Border-like cell formation mediated by SgPG1 confers aluminum resistance in Stylosanthes guianensis

Stylosanthes is an important forage legume in tropical areas with strong resistance to aluminum (Al) toxicity, though knowledge of mechanisms underlying this resistance remains fragmentary. We found that border-like cells (BLCs) were constitutively produced surrounding the root tips of all 54 examined Stylosanthes guianensis genotypes, but not the Stylosanthes viscose genotype TF0140. In genotypic comparisons under Al conditions, the S. guianensis genotype RY#2 retained significantly more Al in BLCs and thereby showed higher relative root growth than TF0140. Formation of BLCs accompanied changes in cell wall pectin epitopes and differential expression of genes involved in pectin metabolism, including a polygalacturonase (SgPG1). The expression pattern of SgPG1 was consistent with the formation of BLCs in both RY#2 and TF0140. SgPG1 was localized in cell walls and exhibited high activities mediating demethyl-esterified homogalacturonan degradation. Overexpressing SgPG1 changed cell wall pectin epitopes, enhanced BLCs production, and Al resistance in both Arabidopsis and Stylosanthes hairy roots. Furthermore, combining protein-DNA binding assays in vitro and in vivo, a bHLH transcription factor SgbHLH19 was demonstrated to be the upstream regulator of SgPG1. Our study demonstrates that S. guianensis Al resistance mainly relies on BLCs, whose formation involves cell wall pectin epitope modification by SgPG1.