Ontogeny of the mycoheterotrophic species Afrothismia hydra (Burmanniaceae)

Abstract

The complete ontogeny of the mycoheterotrophic Afrothismia hydra (Burmanniaceae) from seed to seed dispersal is presented. The oblong-ovoidal seeds are up to 0.7 mm long. They germinate with root tissue only, disrupting the seed coat and developing a primary ovoid root tubercle. At the proximal end of the tubercle, a second tubercle arises and further root initials indicate the sequential growth of more root tubercles with filiform extensions resulting in a small root aggregate. The seed coat often remains attached to this structure. When the root aggregate enlarges, a central axis to which all roots are connected becomes visible. This axis has a growth pole where new root tubercles arise. The same growth pole will later develop into a stem with scale leaves finally terminating in a flower. Flowers develop sympodially when the mature plant is only several centimetres long. After anthesis, the corolla tube disintegrates, leaving a pyxidium which opens by means of a peculiar elongating placenta, here called 'placentophore'. The placentophore elevates the placenta with attached seeds above the flowering level and is interpreted as an adaptation to ombrohydrochory. The reduction of hypocotyl, cotyledon and primary shoot is discussed with regard to the classical germination concepts of monocotyledons and with mycoheterotrophic dicotyledons.

Mycoheterotrophic germination of Pyrola asarifolia dust seeds reveals convergences with germination in orchids

“…MH germination results in highly distinctive underground, achlorophyllous organs such as protocorms of orchids (Bernard, 1899; Rasmussen, 1995), or root‐like structures from which shoots later arise as in some Ericaceae (Velenovsky, 1892; Lihnell, 1942) and Burmanniaceae (Imhof & Sainge, 2008). When it emerges above ground, the plant can become autotrophic.…”

“…However, germination can sometimes be triggered by different fungi, for example at forest sites from which conspecific adults are absent (Bidartondo & Bruns, 2005). In other MH lineages, studies on germination have focused solely on morphological aspects (Imhof & Sainge, 2008; Smith & Read, 2008), and fungal identification is pending at this stage.…”

‘Dust seeds’ with undifferentiated embryos and their germination in mycoheterotrophic Monotropoideae (Ericaceae)

“…The only species of mycoheterotrophs besides Monotropoideae and orchids that produces dust seeds with an undifferentiated embryo whose germination and ontogeny have been described in some detail appears to be Afrothismia hydra in the Thismaceae (Imhof and Sainge, 2008; Imhof et al, 2013). In this species, germination consists of primary root tubercle (with a filiform extension) development splitting through the seed coat and emerging free of it.…”

“…In this species, germination consists of primary root tubercle (with a filiform extension) development splitting through the seed coat and emerging free of it. In the next stage, ‘ … this initial tubercle has grown a short tail-like extension (only about 150 μm long) and is fully packed with whitish material, which can be interpreted as mycorrhizal fungus … ’ (Imhof and Sainge, 2008). Eventually, the successive development of root tubercles elongated by a filiform extension results in dense aggregates of small ‘tuberous roots’, a common axis that gives rise to a shoot axis (rhizome) terminated in a 1-cm-long flower/fruit.…”

Marking the end of the International Year of Biodiversity

“…We live in worrying times, with climate change and other factors threatening the continued existence of the biodiversity that surrounds us and upon which we depend for our own survival. Many plants depend on a complex web of other organisms, including vertebrate and invertebrate pollinators (Micheneau et al ., 2009), fungal symbionts (Wickett & Goffinet, 2008; Imhof & Sainge, 2008) and, in some cases [notably carnivorous plants (Chase et al ., 2009) and parasites (Fay et al ., 2010)], other plants and animals for their sustenance. The decisions taken at COP 10 in Japan are internationally significant and important: let us hope that they have the desired effect of halting loss of biodiversity, so that these complex webs of interactions will survive and continue to fascinate and to secure life on earth.…”