Rice Genomics and Genetics 2024, Vol.15, No.3, 121-131 http://cropscipublisher.com/index.php/rgg 122 2The SD1 Gene and Its Role in Rice 2.1 Genetic and molecular basis of SD1 The SD1 gene, known as the "green revolution gene", encodes the enzyme gibberellin 20-oxidase-2 (GA20ox-2), which is crucial in the biosynthesis of gibberellins (GAs), a class of plant growth hormones. The gene consists of three exons and two introns, with mutations in this gene leading to a semi-dwarf phenotype due to reduced GA levels (Ashikari et al., 2002; Monna et al., 2002; Peng et al., 2021). The most common mutations include deletions and single nucleotide substitutions that result in a nonfunctional enzyme, thereby reducing the plant's height (Monna et al., 2002; Spielmeyer et al., 2002). The SD1 gene plays a pivotal role in the final steps of GA biosynthesis. Mutations in SD1 lead to a decrease in bioactive GA levels, which in turn affects plant height and other growth parameters. The gene's expression is regulated by feedback mechanisms involving GA levels, ensuring that GA biosynthesis is tightly controlled (Ashikari et al., 2002; Sasaki et al., 2002). Additionally, interactions with other hormones such as brassinosteroids (BR) further modulate the effects of SD1on plant growth (San et al., 2020). 2.2 Impact on plant architecture The introduction of SD1 alleles has been instrumental in creating semi-dwarf rice varieties, which are shorter and more robust compared to their wild-type counterparts. This semi-dwarfism is primarily due to the reduced levels of bioactive GAs resulting from the loss-of-function mutations in the SD1 gene (Figure 1) (Asano et al., 2007; Jia et al., 2020; Peng et al., 2021). These semi-dwarf varieties have been widely adopted in rice breeding programs to improve yield and plant stability (Sasaki et al., 2002; Asano et al., 2007). The SD1 gene significantly influences stem length and strength. Semi-dwarf varieties with SD1 mutations exhibit shorter and sturdier stems, which are less prone to lodging (falling over) under high nitrogen fertilization and heavy grain loads (Ashikari et al., 2002; Spielmeyer et al., 2002; San et al., 2020). This enhanced stem strength is a direct consequence of the reduced GA levels, which limit excessive stem elongation and promote a more compact plant structure (Monna et al., 2002; Peng et al., 2021). Figure 1 Allelism test of plant heights between Dontokoi and either IR8 (A) or Hokuriku100 (B) (Adopted from Asano et al., 2007) Image caption: F1 plants from crosses between the parental plants on the left and right are shown in the center (Adopted from Asano et al., 2007)
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