Geometric morphometrics reveals shifts in flower shape symmetry and size following gene knockdown of CYCLOIDEA and ANTHOCYANIDIN SYNTHASE

BACKGROUND

While floral symmetry has traditionally been assessed qualitatively, recent advances in geometric morphometrics have opened up new avenues to specifically quantify flower shape and size using robust multivariate statistical methods. In this study, we examine, for the first time, the ability of geometric morphometrics to detect morphological differences in floral dorsoventral asymmetry following virus-induced gene silencing (VIGS). Using Fedia graciliflora Fisch. & Meyer (Valerianaceae) as a model, corolla shape of untreated flowers was compared using canonical variate analysis to knockdown phenotypes of CYCLOIDEA2A (FgCYC2A), ANTHOCYANIDIN SYNTHASE (FgANS), and empty vector controls.

RESULTS

Untreated flowers and all VIGS treatments were morphologically distinct from each other, suggesting that VIGS may cause subtle shifts in floral shape. Knockdowns of FgCYC2A were the most dramatic, affecting the position of dorsal petals in relation to lateral petals, thereby resulting in more actinomorphic-like flowers. Additionally, FgANS knockdowns developed larger flowers with wider corolla tube openings.

CONCLUSIONS

These results provide a method to quantify the role that specific genes play in the developmental pathway affecting the dorsoventral axis of symmetry in zygomorphic flowers. Additionally, they suggest that ANS may have an unintended effect on floral size and shape.

The relationship between cell division and elongation during development of the nectar-yielding petal spur in Centranthus ruber (Valerianaceae)

BACKGROUND AND AIMS

Floral spurs are hollow, tubular outgrowths that typically conceal nectar. By their involvement in specialized pollinator interactions, spurs have ecological and evolutionary significance, often leading to speciation. Despite their importance and diversity in shape and size among angiosperm taxa, detailed investigations of the mechanism of spur development have been conducted only recently.

METHODS

Initiation and growth of the nectar-yielding petal spur of Centranthus ruber 'Snowcloud' was investigated throughout seven stages, based on bud size and developmental events. The determination of the frequency of cell division, quantified for the first time in spurs, was conducted by confocal microscopy following 4',6-diamidino-2-phenylindole (DAPI) staining of mitotic figures. Moreover, using scanning electron microscospy of the outer petal spur surface unobstructed by trichomes, morphometry of epidermal cells was determined throughout development in order to understand the ontogeny of this elongate, hollow tube.

KEY RESULTS

Spur growth from the corolla base initially included diffuse cell divisions identified among epidermal cells as the spur progressed through its early stages. However, cell divisions clearly diminished before a petal spur attained 30 % of its final length of 4·5 mm. Thereafter until anthesis, elongation of individual cells was primarily responsible for the spur's own extension. Consequently, a prolonged period of anisotropy, wherein epidermal cells elongated almost uniformly in all regions along the petal spur's longitudinal axis, contributed principally to the spur's mature length.

CONCLUSIONS

This research demonstrates that anisotropic growth of epidermal cells - in the same orientation as spur elongation - chiefly explains petal spur extension in C. ruber. Representing the inaugural investigation of the cellular basis for spur ontogeny within the Euasterids II clade, this study complements the patterns in Aquilegia species (order Ranunculales, Eudicots) and Linaria vulgaris (order Lamiales, Euasterids I), thereby suggesting the existence of a common underlying mechanism for petal spur ontogeny in disparate dicot lineages.