Testcross Analysis of Pl-1 Marker Expression and Seedling Vigor in Thai Maize Germplasm for Doubled Haploid Breeding Applications

Doubled haploid (DH) technology accelerates maize breeding by generating completely homozygous lines within two generations, but its efficiency depends on reliable haploid identification markers. The Purple plant 1 (<i>Pl-1</i>) root pigmentation marker has emerged as a promising alternative to R1-Navajo (<i>R1-nj</i>), which suffers from frequent suppression in tropical germplasm. This study evaluated <i>Pl-1</i> expression and seedling vigor in 298 diverse Thai maize genotypes across four market classes under controlled growth chamber conditions (24 ± 1 °C, 85-90% humidity, and standardized lighting), followed by testcross analysis with 89 representative genotypes crossed with BHI306 to distinguish between allelic absence and epistatic suppression mechanisms. Complete absence of <i>Pl-1</i> expression was observed in 99.3% of Thai genotypes, contrasting with consistent expression in the temperate-derived control (BHI306). Testcross F₁ progeny from 89 Thai × BHI306 crosses exhibited intermediate expression levels (1.57-2.05) across all market classes, confirming allelic absence rather than suppressor-mediated inhibition. Substantial genetic diversity was detected in seedling vigor traits independent of <i>Pl-1</i> status, with root length varying 43-fold and fresh weight 20-fold, showing highly significant genotypic effects. The uniform lack of <i>Pl-1</i> expression across Thai germplasm eliminates background interference, highlighting its utility as a complementary marker when introgressed from inducer lines. These findings establish the genetic foundation for implementing optimized DH breeding strategies in tropical maize through marker-assisted backcrossing approaches.