subtropical or temperate regions, while qHI8.1 might be involved in controlling the physiological processes of source ability or the translocation of photosynthesis products from vegetative organs to grains depending on environmental conditions during the maturing stage. These QTLs will be useful genetic resources for future breeding programs to break through the ceiling of maximum yield in Indica Group varieties.
Highlights
The harvest index (HI) is a measure of the biological success of forming harvestable products
HI and Yield-Related Traits in the Two Parental Lines The heading dates of IR 64 at Ishigaki were on May 30 in 2016, June 3 in 2017, and May 30 in 2018; on the other hand, the heading dates of YTH183 at Ishigaki were on May 28 in 2016, June 2 in 2017, and May 28 in 2018 (Supplemental Table 1)
In the yield-related traits, there was no significant difference in PN or Total number of spikelets per panicle (TS) between YTH183 and IR 64, grain weight (GWt) and Fertility rate 5 (FR) were higher in YTH183 than in IR 64, resulting in a higher Panicle weight (PW) in YTH183 (Fig. 1c)
Summary
The harvest index (HI) is a measure of the biological success of forming harvestable products. YTH183 is a high-yielding line with large panicles and high HI derived from a cross between the Indica Group variety IR 64 and the NPT line IR 69093-41-2-32 (YP5). NPTs are varieties harboring low tiller number, few unproductive tillers, and a large number of grains per panicle, and a phenotype expected to result in high yield potential (Peng et al 1994). NPTs developed from the crosses between the tropical Japonica Group varieties and the improved Indica Group varieties have been released since 1993; grain yields have been disappointing because of low biomass production and poor grain filling (Peng et al 2008). To achieve high grain filling and increase yield potential, in terms of improvement of the sink-source balance, it is necessary to simultaneously improve both sink capacity (a large number of spikelets per panicle, heavy grains, and stable grain filling) and source ability (carbohydrate supply) to accommodate the large sink size under high temperatures
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