Hormonal Interplay of GAs and Abscisic Acid in Rice Germination and Growth Under Low-Temperature Stress

Seed germination and early growth in rice are critical stages influenced by the hormonal balance between gibberellins (GA) and abscisic acid (ABA), particularly under low-temperature stress. This study investigated the effects of GA₃ and ABA on seed germination, embryonic growth, gene expression, and biochemical activities in rice cultivars with contrasting tolerance to low temperatures. GA₃ markedly improved germination in resistant cultivars Nagdong and CNDH77, whereas susceptible cultivars showed minimal improvement, while ABA strongly inhibited germination, especially under higher concentrations. GA₃ also promoted embryonic growth, with resistant cultivars displaying the longest embryo cells (10.10 µm and 13.49 µm, respectively), whereas ABA suppressed embryonic growth and completely inhibited germination in susceptible cultivars. Upregulation of GA biosynthesis (<i>OsCPS1</i> and <i>OsKS1</i>) and signaling genes (<i>OsGID1</i> and <i>OsGID2</i>) in resistant cultivars correlated with enhanced germination and growth, whereas ABA-induced <i>ABI5</i> expression suppressed germination, particularly in susceptible cultivars. Hormone quantification confirmed increased endogenous GA₃ after GA₃ treatment and reduced ABA levels under ABA treatment. Additionally, GA₃ modulated ABA signaling genes, upregulating <i>OSK3</i>, <i>ABI3</i>, <i>ABI4</i>, and <i>ABI5</i>, while ABA treatment had contrasting effects, particularly between resistant and susceptible cultivars. GA₃ treatment also enhanced the expression of GA biosynthesis and signaling genes (<i>OsCPS1</i>, <i>OsKS1</i>, <i>OsGID1</i>, and <i>OsGID2</i>), whereas ABA treatment upregulated ABA catabolic genes (<i>OsABA8ox2</i>). GA₃ also enhanced amylase activity and sugar-related gene expression, supporting its role in energy mobilization during germination. Conversely, ABA suppressed cell elongation, reducing it to 4.45 µm in CNDH77 under 100 µM ABA. These findings provide valuable insights into the hormonal regulation of rice seed germination and growth under low-temperature stress, offering potential strategies to enhance seed vigor and stress tolerance in rice breeding.