1 Control of flowering in strawberries

Strawberries (Fragaria sp.) are small perennial plants capable of both sexual reproduction through seeds and clonal reproduction via runners. Because vegetative and generative developmental programs are tightly connected, the control of flowering is presented here in the context of the yearly growth cycle. The rosette crown of strawberry consists of a stem with short internodes produced from the apical meristem. Each node harbors one trifoliate leaf and an axillary bud. The fate of axillary buds is dictated by environmental conditions; high temperatures and long days (LDs) promote axillary bud development into runners, whereas cool temperature and short days (SDs) favor the formation of branch crowns. SDs and cool temperature also promote flowering; under these conditions, the main shoot apical meristem is converted into a terminal inflorescence, and vegetative growth is continued from the uppermost axillary branch crown. The environmental factors that regulate vegetative and generative development in strawberries have been reasonably well characterised, and are reviewed in the first two chapters. The genetic basis of the physiological responses in strawberries is much less clear. To provide a point of reference for the flowering pathways described in strawberries so far, a short review on the molecular mechanisms controlling flowering in the model plant Arabidopsis is given. The last two chapters will then describe the current knowledge on the molecular mechanisms controlling the physiological responses in strawberries. 1.1 The yearly growth cycle Strawberries are small perennial plants that reproduce both sexually through seeds and clonally via runners. Strawberry stem is called rosette crown that consists of short internodes produced from the apical meristem. Each node harbors one trifoliate leaf and an axillary bud. Axillary buds may develop into runners, which are elongated stems with two long internodes followed by a daughter plant, into axillary leaf rosettes called branch crowns, or remain dormant. The apical meristem may keep on producing new nodes or develop into a cymose inflorescence. When the apical meristem turns into an inflorescence meristem, the uppermost axillary bud develops into a branch crown that sustains vegetative growth in a sympodial fashion (Figure 1.1.1; Darrow 1966). The fate of the apical and axillary meristems is dictated by seasonal changes in photoperiod and temperature. During the summer months, strawberries grow vegetatively and axillary buds differentiate into runners. In autumn, axillary buds develop into branch crowns instead of runners, and the apical meristems of the main crown and bigger branch crowns form terminal inflorescences. Short days (SDs) in autumn also cause an overall halt in vegetative development, and the plants enter a state of semi-dormancy characterised by shorter petiole length and stunted growth (Sønsteby and Heide 2011). Winter chilling is needed to break this dormant state, and in the following spring, inflorescences initiated the preceding autumn complete their development, and the next seasonal cycle follows (Figure 1.1.1; Darrow 1966). The seasonal growth cycle described above holds true for both the diploid woodland strawberry Fragaria vesca (L.), and its cultivated octoploid relative Fragaria × ananassa (Duch.), although exceptional environmental responses have been described in both species (Chapter 1.2). In both species, flowering is induced in SDs at cool to medium temperatures, and inhibited by long days (LDs) and high temperature. Because of the close resemblance of physiological responses, it has long been speculated that the molecular regulatory networks controlling environmental responses may be similar in the two species (e.g. Battey et al. 1998). Indeed, this view recently gained empirical evidence when Koskela et al. (2016) showed that some of the core genes involved in photoperiod and temperature regulated flowering responses function similarly in the two strawberry species. Due to these similarities, it is feasible to present findings on diploid and octoploid strawberry physiology and molecular pathways side-by-side. In some cases, this is also necessary, as physiological responses have been examined at a greater depth in octoploid than in diploid strawberries, while molecular studies into flowering regulation have mostly focused on the diploid F. vesca. Figure 1.1.1. Plant structure and typical seasonal growth cycle of strawberries grown in temperate zone. During the summer months, strawberries grow vegetatively and axillary buds produce runners, whereas in autumn, young axillary buds differentiate into branch crowns. Floral induction occurs in autumn, and apical meristems of main and branch crowns develop into inflorescence meristems. Inflorescence development is completed in the following spring and summer when plants flower and produce fruits. The axillary buds that were too young to be induced to flower the preceding autumn (highlighted with black rectangles) grow out as new branch crowns, thus completing the perennial life cycle. To understand flowering, i.e. generative development, it is important to comprehend vegetative development. These two processes are often interrelated; for instance, the number of floweringcompetent apical meristems in strawberry depends on the number and age of branch crowns on the plant, and the timing of branch crown formation (Hytönen et al. 2004). Because of the interdependency between vegetative and generative development, this review aims to give a thorough picture of the effects that the seasonally changing environment imposes on the yearly growth cycle of strawberries. The latter chapters will then describe the current knowledge on the molecular mechanisms controlling these physiological responses.