Plant hormones including ethylene are recruited in calyx inflation in Solanaceous plants
Introduction
Hormones play a central role in the integration of diverse environmental cues with the plant genetic program and in shaping the morphological structures. However, interactions between developmental control genes and hormones involved in the origin of morphological novelties are not well understood. During inflated calyx syndrome (ICS) formation, a spectacular floral morphological novelty exhibited by a few genera of Solanaceae such as Physalis, two plant hormones, cytokinins and gibberellins, have been shown to be essential for calyx inflation, provided a MADS-box transcription factor MPF2 is available (He and Saedler, 2005, He and Saedler, 2007a, He and Saedler, 2007b). In sharp contrast with Physalis, no calyx inflation in response to hormone application is observed in Solanum tuberosum. However, Solanum and Physalis are distant relatives and may not provide the appropriate material for comparative study of hormonal and molecular interactions. Interestingly, Withania and Tubocapsicum, which are closely related to Physalis and sisters to each other, may offer the most suitable materials for a comparative study (Khan et al., 2009). Withania exhibits a variety of inflated calyces, while Tubocapscium is an evolutionary natural loss mutant of the ICS.
5–10 plant hormones have been discovered so far. These hormones interact with one or more additional hormones through change in the level or response (Santner et al., 2009). For example, in addition to being linked to flower senescence and abscission, there is a synthesis of the gaseous hormones ethylene shortly after fertilization and wounding of floral organs. Therefore, in this study, in addition to exploring the roles of auxin, cytokinins and gibberellins in the ICS development in the Withaninae, we focused on elucidating a potential role of ethylene and its possible interaction with other hormones. Scanning of the calyx revealed that epidermal cells show enlargement and lobation after fertilization in Withania in comparison with Tubocapsicum, where no such morphological features are visible. By synchronizing calyx and fruit growth, the exogenous cytokinins and gibberellins mimic fertilization signals in Withania. For the first time, we report that ethylene seems to be involved in the process of maturation of ICS. Moreover, interactions between hormones and MPF2-like genes seem not to occur at transcriptional level.
Section snippets
Materials and methods
All together 13 accessions of Withania, including 5 species (W. aristata, W. frutescens, W. riebeckii, W. coagulans and W. somnifera) and 2 accessions of Tubocapsicum anomalum and one accession each for Iochroma australe, Vassobia breviflora and Physalis peruviana, were used for this study (see Khan et al., 2009). All of the accessions were grown in glasshouses of the Max-Planck-Institute for Plant Breeding Research (MPIPZ), Cologne, as described in Khan et al. (2009).
Withania displays a diversity of inflated calyces and shows variations in the inflation degree and architecture of the calyx
Fig. 1A shows that Withania displays a wide variety of inflated calyces in terms of calyx size, shape and architecture. It ranges from the half-open balloon of W. aristata and W. frutescens containing needle-like and teeth-like projections, respectively, to the open fleshy “lanterns” of W. riebeckii, to completely closed, semi-succulent papery lanterns in W. somnifera and W. coagulans. In sharp contrast to Withania, Tubocapsicum features only a collar-like rudimentary calyx. In order to examine
Morphological development of the inflated calyx
Morphological novelties such as ICS provide plants with prospects for the exploitation of new ecological niches that might lead to better adaptive radiation. The genus Withania displays a wide variety of inflated calyces ranging from the half open balloon of W. aristata and W. frutescens to open fleshy lanterns of W. riebeckii, to completely closed, semi-succulent papery lanterns in W. somnifera and W. coagulans. Tubocapsicum on the other hand features only collar-like rudimentary calyx (Khan
Acknowledgements
We thank Dr. Daniela Liebsch for valuable discussions and comments. This work was supported by a HEC-DAAD fellowship awarded to RMK, partially by the MPIZ, Max-Planck-Society and partially by the Hundred Talents Project of the CAS to CYH.
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