Involvement of Gibberellins during Fruit Development in Cloudberry (Rubus Chamaemorus)
G. Nilsen, I. Martinussen A. Erntsen T.V. Bhuvaneswari and O. Junttila
Department of Biology,
University of Tromsø,
Keywords: parthenocarpic, seedless, unpollinated, in vitro, plant growth regulators, NAA
The purpose of this study was to test the activity of gibberellins and auxins for induction of parthenocarpic fruit development in cloudberry, Rubus chamaemorus L., a dioecious, northern wild berry with potential for cultivation. Application of synthetic hormones is a well-known horticulture procedure, which has been successful in promoting parthenocarpic fruit development in many plant species. Our studies indicate that external application of some GAs induces parthenocarpic fruit development in cloudberry. Unpollinated ovaries were incubated on MS medium containing combinations of the plant hormones BAP, NAA and GA3. Higher proportion of ovaries showed parthenocarpic development in treatments containing GA3 combined NAA, than in treatment containing GA3 alone. Fruits were also larger in size in treatments containing both GA3 and NAA. There seems to be a difference in hormone requirements between in vitro and in planta parthenocarpic fruit development. This difference suggests that hormones, or precursors for biosynthesis of hormones may have to be transported from other parts of the plant to the developing ovaries for parthenocarpic development.
Cloudberry (Rubus chamaemorus L.) is a circumpolar, perennial, dioecious plant growing on bog areas (Hulten, 1971; Taylor, 1971). Flowering shoots are annual, but the plants have an extensive rhizome system with over-wintering generative and vegetative buds. The plant species can be propagated by rhizome cuttings as well as through in vitro micropropagation from apical meristem (I. Martinussen, pers. commun.). The female flowers may contain up to 30 ovaries, and each pollinated ovary gives rise to one small fruit, leading to a combined fruit much like the raspberry. Ripe cloudberries are yellowish-orange or almost red in colour, depending on the genotype. Due to the high content of vitamin C, cloudberries have traditionally been a very important contribution to the diet of people living in the northernmost parts Europe. Today it is still highly valued as a delicacy in the Nordic countries. Cloudberries are traditionally harvested from natural stands, but there is an increasing interest for cultivation, Two cultivars of cloudberry have been released (Rapp, 1989; 1991).
Because male and female flowers are borne on separate plants, good yield is dependent on an optimal ratio of male to female plants, presence of pollinating insects and favourable conditions for pollination, and these factors currently limit the production.
Inducing parthenocarpic fruit development is an attractive solution to solve all of the problems mentioned above. Parthenocarpic fruit development has been reported in many Rosaceae genera, including Malus, Prunus, Rosa, Rubus by application of either auxins and/or gibberellins (for references, see Crane, 1964; Moore and Ecklund, 1975).
Materials and methods
Experiments were conducted with potted plants of cloudberry cv. Fjellgull. Plants were taken directly from the cold storage to the phytotron compartment and grown at 15C under 24-h photoperiod.
Open female flowers were harvested, and the carpels were surgically separated before sterilising for 1 min in 70% ethanol and 3 min in 1.2 % Na-hypochlorite, followed by extensively rinsing in destilled sterile water. Number of druplets was 15-20 in each treatment.
Sterilised carpels were placed in Petridishes containing MS medium with 3% sucrose, 0.7% agar and plant growth regulators as shown in figure 1, and incubated in growth chamber at 20C with continuous light. Development of parthenocarpic fruitlets was observed during 6 weeks, before fresh weight were measured and visual data collected.
In the first experiment when combined fruits were used, only carpels which were in direct contact with the medium developed further. Later experiments were therefore performed with separated individual carpels. Within each treatment there was a large variation in the extent of fruit development, ranging from no response to fully developed fruit. In spite of this large variation, a clear difference could be discerned between treatments. These in vitro experiments show better colour development and higher freshweight per carpel when gibberellins are used in combination with auxin.
Exogenous applications of GA, auxin, or both, have been found to be effective for induction of parthenocarpic fruit growth (Moore and Ecklund, 1975). In the first reports using transgenic technology to obtain parthenocarpic fruit, eggplant and tobacco were transformed to give ovule specific expression of the auxin biosynthetic enzyme, tryptophan monooxygenase (Rotino et al., 1997). In these plants the biosynthesis of auxin was sufficient for full fruit development.
In cloudberry there seem to be a difference in hormone requirements for parthenocarpic fruit development in vitro and in planta. Applying 3-hydroxylated GAs to open unpollinated female flowers can effectively induce parthenocarpic fruit development in cloudberry while application of auxins appears not to be as effective (Junttila et al., 2001). The results of the in vitro experiments presented here show that GA3 alone was not enough to give full development of parthenocarpic fruits. The treatment containing GA3 resulted in partly developed carpels that were small and failed to develop the characteristic orange cloudberry colour. The treatments containing a combination of GA3 and NAA, on the other hand, resulted in parthenocarpic fruits with higher fresh weight and orange colour. This difference suggests that hormones, or precursors for biosynthesis of hormones, may have to be transported from other parts of the plant to the developing ovaries for proper parthenocarpic development.
Quality of parthenocarpic and pollinated fruits of cloudberry has not yet been compared, but we observed no obvious differences in the rate of ripening process and development of fruit colour. The in vitro induced parthenocarpic fruits also possessed the characteristic odour of cloudberries. Chemical analysis of the parthenocarpic fruits is needed to determine the levels of plant growth regulators in treated fruits.
Further work will address the interaction between gibberellins and auxins in parthenocarpic development of fruits in cloudberry.
This work was funded by The Research Council of Norway.
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