Rht genes associated with frost QTL
Significant frost QTL were detected in the proximity of the Rht1and Rht2 gene regions in the G7A and St3B populations, in which the two genes were segregating. In both populations, the frost QTL were contributed by the mutated type of Rht-B1b and Rht-D1b,the semi-dwarf alleles that are associated with short plant height. This indicates that the mutated or short plant genotype is more prone to frost. On the other hand, the wild type Rht-B1a andRht-D1a are relatively frost tolerant. In sum, shorter plants are frost susceptible while the taller plants are relatively frost tolerant. It is well known that lines with Rht-B1b and Rht-D1balleles have reduced sensitivity to gibberellic acid (GA), and GA-responsive growth is therefore repressed (Harberd et al., 2009). This may affect GA levels among the different combinations of dwarfing genes, and thus lead to varying flowering times. It has been reported that GA can increase the transcription level of SOC1 including a MADS-box gene, promoting flowering (Shi et al., 2019). In a study of ethylene effects in the GA-GID1-DELLA signalling pathway, ethylene leads to a reduction of GA, which delays floral induction (Harberd et al., 2009). Nevertheless, in the current study, no flowering QTL were detected in the Rht1 and Rht2 gene regions. This suggests that the regulatory function of the Rht-B1b and Rht-D1b genes on anthesis is minor. Selecting taller or wild-type rht alleles for frost tolerance in breeding programs will not cause delayed flowering time or maturity, thus these rht alleles can be recommended for frost tolerance breeding.
In previous studies, the semi-dwarfing alleles Rht-B1b andRht-D1b did not lead to higher grain yields in drought environments (Butler et al., 2005; Zhang et al., 2013). The QTL analysis showed that the rht alleles are associated with higher grain yields while the semi-dwarfing alleles are associated with lower grain yields (Zhang et al., 2013). Our results further demonstrated the disadvantages of the Rht-B1b and Rht-D1b alleles in frost-prone environments. From 1967 to 2015, the frequencies of theRht1 and Rht2 alleles have increased from 0 to 0.6 and 0.4 in Australian wheat varieties, respectively. This is in line with the first green revolution in that the dwarfing genes are utilised widely. However, a recent study has revealed that the short plant phenotype has become a bottleneck for wheat grain yield improvement (Butler et al., 2005; Zhang et al., 2013). Achieving high grain yield by increasing plant height has become a newly established breeding approach. Our study indicates that eliminating the dwarfing alleles Rht-B1b andRht-D1b in breeding can increase frost tolerance in wheat.
In summary, six DH populations planted at two different locations in 2018 were hit by a severe frost event at the critical reproductive phase. Evaluation of frost damage showed that the plant growth stage most susceptible to low temperature (<2 ⁰C) was during the young microspore stage (10-18 days before anthesis). Out of the 30 frost QTL detected, 18 major QTL were mapped onto 13 chromosomes (2A, 2B, 2D, 3A, 4A, 4B, 4D, 5A, 5D, 6D, 7D, 7A, and 7B). Most frost QTL overlapped or were closely linked to the QTL for anthesis and maturity Zadok stages as well as to anthesis-related genes. However, most of these QTL represented are related to escape mechanisms, i.e., the frost tolerance contribution by these QTL clearly stems from the late flowering alleles, indicating that they are not useful in wheat breeding for water-limited environments. The mutated VrnA1a , VrnD1a , Rht1 , andRht2 alleles and a high-copy number Ppd-B1 allele contributed significantly to frost damage. Nevertheless, QTL or genetic factors outside the escape mechanisms were detected in the current study. Anthesis QTL were repeatedly detected in the proximity of theVrnB1a region and on chromosome 1B, whereas no frost QTL were detected on these two chromosomes. These striking results strongly imply that the VrnB1a andTaFT3_1B alleles on 5B and 1B should be utilised in breeding for frost tolerance, as the early-flowering phenotype associated with these two genes is frost tolerant. Meanwhile, the recessive non-dwarfing alleles Rht-B1a(rht1 ) and Rht-D1a (rht2 ) associated with normal plant height could also be used in breeding for reproductive frost tolerance without delaying the flowering time.