Abstract
Pseudonectaries, or false nectaries, the glistening structures that resemble nectaries or nectar droplets but do not secrete nectar, show considerable diversity and play important roles in plant-animal interactions. The morphological nature, optical features, molecular underpinnings and ecological functions of pseudonectaries, however, remain largely unclear. Here, we show that pseudonectaries of Nigella damascena (Ranunculaceae) are tiny, regional protrusions covered by tightly arranged, non-secretory polygonal epidermal cells with flat, smooth and reflective surface, and are clearly visible even under ultraviolet light and bee vision. We also show that genes associated with cell division, chloroplast development and wax formation are preferably expressed in pseudonectaries. Specifically, NidaYABBY5, an abaxial gene with ectopic expression in pseudonectaries, is indispensable for pseudonectary development: knockdown of it led to complete losses of pseudonectaries. Notably, when flowers without pseudonectaries were arrayed beside those with pseudonectaries, clear differences were observed in the visiting frequency, probing time and visiting behavior of pollinators (i.e., honey bees), suggesting that pseudonectaries serve as both visual attractants and nectar guides.
Highlights
Pseudonectaries, or false nectaries, the glistening structures that resemble nectaries or nectar droplets but do not secrete nectar, show considerable diversity and play important roles in plant-animal interactions
Pseudonectaries, which are located at the distal geniculate bend of the lower petal lip, are hemispherical, emerald, glistening, and nectar droplet-like, and about 700 μm in diameter and 500 μm in height (Fig. 2a, b)
Under X-ray microcomputed tomography, the pseudonectaries are obviously thicker than the other regions of the lower petal lip (Fig. 2c, d), suggesting that they were caused by regional thickening rather than simple surface curving
Summary
Pseudonectaries, or false nectaries, the glistening structures that resemble nectaries or nectar droplets but do not secrete nectar, show considerable diversity and play important roles in plant-animal interactions. The family Ranunculaceae is an excellent system for the study of pseudonectary development and evolution, for three reasons Pseudonectaries, especially those that are located on petals, have been documented in at least four genera (i.e., Nigella, Trollius, Eranthis, and Xanthorhiza), and show considerable diversity in number, colour, morphology, and functions[24,32,38,39] (Fig. 1). This provides an excellent opportunity for a comprehensive understanding of the generalities and peculiarities of pseudonectaries. Our results provide a comprehensive portrait of pseudonectaries, and clarify the differences between pseudonectaries and nectaries
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