Quantitative genetic analysis reveals potential to breed for improved white clover growth in symbiosis with nitrogen-fixing Rhizobium bacteria

Abstract

White clover (Trifolium repens) is integral to mixed pastures in New Zealand and temperate agriculture globally. It provides quality feed and a sustainable source of plant-available nitrogen (N) via N-fixation through symbiosis with soil-dwelling Rhizobium bacteria. Improvement of N-fixation in white clover is a route to enhancing sustainability of temperate pasture production. Focussing on seedling growth critical for crop establishment and performance, a population of 120 half-sibling white clover families was assessed with either N-supplementation or N-fixation via inoculation with a commercial Rhizobium strain (TA1). Quantitative genetic analysis identified significant (p < 0.05) family additive genetic variance for Shoot and Root Dry Matter (DM) and Symbiotic Potential (SP), and Root to Shoot ratio. Estimated narrow-sense heritabilities for above-ground symbiotic traits were moderate (0.24–0.33), and the strong (r ≥ 0.97) genetic correlation between Shoot and Root DM indicated strong pleiotropy or close linkage. The moderate (r = 0.47) phenotypic correlation between Shoot DM under symbiosis vs. under N-supplementation suggested plant growth with mineral-N was not a strong predictor of symbiotic performance. At 5% among-family selection pressure, predicted genetic gains per selection cycle of 19 and 17% for symbiotic traits Shoot DM and Shoot SP, respectively, highlighted opportunities for improved early seedling establishment and growth under symbiosis. Single and multi-trait selection methods, including a Smith-Hazel index focussing on an ideotype of high Shoot DM and Shoot SP, showed commonality of top-ranked families among traits. This study provides a platform for proof-of-concept crosses to breed for enhanced seedling growth under Rhizobium symbiosis and is informative for other legume crops.

Screening and field evaluation of white clover rhizobia for New Zealand pastures

“…Recent work by Weith et al . (2022) in a white clover breeding pool characterised the clover quantitative genetics of this symbiosis, indicating opportunities to breed for improved white clover growth in symbiosis with Rlt . In contrast, a study by Moeskjær et al .…”

“…Seedlings were inoculated with individual rhizobium isolates and grown in a controlled temperature growth room using methods described in Weith et al . (2022). Briefly, 500 g of Grade 3 vermiculite was packed into a 4.4 L container and saturated with 2 L of low-N McKnight’s solution.…”

“…A total of 16 germinated seeds per cultivar were transplanted individually into the vermiculite of the allocated quadrant at 1 × 1 cm spacing in four rows of four seedlings in each container. Suspensions of rhizobium inoculants (1 × 10 7 cells mL −1 ) were prepared using sterile deionised water as described previously (Weith et al . 2022), and a 0.5 mL aliquot of the appropriate inoculant was added to the base of each seedling.…”

“…A total of 16 germinated seeds per cultivar were transplanted individually into the vermiculite of the allocated quadrant at 1 × 1 cm spacing in four rows of four seedlings in each container. Suspensions of rhizobium inoculants (1 × 10 7 cells mL −1 ) were prepared using sterile deionised water as described previously (Weith et al 2022), and a 0.5 mL aliquot of the appropriate inoculant was added to the base of each seedling. Seedlings in the negative control treatment received a 0.5 mL aliquot of sterile deionised water, and seedlings in the N positive control received three separate 1 mL aliquots of 15 mmol L −1 NH 4 NO 3 (1.2 g L −1 ), with the first dose applied 9 days after sowing and weekly thereafter.…”

White Clover (Trifolium repens L.) Benefits in Grazed Pastures and Potential Improvements

“…Spill-over benefits of rhizobium have been demonstrated, with the white clover Rhizobium strain TA1 conferring tolerance against Cd toxicity, an impurity in phosphate fertilisers which may have toxic effects on both plant growth and rhizobia activity [167]. Whether variation in nitrogen fixation capacity by genotypes within populations of white clover can be exploited for more effective symbiotic outcomes is yet to be determined [168].…”