Effective Pollen-Fertility Restoration Is the Basis of Hybrid Rye Production and Ergot Mitigation

Abstract

Hybrid rye breeding leads to considerably higher grain yield and a higher revenue to the farmer. The basis of hybrid seed production is the CMS-inducing Pampa (P) cytoplasm derived from an Argentinean landrace and restorer-to-fertility (Rf) genes. European sources show an oligogenic inheritance, with major and minor Rf genes, and mostly result in low-to-moderate pollen-fertility levels. This results in higher susceptibility to ergot (Claviceps purpurea) because rye pollen and ergot spores are in strong competition for the unfertilized stigma. Rf genes from non-adapted Iranian primitive rye and old Argentinean cultivars proved to be most effective. The major Rf gene in these sources was localized on chromosome 4RL, which is also a hotspot of restoration in other Triticeae. Marker-based introgression into elite rye materials led to a yield penalty and taller progenies. The Rfp1 gene of IRAN IX was fine-mapped, and two linked genes of equal effects were detected. Commercial hybrids with this gene showed a similar low ergot infection when compared with population cultivars. The task of the future is to co-adapt these exotic Rfp genes to European elite gene pools by genomic-assisted breeding.

Improving Yield and Yield Stability in Winter Rye by Hybrid Breeding

“…The selection for individuals with a minimum of donor DNA was successful and the yield disadvantage associated with Rfp1 could be significantly reduced, but not completely eliminated [ 68 ]. As a consequence of this yield loss, a restricted integration of Rfp genes from weedy rye into elite pollinator synthetics resulting in a restorer index of ~50% is considered a viable practice [ 71 , 72 ]. However, CMS-based hybrids with an unsatisfactory restoration level and reduced pollen shedding are notably susceptible to ergot as the fungal spores have no competitors during the infection of the ovary [ 69 ].…”

“…It needs to be emphasized that rainy weather at flowering time reduces pollen shedding and pollen movement. A suggested restorer index of ~50% [ 71 , 72 ] is a tribute to the undesired impact of major restorer genes such as Rfp1 on grain yield but may result in insufficient quantities of pollen to combat the fungus adequately in farmers’ fields, as wet pollen agglutinates and distributes over short distances only. In fact, multi-environmental data on natural ergot infection provided by the Polish Research Centre for Cultivar Testing (COBORU) confirm this assumption ( Figure 7 ).…”

“…The median ergot severity of all hybrids in 2020 is only slightly below the median of OPVs. Notably, the median ergot severity of hybrids traded as Rfp1 carriers [ 72 ] was greater in 2017, 2018, and 2019 as compared to hybrids without this restorer gene, the latter of which were evaluated without a blend of population rye. More precisely, in the period from 2017 to 2021 the level of ergot sclerotia in unprocessed rye was above the upcoming threshold of 0.2 g/kg in 83%, 40%, 42%, 100% and 20% of the evaluated Rfp1 hybrids.…”

“…According to the German descriptive variety list, these two hybrids were rated with a medium expression of ergot susceptibility, but with the hybrid released in 2016 ( Figure 6 ) counting among the eight cultivars with very high yield potential. To conclude, this comprehensive multi-environmental dataset contravenes observations of ergot severity upon natural infections in eight other locations [ 72 ]. As a consequence, this extended assessment enables an open, evidence-based, and balanced discussion of the current strategy to use Rfp1 for ergot defense in rye.…”

“…Another reason for the distinguished ergot defense pattern between hybrid cultivars might refer to different selection strategies in the pollinator pool. In hybrids traded with Rfp1 , selection against adapted restorer genes from European rye germplasm in the pollinator pool likely has contributed to weakening ergot defense upon natural infection, as, in hybrids with a restorer index of ~50% [ 71 , 72 ], the frequency of male sterile plants is very high. The driving force for this selection strategy refers to an increased selection efficiency for strong pollen-fertility restoration by Rfp1 [ 72 ].…”