24-Epibrassinolide Mediated Interaction among Antioxidant Defense, Lignin Metabolism, and Phytohormones Signaling Promoted Better Cooperative Elongation of Maize Mesocotyl and Coleoptile under Deep-Seeding Stress

Both mesocotyl and coleoptile synergistic elongation of maize are the major causes of its deep-seeding tolerance. To elucidate the physiological mechanism of their elongation induced by brassinosteroid under deep-seeding stress, seeds of deep-seeding tolerant N192, W64 and intolerant Ji853, K12 were treated with 4.16 × 10–3 M exogenous 24-epibrassinolide (EBR) under 3 or 20 cm seeding depths. The cumulative mesocotyl/coleoptile length reached a maximum of 8.37/5.02 cm on the 12th day of growth at lack of seeding depths in darkness, maximum cumulative seedling length was 21.25 cm on the 22th day; There were complex linear relationships between the four growth parameters of seedlings (maximum cumulative mesocotyl/coleoptil/total mesocotyl and coleoptile/seedling length) and seed characteristics (seed size, weight, physical properties, reserves) by Pearson correlation analysis and path analysis, further using the seed characteristics to establish four optimal regression equations to predict their growth parameters. 4.16 × 10–3 M exogenous EBR application changed both mesocotyl and coleoptile enzymes system, alleviated the reactive oxygen species damage, decreased lignin accumulation, and regulated phytohormones level and polar transport in maize under 20 cm stress, resulting in 26.20, 4.37, 17.53, and 16.33% increase on mesocotyl length, coleoptile length, mesocotyl and coleoptile total length, and ratio of mesocotyl to coleoptile to normally germinate of seedlings under deep soil layer. Additionally, the expression of Zm00001d033286 (PAL), GRMZM2G159393 (YUCCA2), GRMZM2G017187 (ARF9), MSTRG.30577 (BIN2), and GRMZM2G413006 (TCH4) changed in parallel with mesocotyl and coleoptile elongation under different treatments. Therefore, EBR, regulatory role in maize mesocotyl and coleoptile elongation, can promote its safety production in arid regions.