Relationships between shoot and root growth support indirect selection for increased root biomass

Authors: Pinson, Shannon; Barnaby, Jinyoung; BUI, LIEM - Cuu Long Delta Rice Research Institute; CHUN, JAEBUHM - Rural Development Administration - Korea

Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/19/2018
Publication Date: 10/16/2018
Citation: Pinson, S.R., Barnaby, J.Y., Bui, L.T., Chun, J. 2018. Relationships between shoot and root growth support indirect selection for increased root biomass. Rice Technical Working Group Meeting Proceedings. February 19-22, 2018, San Diego, California. Electronic Publication.

Interpretive Summary:

Technical Abstract: The size and vigor of below-ground tissues (roots) impact the health and growth of above-ground plant tissues, and vice versa. One would therefore expect covariance between the growth dynamics of shoot and root structures. Furthermore, the root system of a rice plant is largely composed of nodal roots which are produced along with tillers from the same individual nodes. With tiller and root development being synchronized and physically connected in this manner, one would expect that genetic and environmental factors that stimulate the production of tillers in a rice seedling would also increase root biomass. The converse is also expected. By determining rates of water and nutrient acquisition, the size and architecture of a root system would be expected to impact shoot growth and development. Numerous genes and QTLs associated with the number and timing of rice tiller production have been reported. There have also been several studies defining optimal rice tillering ability as a key yield component. Other studies have identified QTLs affecting root structure and development in rice. However, because the tillering and root QTL studies were conducted independently in unrelated mapping populations, they could not evaluate covariance and trade-offs between tiller number (TN), shoot biomass (SB), and root biomass (RB). We examined relationships between tillering, root growth, and shoot growth in 6-wk-old seedlings of three populations of rice recombinant inbred lines (RILs): Lemont x TeQing RILs (LT-RILs), Kaybonnet-lpa x Zhe733 RILs (KZ-RILs), and Francis x Rondo RILs (FR-RILs). Six-wk old seedlings were evaluated based on previous LT-RIL and KZ-RIL TN QTL studies which showed that TN QTLs revealed by TN counts at 5- to 6-wks were more predictive of final panicle number and grain yield than were TN QTLs that became detectable only in later stages. Measurement of RB is difficult and laborious. With the known physical synchrony between tiller and root initiation, we were particularly interested in determining if TN was sufficiently predictive of RB for it to serve as a non-destructive surrogate in future research and breeding. Relationships between TN, SB, and RB were therefore evaluated by first selecting divergent high-TN versus low-TN subgroups among the three RIL populations, then determining the impacts of divergent TN selection on SB and RB. The LT-RILs and KZ-RILs had been previously characterized and mapped for TN QTLs at multiple seedling and mature plant stages; previously collected TN data were used to select high vs low TN subsets from the LT-RIL and KZ-RIL populations. FR-RIL selections were based on TN from 2 replications of 6-wk-old FR-RILs. Selected RILs plus parents of each population were then evaluated for TN, SB, and RB using 3 replications of greenhouse-grown single plants. At 6-wks after seeding, TN was counted then seedlings were gently uprooted, rinsed of soil, divided into shoots and roots, dried, then weighed to obtain SB and RB per plant. Our results showed that TN, SB, and RB were positively correlated in all three RIL populations. This suggests that laborious evaluation of RB could be avoided by using indirect selection for either TN or SB. TN offers the added benefit of providing non-destructive data prior to flowering, which speeds breeding by allowing for both selection and new crosses to be accomplished in a single generation. Our results further indicated impact of Sd1/sd1 on relationship between tillering and root growth traits. The sd1 semidwarf gene is known to improve grain yields by diverting carbohydrates and biomass from above-ground vegetative tissues and into grain. By selecting four progeny subsets from the KZ-RIL population, specifically, high and low TN selections of both Sd1/Sd1 and sd1/sd1 genotypes, we were able to ask if sd1 also alters the partitioning of re