Effects Of Dwarfing Gene Rht18 On Agronomic Traits And Its Response To Exogenous GA₃ In Common Wheat

The introduction of semi-dwarfing genes Rht-B1b?Rht1? and Rht-D1b?Rht2? into wheat was a crucial component of the “Green Revolution” in the 1960 s, which led to impressive increases in wheat yield worldwide. However, only a few dwarfing genes, mainly the GA-insensitive Rht-B1 b and Rht-D1 b, were widely used in wheat breeding program so far,which was not good for the sustainable development of wheat dwarf breeding. In deed, the GA-insensitive dwarfing genes Rht-B1 b and Rht-D1 b also contributed to reduced seedling vigor and coleoptile length, which limited their use in arid area. Thus, development and utilization of new dwarfing genes was very important for wheat breeding. Previous studies showed that the GA-responsive dwarfing genes, including Rht18, have potential to reduce plant height without compromising seedling vigor. The objective of this work was to obtain a comprehensive understanding on the effects of Rht18 from tetraploid wheat on agronomic traits and photosynthetic traits in common wheat, so as to promote the utilization of Rht18 in wheat breeding, and to understand wheather deficiency in GA biosynthesis or signaling pathway was involved in the dwarfism induced by Rht18.Three populations were developed by making separate crosses of three Chinese winter wheat cultivars, Xifeng20, Fengchan3 and Jinmai47, with Icaro, the Rht18 donor?tetraploid wheat?. The homozygous Rht18 individuals with hexaploid status of F2 population were selected based on the corresponding molecular marker detection and root-tip squash method,and to develop the F2:3, F3:4, F4:5 and F5:6 lines. Some stable common wheat dwarf lines containting Rht18 were breeding. Plant height, coleoptile length, seedling root characters,yield-related traits and other agronomic traits and photosynthetic traits were investigated to evaluate the effect of Rht18 on common wheat. Meanwhile, dwarf lines of Xifeng20/Icaro and Jinmai47/Icaro were treated with exogenous GA₃ for examining the response of Rht18 to exogenous GA₃ on coleoptile length, agronomic traits and yield components and to explore the possible role of Rht18 in GA biosynthesis or signaling. Additionally, the process of spike development in the Rht18 dwarf lines with dominant Vrn-A^t1 allele of tetraploid donor Icaro were investigated and compared with their tall parents. Further to select some Rht18 dwarf lines having potential to increase grain yield with proper flowering time. The main resultswere as follows:1. Genetic effects of Rht18 on agronomic traits and photosynthetic traitsAgronomic traits were investigated and compared in field experiments using F3:4, F4:5and F5:6 hexaploid dwarf lines with Rht18 derived from three crosses between its tetraploid donor Icaro and three tall Chinese winter wheat cultivars, Xifeng20, Fengchan3 and Jinmai47.The results showed that Rht18 significantly reduced plant height by 24.6%?12.6% and 14.0%,on average, in the dwarf lines of Xifeng20, Fengchan3 and Jinmai47 compared with their tall parents, respectively. Rht18 reduced plant height through shortening the length of internodes without affecting the number of internodes, and the wall thickness of the basal internodes was significantly increased, which was benefit for improving lodging resistance. The effects of Rht18 on yield components varied with the genetic background of the tall parents used. For the dwarf lines of Xifeng20, spikelet number spike-1 was increased by 5.1%, and grain number spike-1was significantly increased by 8.6% on average compared with Xifeng20, but1000-kernel weight was significantly decreased by 14.2%. In contrast, decreased spikelet number spike-1?-8.2% and-20.7%? and grain number spike-1?-6.4% and-2.3%? but increased1000-kernel weight?8.6% and 6.0%? were observed in the Rht18 dwarf lines of both Fengchan3 and Jinmai47, respectively. Biomass plant-1 and grain yield plant-1 were decreased in the Rht18 dwarf lines of all three crosses but harvest index was increased compared to the tall parents. Additionally, Rht18 had some adverse effects on kernel quality traits.Rht18 had no negative effects on coleoptile length and seedling root characters in the dwarf lines of Xifeng20/Icaro and Fengchan3/Icaro. For the dwarf lines of Jinmai47/Icaro,Rht18 restrained the elongation of coleoptiles while improved seedling root growth.The influences of Rht18 on photosynthetic traits in different growth stage were not obvious. SPAD values of flag leaf were significantly increased in the Rht18 dwarf lines compared with the tall parents, which may be benefit for keeping high photosynthetic capacity.2. The response of Rht18 to exogenous GA₃ in common wheatThe responses of Rht18 to exogenous GA₃ were investigated