Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2015
Publication Date: 3/1/2016
Citation: Pinson, S.R., Jia, Y. 2016. QTLs for early tiller production and relationships with rapid seedling growth and increased panicle number in rice. Crop Science. 56:505-519. doi: 10.2135/cropsci2014.09.0667.
Interpretive Summary: Tiller number (TN) is a key determinant of final grain yield in the small grains crops, including rice (Oryza sativa L.), and is desired to enhance competition with weeds. Quantitative trait loci (QTLs) associated with TN were mapped within a set of recombinant inbred lines developed from a cross between the low tillering tropical japonica cultivar ‘Kaybonnet-lpa1-1’ and the high tillering, high yielding, early maturing indica cultivar ‘Zhe733’ using data collected from greenhouse potted plants evaluated weekly. . Relationships between the five identified TN QTLs and related traits including panicle number (PN), flag leaf number (FLN), seedling leaf number (SLN), and earliness of tiller production were also evaluated. The three TN QTLs that were linked with early tiller emergence were also associated with PN; two additional TN not associated with PN were also identified. All TN QTLs were also SLN QTLs, though other loci affecting SLN were also identified. Several of the TN and SLN QTLs identified using data from young plants (e.g. = 6 weeks after planting) were not detectible in data collected just before or after heading. The three TN QTLs also associated with PN showed stronger effects (higher LODs and additive effects) on TN as observed in seedling stages than on final TN at heading time, and their effects diminished over time to the point that they were not always detectible in TN data collected at or after heading. The three TN QTLs that were also PN QTLs were robust enough that when TN counts collected once, six weeks after planting, observation of merely two plants in the Arkansas trial detected the QTL effects with 1/4th the labor and resources. The three loci associated with both TN and PN are the TN loci most likely to increase rice yield, and the markers presently found linked to them can be used for marker assisted breeding.
Technical Abstract: The ideal plant type, or idiotype, for rice is to create a moderate number of tillers, not too few or yield will be decreased by production of too few seed heads, nor too many which will divert energy and nutrients away from grain into excessive stems and leaves. While the rice cultivars grown today throughout Asia (generally of the indica ancestral group) appear to often waste energy by producing so many tillers that seedheads are incompletely filled, studies have shown that yields of the US tropical japonica rice cultivars could be improved with increased tillering. In this study, genetic loci (QTLs) affecting tiller number (TN) at various plant growth stages were identified using japonica x indica mapping populations. In order to distinguish between the TN genes that encouraged excessive tillering as opposed to those that encouraged an increase in panicle (seedhead) number (PN, and thus, hypothetically increase grain yield), QTLs for PN were also studied. Of the five TN QTLs, three had a positive effect on PN, and thus are the TN QTLs desired by rice breeders, whether in the US or Asia, for producing optimum TN. Using data on the speed of plant growth in weekly intervals (monitored leaf number and tiller number weekly), we also learned that the three QTLs with beneficial effects on both TN and PN were associated with very rapid plant growth (leaf number) and very early initiation of tillering, both detectable in the first 2 weeks of plant growth. Now that these positive associations between very early plant growth and production of tillers most likely to increase grain yield have been discovered, breeders can use this information to indirectly select plants most likely to develop increased PN and yield very early in the life cycle. The markers we found linked to the desirable and less desirable TN loci can also be used for marker assisted selection by breeders developing new rice varieties.