Association of Gene Expression Data with Dormancy and Sprout Suppression in Onion Bulbs Using a Newly Developed Onion Microarray

Onions have evolved as a storage organ in order for the plant to overwinter. During the transition of endo-dormancy (dormancy) to eco-dormancy (sprout suppression) to sprout growth, the bulb undergoes change from sink organ to source in order to sustain cell division in the base plate. The mechanisms controlling these stages of development have yet to be elucidated. The experiment herein was carried out in 2007-2008 at a Norfolk, UK site using two UK-grown cultivars 'Sherpa' and 'Wellington' grown according to normal UK commercial practise. Transcriptional changes in onion bulbs throughout postharvest storage were investigated using a newly developed onion microarray. Probes were functionally categorised and clustered using K-means clustering analysis according to altered gene expression analysed throughout storage; before curing, after curing and at two further time points during cold storage. Probes with tentative annotations relating to defence/stress formed the second largest functional category in a cluster highly up-regulated. In contrast, ca. 20% of a cluster highly down-regulated was placed in the functional category related to photosynthesis. Bulb formation is well known to be influenced by photoperiod, and as light is perceived by the leaves and the signals transmitted to other plant parts, then the down-regulation of these light responsive elements could follow the die back of onions and subsequent removal of the leaves at harvest. It would be expected that expression of genes related to cell cycle regulation would increase during storage, as the meristematic cells begin to divide and elongate to form new sprout tissue. Indeed, three clusters contained a high proportion of probes with tentative annotation for genes including histones and tubulins. It is likely that the transition from dormancy to sprout suppression will also involve changes in the cell walls, as modifications are made to control the transport of reserve compounds and cell division resumes at the meristem. In general, cellulose synthase decreased postharvest. Our transcriptional profiling identified probes with homology to genes involved in cell wall modification including, polygalacturonase, pectinesterase, beta galactosidase and pectate lyase, as being up-regulated after harvest. Taken together, this suggests that the cell walls are being degraded and the bulb is becoming softer.