Developmental Study of Branched Rhizophores in Three Selaginella Species

Abstract

A morphogenetic investigation was made of the rhizophore of three large‐sized tropical Selaginella species. The rhizophores of Selaginella delicatula, S. caudata, and S. plana arise exogenously at the points of branching of the main stems. In S. delicatula they are initiated at the junction of the second youngest branching. The rhizophore apical meristem has a tetrahedral apical cell and is capless. The rhizophores are usually three or four times dichotomously branched in S. delicatula and S. plana and four or five times in S. caudata. In S. delicatula, dichotomous branching of the rhizophore involves formation of two new apical cells subsequent to loss of an original apical cell. A pair of roots is formed endogenously from inner cells below the dermal layer at the apex of ultimate rhizophore branches. The finding that the rhizophore is an autonomously branched, leafless, and capless axis leads us to argue that Selaginella rhizophores, like lepidodendrid rhizomorphs, are fundamental axial organs that coordinate with the stem and root.

Specific intercalary growth of rhizophores and roots in Selaginella kraussiana (Selaginellaceae) is related to unique dichotomous branching

“…In this phase of growth the rhizophores show some resemblance to the shoot, e.g. they are capless and lack root hairs (Imaichi, 2008;Imaichi and Kato, 1991;Jernstedt et al, 1992;Kato and Imaichi, 1997;Lu and Jernstedt, 1996;Webster, 1992;Webster andSteeves, 1964, 1967). After cutting off several segments (=merophytes), prior to terminal branching of the axis, the rhizophore IC undergoes segmentation, losing its IC identity.…”

“…In shoot apical meristems (SAM), with multiple initial cells, such as in lycopods and a few seed plants that may branch dichotomously, new apices for twin axes are selected from the pool of meristematic cells formed prior to division. In meristems with a single initial cell (IC), such as in Selaginella or in ferns, formation of new meristematic centers is rarely associated with direct division of the original IC (Bierhorst, 1977); more often it is related to its disappearance due to segmentation or inactivation (Hagemann, 1980;Hagemann and Schulz, 1978;Hébant-Mauri, 1993;Imaichi, 2008;Imaichi and Kato, 1991;Jernstedt et al, 1994;Mueller, 1982). Cells, which are located between two new meristematic centers, undergo vacuolization and differentiation, giving rise to a furrow separating the twin axes (Hagemann, 1980;Nolan, 1969;Piche et al, 1982;Tomlinson, 1970Tomlinson, , 1971.…”

“…Selaginella uncinata (Imaichi and Kato, 1989) and S. kraussiana (Imaichi, 2008), formation of root primordia starts with the first dichotomy. Newly created ICs for root primordia are tetrahedral, similar to rhizophore IC, but they also cut off a distal merophyte to form a root cap (Imaichi, 2008;Imaichi and Kato, 1991;Jernstedt et al, 1992;Kato and Imaichi, 1997;Lu and Jernstedt, 1996). Root primordia reside within the rhizophore apex, covered by parenchyma tissues forming the so-called calyptra, yet are produced by the rhizophore superficial IC.…”

The Selaginella rhizophore has a unique transcriptional identity compared with root and shoot meristems

“…In subsequent stages, the outgrowth that originated as a rhizophore acquires root characters (root cap and hairs) usually just before or upon reaching the soil. The organ identity of the rhizophore has been debated since its first description in 1868 (N€ ageli & Leitgeb, 1868), with botanists claiming that the structure is either a modified shoot (Pfeffer, 1871;Treub, 1874;Bruchmannm, 1905;Worsdell, 1910;Cusick, 1953Cusick, , 1954, a root throughout development (Webster & Steeves, 1964;Webster, 1969Webster, , 1992Wochok & Sussex, 1974, 1976, or a distinct organ before its transition to a root (N€ ageli & Leitgeb, 1868;Bower, 1908;Jernstedt & Mansfield, 1985;Imaichi & Kato, 1991;Lu & Jernstedt, 1996).…”

Class 1 KNOX Gene Expression Supports the Selaginella Rhizophore Concept

“…Rhizophores are initiated exogenously at the junctions of branching stems and endogenously give rise to roots (Imaichi and Kato 1991). The expression pattern of SuKNOX1 was investigated using in situ hybridization.…”