Abstract
Members of the genus Monophyllaea are unique in that they produce no new organ during the vegetative phase in the shoot; instead, one of the cotyledons grows indeterminately. The mechanism of this unique trait is unclear, in part because of the lack of suitable assessment techniques. We therefore established a whole-mount in situ hybridization technique, a powerful means of examining spatial patterns in gene expression, for Monophyllaea glabra. By using this, we examined the expression pattern of a SHOOT MERISTEMLESS (STM) ortholog, which is indispensable for the formation and maintenance of the shoot apical meristem (SAM) in typical angiosperms. Expression was confined to the groove meristem (GM), which corresponds to the SAM. We also assessed the expression pattern of ANGUSTIFOLIA3 (AN3), a key promoter for cell division in the leaf meristem. It was expressed not only in the basal meristem (BM) tissue with active cell division in the basal part of the growing cotyledon but also in the GM. The findings suggest that the unusual gene expression pattern of the GM underpins the fuzzy morphogenesis of Monophyllaea.
Highlights
Plants can produce new organs from the meristem throughout their lives, whereas animals complete most morphogenesis during the embryonic stages (Steeves and Sussex, 1989; Graham et al, 2000; Wolpert and Tickle, 2011)
One meristem resided in the most proximal part of the macrocotyledon around 100 μm from the mediolateral axis and adjacent to five rows of differentiated cells in the distal part of the meristem. This meristem is the groove meristem (GM)—inflorescence was produced at this position, as evidenced by the presence of a bulge (Figures 1D, E and 3I)
Hereafter we regard the basal part of the tissue inside these points as the GM and the basal part of the tissue outside it as the basal meristem (BM)
Summary
Plants can produce new organs from the meristem throughout their lives, whereas animals complete most morphogenesis during the embryonic stages (Steeves and Sussex, 1989; Graham et al, 2000; Wolpert and Tickle, 2011). The aerial part of typical seed plants, the shoot, is composed of repeating phytomere units (Gray, 1879), each of which consists of a leaf, stem, and axillary bud. All of these components are produced from the indeterminate meristem, the shoot apical meristem (SAM), at the tip of the shoot. The indeterminate nature of the SAM is dependent on the maintenance of stem cells. Because of the indeterminate nature of the SAM, the shoot system is indeterminate.
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