Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK.
Nuclear Institute for Agriculture and Biology, Faisalabad, Punjab, Pakistan.
BMC Plant Biol. 2024 May 9;24(1):384. doi: 10.1186/s12870-024-05098-1.
Semi-dwarfing alleles are used widely in cereals to confer improved lodging resistance and assimilate partitioning. The most widely deployed semi-dwarfing alleles in rice and barley encode the gibberellin (GA)-biosynthetic enzyme GA 20-OXIDASE2 (GA20OX2). The hexaploid wheat genome carries three homoeologous copies of GA20OX2, and because of functional redundancy, loss-of-function alleles of a single homoeologue would not be selected in wheat breeding programmes. Instead, approximately 70% of wheat cultivars carry gain-of-function mutations in REDUCED HEIGHT 1 (RHT1) genes that encode negative growth regulators and are degraded in response to GA. Semi-dwarf Rht-B1b or Rht-D1b alleles encode proteins that are insensitive to GA-mediated degradation. However, because RHT1 is expressed ubiquitously these alleles have pleiotropic effects that confer undesirable traits in some environments.
We have applied reverse genetics to combine loss-of-function alleles in all three homoeologues of wheat GA20OX2 and its paralogue GA20OX1 and evaluated their performance in three years of field trials. ga20ox1 mutants exhibited a mild height reduction (approximately 3%) suggesting GA20OX1 plays a minor role in stem elongation in wheat. ga20ox2 mutants have reduced GA content and are 12-32% shorter than their wild-type segregants, comparable to the effect of the Rht-D1b 'Green Revolution' allele. The ga20ox2 mutants showed no significant negative effects on yield components in the spring wheat variety 'Cadenza'.
Our study demonstrates that chemical mutagenesis can expand genetic variation in polyploid crops to uncover novel alleles despite the difficulty in identifying appropriate mutations for some target genes and the negative effects of background mutations. Field experiments demonstrate that mutations in GA20OX2 reduce height in wheat, but it will be necessary to evaluate the effect of these alleles in different genetic backgrounds and environments to determine their value in wheat breeding as alternative semi-dwarfing alleles.
半矮秆基因被广泛应用于谷类作物，以提高抗倒伏能力和同化分配。在水稻和大麦中应用最广泛的半矮秆基因编码赤霉素（GA）-生物合成酶 GA20 氧化酶 2（GA20OX2）。六倍体小麦基因组携带三个同源 GA20OX2 拷贝，由于功能冗余，单个同源基因的功能丧失突变不会在小麦育种计划中被选择。相反，大约 70%的小麦品种携带 REDUCED HEIGHT 1（RHT1）基因的功能获得性突变，这些基因编码负生长调节剂，并且对 GA 有反应而被降解。半矮化 Rht-B1b 或 Rht-D1b 等位基因编码对 GA 介导的降解不敏感的蛋白质。然而，由于 RHT1 广泛表达，这些等位基因具有多效性，在某些环境下赋予了不理想的性状。
我们应用反向遗传学方法，结合小麦 GA20OX2 及其旁系 GA20OX1 三个同源基因的功能丧失等位基因，并在三年的田间试验中对其性能进行了评估。ga20ox1 突变体表现出轻微的高度降低（约 3%），表明 GA20OX1 在小麦茎伸长中起次要作用。ga20ox2 突变体 GA 含量降低，比其野生型分离株矮 12-32%，与 Rht-D1b“绿色革命”等位基因的效果相当。ga20ox2 突变体在春小麦品种“Cadenza”中对产量构成因素没有显著的负面影响。
我们的研究表明，化学诱变可以在多倍体作物中扩展遗传变异，尽管对于某些目标基因来说，识别合适的突变是困难的，并且背景突变的负面影响也存在，但仍可以发现新的等位基因。田间试验表明，GA20OX2 突变降低了小麦的高度，但有必要评估这些等位基因在不同遗传背景和环境中的作用，以确定它们作为替代半矮化等位基因在小麦育种中的价值。
