GA2 and GA20-oxidase expressions are associated with the meristem position in Streptocarpus rexii (Gesneriaceae)
We examined genes involved in the regulatory pathway of gibberellin (GA) in meristems of Streptocarpus rexii. The plants do not possess a typical shoot apical meristem (SAM) and form unique meristems: the basal meristem extends the lamina area of one cotyledon to produce anisocotylous seedlings; the groove meristem forms new leaves at the base of the macrocotyledon. Exogenous application of GA significantly suppresses the basal meristem activity in developing cotyledons and the seedlings remain isocotyl. To examine the role of endogenous GA on these meristems in vivo, we isolated homologs of GA2-oxidase responsible for degrading active GAs (SrGA2ox), and GA20-oxidase regulating the rate limiting step of active GA synthesis (SrGA20ox). During embryogenesis, while first partly overlapping, the expression of SrGA2ox and SrGA20ox became more differentiated and mutually exclusive, ending with SrGA2ox being expressed solely in the adaxial–proximal domain of the embryo in regions with meristem activity, whereas SrGA20ox was restricted to the fork between the two cotyledons. The latter may be responsible for suppressing the formation of an embryonic SAM in S. rexii. In developing seedlings, SrGA2ox expression also followed the centers of meristem activity, where SrGA20ox expression was excluded. Our results suggest that low levels of GA are required in S. rexii meristems for their establishment and maintenance. Thus, the meristems in S. rexii share similar regulatory pathways suggested for the SAM in model plants, but that in S. rexii evolutionary modifications involving a lateral transfer of function, from shoot to leaves, is implicated in attaining the unusual morphology of the plants.
KeywordsBasal meristem Gibberellin20-oxidase Gibberellin2-oxidase Gibberellins Macrocotyledon Streptocarpus
This work was supported, in part, by a Taiwan-Italy Scientific Research Cooperation grant from the National Science Council (NSC) in Taiwan and National Research Council (CNR) in Italy (Grant Number 99-2923-B-002-007-MY2) and the Excellent Research Program from the National Taiwan University (10R30701, NTU) to CW. KN is supported by the NSC funding NSC 101-2811-B-002-150 and Sibbald Trust at Royal Botanic Garden Edinburgh (UK). We thank Dr. Min-Liang Kuo (NTU), Dr. Shin-Tong Jeng (NTU), Dr Tsan-Piao Lin (NTU) and Dr. Shih-Ying Hwang (National Taiwan Normal University, Taiwan) for their research funding support and helpful comments on this study. We thank Dr. K.-J. Tang and Ms. Y.-Y. Gao (TechComm, NTU) for technical support and enabling access to real-time PCR facilities. We thank the Science Division of RBGE for supporting this work. RBGE is supported by the Rural and Environment Science and Analytical Services division (RESAS) in the Scottish Government.
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