VRN and flowering gene effects
Since the reproductive stage is the period most sensitive to frost, flowering time becomes very critical. Ideally, late flowering is helpful in avoiding frost events, which happens mostly during early spring. This is, however, contradictory to the needs of avoiding terminal drought, which requires early maturity (Nazim Ud Dowla et al., 2018). Being able to combine early flowering with reproductive frost tolerance is thus highly desirable for wheat breeding. In the previous study, frost QTL were detected in the same region as the Vrn1_5A (VrnA1a ) allele on chromosome 5A (Sutka & Snape, 1989; Galiba et al., 1995). It has been reported that plants with VRN1 copy showed normal flowering but reducing frost tolerance (Dhillon et al., 2010). In our study, significant frost QTL were detected close to the Vrn1_5Aallele in both the Sp7A and SpB populations at the Williams location. In the SpM population grown in Muresk, significant frost QTL were detected close to the Vrn1_5D (VrnD1a ) allele, where the QTL for anthesis and maturity were located for all three environments, which further validated the tight connections between frost damage and the early anthesis alleles VrnA1a and VrnD1a . TheVrnB1a allele, on the other hand, segregated in the BW, SpB, Sp7A, and St3B populations, with anthesis QTL closely linked to theVrnB1a gene in the SpB and St3B population. Strikingly, no frost QTL was detected on chromosome 5B in any of the six populations studied at the two locations. The link between frost and anthesis was not observed on 5B. In our previous study on days to anthesis, the contribution of vrnb1 was less than that of vrnd1 andvrna1 , whereas VrnB1a made a larger contribution to days of grain filling than VrnD1a (Zhang et al., 2014). In the historical lines studied, the frequency of spring alleles VrnA1aand VrnD1a in recent varieties amounted to 0.62 and 0.40, respectively, while the frequency of VrnB1a diminished from 0.85 to 0.52 over time. An increased proportion of VrnA1a andVrnD1a may increase the risk of frost damage. The most popular variety, Mace, only possesses theVrnB1a allele and shows good frost tolerance. This clearly demonstrates that the early flowering phenotype induced by VrnB1a is associated with higher frost tolerance. Although the underlying mechanism is unknown, it can be speculated that the metabolites associated with the VrnB1afunctional network may contribute to the increased tolerance (Cheong et al., 2019). Because of the global warming, the earlier flower lines without frost damage are highly demanded (Zheng et al., 2016). Our results revealed that the VrnB1a allele can be utilised in breeding for frost tolerance.
Another flowering time related gene, VRN-B3 , was mapped to the short arm of chromosome 7B (7B: 9.7 Mb) (Yan et al., 2006). In the SpM population, VRN-B3 is tightly linked to a 7B short arm frost QTL that mapped on the top region (7BM55522) of the chromosome. This implies that VRN-B3 contributes to the frost phenotype in the SpM population. In addition, the TaVRT genes on group 7 appeared to be tightly linked to the QTL of anthesis and frost. The TaVRTgene is independently modulated by photoperiod and vernalization. It can inhibit VRN1 activity through binding to the CArG box of theVRN1 promoter in vivo. After vernalization, both TaVRT2and VRN2 functions are repressed, which causes VRN1accumulation for the transition from vegetative to reproductive phase (Li & Dubcovsky, 2008). The functions of TaVRT genes seem to be critical in the SpM, Sp7A, SpB, and BW populations.