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.