Unveiling a longstanding misinterpretation: Revisiting the pattern formation of testa surface in Astragalus (Fabaceae, Astragaleae)

Abstract

As the largest plant genus among flowering plants, Astragalus requires a thorough evaluation of morphological and anatomical evidence to understand its evolutionary history, species relationships, and systematic placements. This study assesses seed surface patterns across various species and evaluates the variation and formation processes of testa ornamentation for taxonomic and phylogenetic purposes. Previous investigations have identified fixed features in several species that aid in species delimitation or characterize related groups. We examined seed surface characteristics, such as roughness under a stereomicroscope, cell shape, and cellular arrangement using Scanning Electron Microscopy, in 181 specimens representing 124 species and 55 sections of Astragalus, along with five species from related genera for comparison. We propose standardized terminology for seed surface characters to establish a consensus framework for future studies. Our results reveal distinct testa surface patterns within a single species and even within individual seeds as they develop. Initially, testa cells exhibit a polygonal isodiametric or elongated shape, which progressively narrows and spaces out, leading to the expansion and deepening of areas at the anticlinal walls. This pattern develops into an irregularly rugulate and ultimately a multi-reticulate form. Notably, A. annularis, A. epiglottis, and A. pelecinus display a rare cellular pattern akin to Phylolobium and Podlechiella vogelii, supporting their phylogenetic placement as the most basal clade in Astragalus. The verrucose-walled multi-reticulate pattern is characteristic of the Phaca clade, while an accelerated emergence of a multi-reticulate pattern in the Astracantha and Hypoglottis clades may indicate an advanced state. Future studies on seed surface patterns should consider these testa surface changes to address taxonomic issues within Astragalus and other Fabaceae genera.