Blackcurrant (Ribes nigrum
L.) is grown widely throughout temperate areas of Europe and New Zealand, with an annual production of 160,000 tonnes in Europe and 185,000 tonnes globally [1
]. The centres of genetic diversity for blackcurrant are situated in northern Scandinavia and Russia, although species are also found in North and South America, Asia and northwest Africa [2
]. The fruit has very high nutritional value, in terms of the content of ascorbic acid and other antioxidants [3
], and its primary commercial use is processing for juice. In the UK, varieties are chosen to give a spread of cropping seasons. Timing of leaf budbreak is an important physiological factor in the developmental processes leading to cropping of blackcurrant, as uniformity of development is essential for both successful harvesting and for optimum fruit quality and yield.
The control of budbreak in many woody perennial plants, including berry fruits such as Ribes
, is dependent on exposure to chilling temperatures during the winter for a sufficient duration to release dormancy, followed by appropriate warmer temperatures in the spring to induce growth [4
]. An insufficient amount of chilling during the dormant period can seriously impact key phenological traits, notably time of budbreak, time and duration of flowering and fruit quality at harvest [5
]. The chilling requirement, in terms of both duration and range of suitable temperatures, varies between species, between cultivars within species [6
], and even between different buds of the same plant [8
]. Blackcurrant has a relatively high chilling requirement for dormancy break, from ca
. 1,300 h in the case of New Zealand-selected germplasm [10
] to over 2,000 h for some late-flowering types such as 'Ben Lomond' from Scotland and most Nordic cultivars [7
]. As warmer winters are likely to become increasingly prevalent [11
], there is a risk of insufficient chilling becoming more widespread in cultivars requiring high levels of winter chilling, so breeding strategies to address this using low-chill parental material are now required [7
However, these must be balanced against the risk of spring frost damage at flowering in cultivars that break bud too early after minimal chilling and whilst the incidence of spring frosts is declining across Europe [5
], there can still be occasional damage in some areas. Also, the risk of spring frost damage at flowering may depend on temperature variance as well as on specific mean temperatures, and climate projections are predicting a future rise in such variance [12
Dormancy in woody perennials is an adaptive mechanism for the survival of winter conditions [9
] and was classified by Lang [13
] into three major types: (i) ecodormancy, where growth inactivity is caused by unfavourable environmental conditions, with a resumption of growth when conditions improve; (ii) endodormancy, caused by endogenous factors within the dormant buds that cannot be overcome even by favourable environmental conditions; and (iii) paradormancy, or correlative inhibition [14
], where the dormancy status of buds is influenced by physiological factors in other parts of the plant (eg. apical dominance). The effects of winter chilling on dormancy break at bud-burst are in endodormant tissues, and further investigation of winter chilling in blackcurrant is structured around improved phenotyping of diverse germplasm [7
] coupled with genetic analysis of Quantitative Trait Loci (QTL) and genes associated with dormancy-related processes. Putative QTL linked to key developmental traits, including time of budbreak, have recently been identified by Brennan et al.
Dormancy break and subsequent physiological events are controlled through the coordinated action of large numbers of genes in woody plants [9
] and whilst there have been many studies of the molecular genetics of dormancy and budbreak, many aspects remain unclear. Microarray studies to examine global gene expression during dormancy and budbreak have been used in a range of woody genera, including Vitis
] and Rubus
]. Most of these studies attempted functional classification of the differentially expressed genes from the transcriptome related to budbreak or, in the case of Mathiason et al.
], to the fulfilment of chilling requirement. Some studies, eg. Mazzitelli et al.
] and Derory et al.
], have identified candidate genes for dormancy-related metabolism, and several of these genes were previously implicated in these processes in other species.
The aim of this study was to examine patterns of gene expression in leaf buds of blackcurrant and to identify the key differential changes in these profiles around the time of budbreak. A custom cDNA microarray was utilised which incorporated blackcurrant Expressed Sequence Tags (ESTs) derived from buds at various stages of dormancy. Expression profiling identified candidate genes associated with budbreak which were mapped onto a blackcurrant genetic linkage map. Initial steps have been taken to develop associated markers that can be deployed in the characterisation of diverse germplasm for inclusion in downstream breeding programmes.