Growth & Development

Staminodes of Aquilegia: how an unusual floral organ develops

Why would a flower want sterile stamens. An investigation into their development yields some clues.

Have you ever taken a close look at the flowers of Aquilegia (also known as granny’s bonnets or columbines) and noticed something unusual? While flowers are generally made up of four key components (petals, sepals, carpels and stamens), many Aquilegia species have extra floral organs known as staminodes. 

The staminodes of Aquilegia appear to be made up of a set of five or six sterile stamens fused together to form a sheath, with two whorls of five staminodes each surrounding the flower’s carpel. A recent study by Meaders and colleagues (2020) aimed to figure out how exactly the staminodes of Aquilegia develop, and what role they might play in the plant’s floral ecology. 

Aquilegia is a genus of approx. 70 species in the family Ranunculaceae. It’s flowers are notable for a fifth type of floral organ: the staminode. Image: Canva.

Through a combination of histological techniques and comparative transcriptomics, the study uncovers various developmental and molecular differences between the staminodes and stamens of Aquilegia coerula ‘Origami’. 

“When their primordia first emerge, the elaborate staminodes of Aquilegia are essentially indistinguishable from those of stamens,” the authors write. “But, they rapidly diverge developmentally, most notably in their lateral expansion and failure to differentiate anthers.”

Examination of the microscopic anatomy of each floral organ showed the staminodes grow to be about three times thicker than stamen filaments, and also tend to be thicker on one side resulting in a curled “C” shape. Each “C”-shaped staminode faces the opposite direction to its neighbour and their edges stick to one another, so that the flower’s cross-section looks a bit like a ring of outstretched arms holding hands around the carpel.

“This creates a physical interlocking along the lateral margins of the organs. Understanding the details of the curling and adherence phenomena will require further study, potentially using more detailed microscopy and transcriptomic approaches,” write Meaders and colleagues.RNA sequencing of the staminodes and stamen filaments of Aquilegia coerula ‘Origami’ revealed the expression of particular genes associated with staminode development. For example, enriched lignin biosynthesis in the later stages of their development matched the observations of staminodes becoming lignified and hardened at maturity.

Aquilegia coerulea ‘Origami’ toluidine blue-stained histological sections from three stages of development, with the staminodes false coloured to highlight both their identity and their alternate marginal curling behaviour. Source: Meaders et al 2020.

By now, you might be wondering, what do Aquilegia staminodes actually do? The authors speculate that the specialised traits of Aquilegia staminodes may provide a protective function for the flower’s carpels. “Although many functional staminodes in other angiosperm lineages play key roles in pollination, we do not believe that this is the case here. While the ecological function of Aquilegia staminodes remains to be determined, these data are consistent with a role in protecting the early carpels from herbivory or pathogen infection.”

One of the authors has made this paper available on their ResearchGate page.

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