Comprehensive morphology and embryology of Pilostyles berteroi (Apodanthaceae): Revisiting floral and seed anatomy in holoparasitic plants

Abstract

Pilostyles berteroi (Apodanthaceae) is an endophytic holoparasite on the stems of Adesmia trijuga (Fabaceae). The presence of flowers and fruits serves as the sole indication of its existence. We analyzed the morphoanatomy of its flowers and seeds and embryological processes. The flowers are gregarious and unisexual, with each flower type distributed on different branches of the same individual A. trijuga. This suggests that populations of P. berteroi growing on A. trijuga may be dioecious. Unisexual flowers have a whorl of bracts and two whorls of tepals. Staminate flowers (SFs) have a synandrium with several pollen sacs arranged in 3–4 rings. Pollen grains are tricolpate with psilate exine and are released by rupture of the pollen sac wall, which lacks the endothecium. Pistillate flowers (PFs) have a semi-inferior and unilocular ovary with parietal-diffuse placentation, and a sessile stigma. Papillae are present between the pollen sacs and stigmatic region in SFs, and cover the stigma's sides in PFs, forming its receptive part. Both flower types have a nectary: embedded at the synandrium base in SFs, and covering the ovary wall base in PFs. The mature seed has a globular embryo, with a suspensor and endosperm. The embryological processes observed in P. berteroi closely resemble those described in other species, indicating a remarkable uniformity among the studied Pilostyles species. These findings underscore the importance of conducting additional taxonomic and molecular investigations to reassess the genus's systematics. Overall, this study provides novel insights into the biology of P. berteroi, which will be valuable for future research on the evolution and ecology of these fascinating plants.