GARDEN DISCOVERY

BY ANNE SUNDERMANN

During ‘No Mow May,’ front lawns throughout Ireland were filled with a bouquet of pollinator-friendly wildflowers. Among such diversity and abundance, there were a few flowers that looked different, slightly deformed. A cluster of multiple dandelion blooms on a single, thickened stem provided an odd bouquet in and of itself. What causes this curious condition, known as fasciation or cristation?

Nature’s Patterns

Like so many life forms, plants often adhere to known and repeated patterns for design and growth, as simple as waves and spirals, or mathematically complex as the Fibonacci curve, fractal branching, or the tiling of Voronoi patterns. These patterns serve a multitude of purposes: promoting reproduction, providing

 Fasciated Common Dog-violet 2

protection, or acting as camouflage.

Occasionally there is a glitch in the pattern design matrix that causes unintended growth forms. But even these abnormal forms often appear in recognizable patterns. Fasciation is one such phenomena found in vascular plants.

Fasciation occurs when plant tissue is flattened and elongated (Fascia comes as a translation of the Latin word for ‘band) and the flattened and coiled stems indeed bear resemblance to ribbon. A fasciated plant may look gigantic compared to the normal counterparts, with increased mass and volume from multiple buds or flowers on a widened, ribbed stem. Species in the Compositae family, including asters, sunflowers, and daisies, are particularly prone to "OCCASIONALLY THERE IS A GLITCH IN THE PATTERN DESIGN MATRIX fasciation. THAT CAUSES UNINTENDED Structural Changes Although the changes wrought by fasciation are quite visible, why it occurs is more of a mystery. There is no one cause GROWTH FORMS. BUT EVEN THESE ABNORMAL FORMS OFTEN APPEAR IN for this abnormal growth RECOGNIZABLE pattern, and it may be tied to PATTERNS. genetic as well as environmental variables such as extreme weather, or exposure to infections from bacteria, fungi, or viruses.

These potential causes are separated into two broad categories: 1) Physiological fasciation, caused by natural environmental factors or applied treatments, and 2) genetic fasciation, based on genes that regulate cell growth. For example, mutations in one gene pathway (the CLAVATA1 gene) promote changes in cell structure so that certain cells are enlarged when compared to unaffected plants.

Regardless of the cause of the changes, it is no mystery as to where, at a cellular or molecular level, fasciated conditions arise: in the apical meristem. This densely packed group of cells located at the tip of a plant shoot or root acts as stem cells in animals in that they are responsible for all cell growth and development in vascular plants. Meristem cells provide critical access to light (via shoots) and water (via roots). A microscopic view shows the shoot apical meristem cells are arranged as tiled polygons, possibly following

 Fasciated Purple-loosestrife

the mathematical dictates of Voronoi patterns, which are often used to provide solutions to growth patterns and other geometric problems.

According to researchers Iliev and Kitin (2011), when fasciation occurs, the activity in shoot apical meristem cells ‘results in a significantly increased circumference of the stem and enlarged proportions of pith and cortical parenchyma [the essential or functional elements of an organ], associated with a delayed differentiation of the vascular tissues. An elliptical or irregular shape of the cross section of a fasciated organ corresponds to a similar shape of the vascular cylinder.’ More specifically, individual plant species may develop ‘deviations from the normal structure of the epidermis, shape of leaves, as well as altered development of axillary buds.’

 Fasciated Daisy

Hormonal Influences

Cells in the shoot apical meristem are particularly influenced by two hormones, cytokinin and auxin, which regulate various plant processes. Cytokinin primarily promotes plant cell division and growth. The plant growth regulator auxin promotes cell elongation and behavioural development of the plant, such as branching.

Research by Schaller and colleagues (2015) likens the complementary interactions of cytokinin and auxin in plant growth to the Chinese concept of yin-yang: ‘…auxin and cytokinin act together dynamically, with roles that can be paradoxically antagonistic and supportive, to provide robustness to developmental processes and to confer distinct cell fates to precursor cells in close proximity, yielding a whole that is greater than the sum of its parts.’

Although plants affected with fasciation are often seen as unappealing, the conditions have relatively no adverse effects on the plant, and the affected plants survive as would a normal plant. In a stressed environment of changing climate conditions and extremes in temperature and rainfall/drought cycles, it is to be expected that abnormal growth patterns like fasciation may become more common. Research into the patterns of fasciation, and their underlying causes will increase awareness of the growth processes and anatomical structure of the meristem. This in turn will lead to better understanding of the interactions between hormones, environmental influences, and plant growth and development.

Sources: Clark SE, Running MP, Meyerowitz EM. (1993) CLAVATA1, a regulator of meristem and flower development in Arabidopsis. Development 119(2): 397-418. Iliev, I, & Kitin, P. (2011) Origin, morphology, and anatomy of fasciation in plants cultured in vivo and in vitro. Plant Growth Regulation 63, 115–129. Schaller GE, Bishopp A, Kieber JJ. (2015) The yin-yang of hormones: cytokinin and auxin interactions in plant development. Plant Cell 27(1): 44-63.