Covariation among Root Biomass, Shoot Biomass, and Tiller Number in Three Rice Populations

Breeding for improved rice (Oryza sativa L.) root systems could improve crop yield by increasing water and nutrient uptake while also protecting the environment by reducing excess nutrients in runoff and methane emissions. Collection and measurement of roots is laborious, preventing direct selection for root biomass (RB) in large breeding populations. Tiller and root development are physiologically connected in rice where the root system is composed predominantly of nodal roots. Each tiller and its associated roots emerge from the same node on an originating stem. Here, we hypothesized that genes affecting tiller number (TN) also have sufficiently large effect on shoot biomass (SB) and RB that selections made on TN can be used to impart improvements in RB and SB. Associations among TN, RB, and SB were evaluated in three recombinant inbred line (RIL) rice mapping populations by first selecting subsets that were divergent for TN to determine if those contrasting RILs exhibited corresponding shifts in SB, RB, SB/RB, RB/TN, or SB/TN. Traits were measured in 6-wk-old potted plants. Selection for high vs. low TN resulted in corresponding shifts in RB and SB in all three populations, suggesting that it might be common for TN genes to also affect RB and SB through pleiotropy or linkage. However, further quantitative trait locus (QTL) study using 62 RILs of one population found instead that just one of the six detected TN-QTL was co-located with a RB-QTL. Gene x gene interactions were detected, with TN selections causing larger shifts in RB and SB among a sd1 (semidwarf) subgroup than among the Sd1 sibling RILs in one of the populations.