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

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.

Condensed Tannins in White Clover (Trifolium repens) Foliar Tissues Expressing the Transcription Factor TaMYB14-1 Bind to Forage Protein and Reduce Ammonia and Methane Emissions in vitro.

Grazing ruminants contribute to global climate change through enteric methane and nitrous oxide emissions. However, animal consumption of the plant polyphenolics, proanthocyanidins, or condensed tannins (CTs) can decrease both methane emissions and urine nitrogen levels, leading to reduced nitrous oxide emissions, and concomitantly increase animal health and production. CTs are largely absent in the foliage of important temperate pasture legumes, such as white clover (Trifolium repens), but found in flowers and seed coats. Attempts at enhancing levels of CT expression in white clover leaves by mutagenesis and breeding have not been successful. However, the transformation of white clover with the TaMYB14-1 transcription factor from Trifolium arvense has resulted in the production of CTs in leaves up to 1.2% of dry matter (DM). In this study, two generations of breeding elevated foliar CTs to >2% of DM. The CTs consisted predominantly of prodelphinidins (PD, 75-93%) and procyanidins (PC, 17-25%) and had a mean degree of polymerization (mDP) of approximately 10 flavan-3-ol subunits. In vitro studies showed that foliar CTs were bound to bovine serum albumin and white clover proteins at pH 6.5 and were released at pH 2.-2.5. Using rumen in vitro assays, white clover leaves containing soluble CTs of 1.6-2.4% of DM significantly reduced methane production by 19% (p ≤0.01) and ammonia production by 60% (p ≤ 0.01) relative to non-transformed wild type (WT) controls after 6 h of incubation. These results provide valuable information for further studies using CT expressing white clover leaves for bloat prevention and reduced greenhouse gas emissions in vivo.

Elevation of Condensed Tannins in the Leaves of Ta-MYB14-1 White Clover (Trifolium repens L.) Outcrossed with High Anthocyanin Lines.

Condensed tannins (CT) are highly desirable in forage as they sequester dietary protein and reduce bloat and methane emissions in ruminants. However, the widely used forage legume white clover (Trifolium repens) only produces CTs in flowers and trichomes and at levels too low to achieve therapeutic effects. Genetic transformation with transcription factor Ta-MYB14-1 from Trifolium arvense was effective in inducing CTs to 0.6% of leaf dry matter. CT synthesis has been elevated further by crossing the primary white clover transgenic line with wild type genotypes producing the related phenylpropanoids, anthocyanins. CT levels in leaves were highest under the anthocyanin leaf marks associated with the "red midrib" trait; however, there was no evidence for CT accumulation in leaf sections with the "red V" anthocyanin marking. Ta-MYB14-1 was stably inherited in two generations of crosses, and T2 progeny produced up to 3.6-fold higher CTs than the T0 parent. The profile of small CT oligomers such as dimers and trimers was consistent in T0, T1, T2, and BC2 progeny and consisted predominantly of prodelphinidins (PD), with lesser amounts of procyanidins (PC) and mixed PC:PD oligomers.

Anthocyanin leaf markings are regulated by a family of R2R3-MYB genes in the genus Trifolium.

Anthocyanin pigments accumulate to form spatially restricted patterns in plants, particularly in flowers, but also occur in vegetative tissues. Spatially restricted anthocyanin leaf markings are poorly characterised in plants, but are common in forage legumes. We hypothesised that the molecular basis for anthocyanin leaf markings in Trifolium spp. is due to the activity of a family of R2R3-MYB genes. R2R3-MYB genes were identified that are associated with the two classic pigmentation loci in T. repens. The R locus patterns 'red leaf', 'red midrib' and 'red fleck' are conditioned by a single MYB gene, RED LEAF. The 'diffuse red leaf' trait is regulated by the RED LEAF DIFFUSE MYB gene. The V locus was identified through mapping two V-linked traits, 'V-broken yellow' (Vby) and 'red leaflet' (Vrl). Two highly similar R2R3-MYB genes, RED V-a and RED V-b, mapped to the V locus and co-segregated with the RED V pigmentation pattern. Functional characterisation of RED LEAF and RED V was performed, confirming their function as anthocyanin regulators and identifying a C-terminal region necessary for transactivation. The mechanisms responsible for generating anthocyanin leaf markings in T. repens provide a valuable system to compare with mechanisms that regulate complex floral pigmentation.