Leaf anatomical traits support the classification of Galphimioid clades in Malpighiaceae

Laticifers represent one of the most functionally striking secretory systems in angiosperms, characterizing entire families and latex being considered a key element in defence against herbivores in plants. However, they are still understudied, especially in clades where the secretions are not as copious, as in Malpighiaceae. Here, we undertake a study combining anatomy, phylogeny and ancestral state reconstruction to unravel the structure, distribution and evolution of laticifers and other leaf anatomical traits in the Galphimioid clade (Malpighiaceae), sampling approximately 90 % of the clade.

Laticifers and other leaf anatomical traits were surveyed comparatively across all genera of the Galphimioid clade using detailed anatomical and histochemical techniques. Leaf characters were mapped onto the most recent phylogenetic framework for Malpighiaceae to reconstruct ancestral states and identify possible synapomorphies to less inclusive clades within the Galphimioid clade. Structural and developmental aspects of laticifers, and the chemical composition of latex were characterized.

Non-anastomosing, articulated laticifers were found in all analysed species, originating from both ground meristem and procambium, representing a remarkable putative anatomical synapomorphy of the clade. Twelve additional synapomorphies were inferred, including absence of trichomes on mature leaves and revolute leaf margins. Ancestral reconstructions indicate that the Galphimioid clade is also characterized by dorsiventral, hypostomatic leaves with concave-convex petioles and flat-convex midribs bearing an open vascular arc. Leaf margin conformation, leaf gland distribution, epidermal features and latex chemistry were informative for distinguishing genera within the clade.

The presence of laticifers represents the main putative synapomorphy of the Galphimioid clade within Malpighiaceae. However, other key anatomical features support the delimitation of less inclusive clades and genera, strengthening the systematics of Malpighiaceae. The study advances our understanding of the evolution of leaf traits and secretory structures, opening new avenues for studies on defence evolution, plant-animal interactions and latex diversification.