Haustorial processes during the female gametophyte formation in Rosularia pallida (Schott & Kotschy) Stapf (Crassulaceae).

Key message: Ultrastructural and cytochemical analyses of the megaspore, embryo sac, and synergid haustoria reveal their roles in nutrition, contributing to the successful development of the megagametophyte in R. pallida. In this paper, we present the first cytochemical and ultrastructural analysis of the megaspores, embryo sac, and synergid haustoria in Rosularia pallida (Schott & Kotschy) Stapf (Crassulaceae) are presented. The haustoria in the ovule of R. pallida primarily function to provide nutrition during megasporogenesis and megagametogenesis. Cytochemical staining reveals a significant increase in the accumulation of insoluble polysaccharides, lipids, and proteins within the megaspores and embryo sac. This increase occurs alongside the progressive degradation of nucellar cells and the growth of haustoria towards the integuments. The direction of haustorial growth within sporophyte tissues and the distribution of nutrients within the ovule complement each other, collectively contributing to efficient nutrition for the developing female gametophyte. Callose is present in the walls of both the megaspores and their haustoria. The functional megaspore (FM) haustorium is the only one that extends beyond the nucellus into the integuments during megasporogenesis. The disappearance of callose in the micropylar portion of the FM haustorium enables apoplasmic transport, particularly in this region. These findings suggest that the FM haustorium supports the development of a specific megaspore in the tetrad, indirectly influencing FM selection through nutrient provision. Furthermore, the removal of callose on the chalazal side of the tetrad likely facilitates the development of the embryo sac from the chalazal megaspore. Ultrastructural analyses of the megaspore, embryo sac, and synergid haustoria reveal the presence of transfer-wall ingrowths. No plasmodesmata were detected in the haustorial walls. Additionally, ultrastructural observations of the synergids indicate that their haustorium significantly elongates toward the micropyle and becomes metabolically active.