using F4:5 and F5:6 dwarf lines of Xifeng20/Icaro and Jinmai47/Icaro. The results showed that the application of exogenous GA₃ significantly increased coleoptile length while it had negative effects on seedling root growth both in the Rht18 dwarf lines and their tall parents. Exogenous GA₃ significantly promoted internodes elongation of dwarf lines and restored plant height to the levels of the tall parents. Plant height was significantly increased by 21.3% and 10.7% on average in the GA₃-treated dwarf lines of Xifeng20 and Jinmai47, respectively. Plant height of the tall parent Xifeng20 and Jinmai47 was not affected significantly by GA₃ treatment,increased only by 5.8cm?5.4%? and 2.5cm?2.7%?, respectively. Exogenous GA₃ increased flag leaf length while decreased flag leaf width in the Rht18 dwarf lines. SPAD values of flag leaves of the GA₃-treated dwarf lines were also decreased compared with the untreated ones.Compared to the untreated dwarf lines, the partitioning of dry matter to ears at anthesis was significantly decreased while the partitioning of dry matter to stems was significantly increased in the GA₃-treated ones. There was no obvious change in dry matter partitioning in the GA₃-treated tall parents. Exogenous GA₃ increased 1000-kernel weight while it significantly decreased grain number spike-1 both in the dwarf lines and their tall parents.1000-kernel weight was increased by 10.4% and 5.0% in the GA₃-treated dwarf lines of Xifeng20 and Jinmai47, respectively. While the grain number spike-1was significantly decreased by 9.3% and 9.0% compared with the untreated dwarf lines. Higher biomass plant-1and lower grain yield plant-1 resulted in a lower harvest index in the GA₃-treated dwarf lines and tall parents compared with the untreated ones. Additionally, exogenous GA₃ increased protein and gluten content of kernels and Zeleny sedimentation value in both the dwarf lines and tall parents while having no effect on moisture, starch and fibre content of kernels. Thus,exogenous GA₃ could restore plant height and other agronomic traits of Rht18 dwarf lines to be normal as tall parents. This study indicated that Rht18 dwarf mutants may be GA-deficient mutants with impaired GA biosynthesis.3. Eeffect of dominant Vrn-A^t1 on development progress of Rht18 dwarf linesProcess of spike development in the hexaploid F5:6 Rht18 dwarf lines with dominant Vrn-A^t1 allele of tetraploid donor Icaro was investigated and compared with their tall parents.The results showed that spike development of the Rht18 dwarf lines was significantly promoted by the dominant Vrn-A^t1 allele compared to their tall parents with recessive vrn-A1.The Rht18 dwarf lines reached double ridge stage more than 70 days earlier than their tall parents. Finally, flowering date of Rht18 dwarf lines was about 2⁶ days earlier than those of the tall parents. Accompany with the fast process of spike development, seedling growth of dwarf lines was also faster than those of the tall parents. Seedling height of dwarf lines was significantly higher than those of their tall parents at early growth stage. This indicated that Vrn-A^t1 promoted both vegetative and reproductive growth of the Rht18 dwarf lines, at least at the early growth stage. The effect of Rht18 on reducing plant height was masked by dominant Vrn-A^t1 until the late jointing stage.Greater florets initiated spike-1 and more fertile florets spike-1 at anthesis resulted in a significant increase in florets fertility and subsequently greater grain number spike-1 in the Rht18 dwarf lines of Xifeng20. Greater florets fertility was also observed in the dwarf lines of Fengchan3 and Jinmai47 although fewer florets initiated spike-1 and fertile florets wereproduced compared with their respective tall parents. The greater florets fertility of dwarf lines was highly related to the greater proportion of dry matter partitioned to the ears at anthesis.In conclusion, Rht18 had a moderate height reduction effect in common wheat and had no adverse effects on coleoptile length and seedling root characters. The effects of Rht18 on yield components varied with the genetic background of the tall parents used. Rht18 had no significant effect on photosynthetic traits in different growth stage. Rht18 was respond to exogenous GA₃ in common wheat, and exogenous GA₃ could restore the phenotypic characters of Rht18 dwarf lines to the levels of the tall parents, which indicated that Rht18 dwarf mutants may be GA-deficient mutants with impaired GA biosynthesis. Additionally,dominant Vrn-A^t1 allele from tetrapliod donor Icaro could significantly promote spike development of the Rht18 dwarf lines, which resulted in earlier flowering than the tall parents.The Rht18 dwarf lines obtained in this study can be used in wheat dwarf breeding.