Sugar beet (Beta vulgaris
L.) is a major crop for sugar production in countries with a temperate climate. The agronomic productivity of sugar beet, as a direct seeded crop, is determined significantly by the uniformity of seedling emergence in the field [1
]. Despite the economic importance of field emergence and a strong desire of growers to improve it, little is known about the molecular mechanisms underlying this trait, mainly due to its genetic complexity and the large environmental effects.
In the current study we concentrated our efforts on the molecular analysis of seed germination, the first stage of field emergence. Germination sensu stricto
commences with the uptake of water by the dry seed – imbibition – and is completed when a part of the embryo, usually the radicle, extends to penetrate the structures that surround it [2
]. Seed germination has been intensively investigated on the molecular level in species having oil-storing seeds, e.g. Arabidopsis
], starch-storing seeds, e.g. cereals [8
], and protein-accumulating seeds, e.g. legumes [10
]. Despite some interspecies differences, regulation of germination is mostly determined by the interaction between the two plant hormones, gibberellins (GAs) and abscisic acid (ABA). Whereas ABA plays a primary regulatory role in seed maturation and dormancy, GAs are essential for the induction of germination. Other plant hormones, especially ethylene and brassinosteroids (BRs), are also involved in the regulation of germination too. The presence of an extensive cross talk between the plant hormone signalling systems is suggested [12
Sugar beet belongs to the Amaranthaceae
and utilizes the starchy perisperm, a diploid maternal tissue originated from the nucellus, as a storage organ of the seed. At seed maturity the perisperm represents a dead tissue surrounded by an embryo, so that only one ('the inner') of the two cotyledons is directly adjacent to it. The 'botanically true' seed consisting of embryo and perisperm covered by a testa, is contained within a thick fruit structure called pericarp [16
]. In addition to the starchy perisperm, which composes around 35% of the seed dry weight, sugar beet embryos accumulate also proteins (16%) and lipids (16%) as reserves [1
]. The unique morphology of the sugar beet seed and the specific proportion of seed storage compounds raise the question, whether germination in sugar beet is similar to the one in the previously investigated species or follows its own program.
Only few studies have investigated sugar beet germination [1
]. Recent analysis of hormone signalling during sugar beet germination [17
] demonstrated that some specific features in the regulation do exist. In contrast to other species, the radicle emergence of sugar beet fruits or seeds is not appreciably affected by a treatment with GAs, BRs, auxins, cytokinins and jasmonates, but is promoted by ethylene or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Other publications address the sugar beet seed storage reserves. Their mobilization attracted much attention because the reserves are the only source of energy for seedling growth until establishment of the photosynthetic apparatus. Elamrani et al. [1
] showed that during early growth, sugar beet cotyledons behaved mainly as a lipid mobilization and gluconeogenic tissue thus providing substrates to the seedling. A subsequent study of de los Reyes et al. [20
] supported the importance of glyoxylate cycle enzymes, which are involved in lipid catabolism, during germination under stress conditions, and suggested, for the fist time, to use these enzymes as biochemical targets for enhanced germination and improved emergence in sugar beet.
Among other potential targets for trait improvement are genes involved in the onset of seed and seedling stress tolerance [21
]. Both seed germination and early seedling growth are very sensitive to biotic and abiotic stresses [22
]. The inability of the seedling to tolerate adverse environmental conditions may result in severe damage and decreased overall field emergence. McGrath et al. [21
] recently showed that sugar beet germplasm differed in germination under salt stress and reported that selection for enhanced stress tolerance during germination appeared to be feasible, since gains were observed in progeny of salt germinated seedlings. Extending our knowledge of genetic and molecular determinants of sugar beet seed germination would facilitate the detection of new targets for breeding sugar beets with an enhanced field emergence potential.
The availability of the complete genome sequence of the model plant Arabidopsis thaliana
, together with the development of high-throughput procedures for global analysis of gene functions has launched the 'post-genomic' era in plant biology [24
]. To gain insight into the sugar beet germination we initiated an analysis of gene expression taking advantage of recently developed genomic tools. Here we present a collection of 2,784 Expressed Sequence Tags (ESTs) derived from dry mature and germinating sugar beet seeds and examine temporal transcriptional profiles of the corresponding genes during germination using dedicated cDNA macroarrays. mRNA profiles were investigated for seeds germinated under standard conditions of temperature and water supply as well as for seeds germinated under multistress conditions combining salt, osmotic, liquid excess and reduced temperature stresses. Based on the obtained transcription data the main metabolic pathways, active during germination, could be described. By comparing the mRNA profiles of seeds germinated under various conditions, we were able to detect stress induced alterations in gene expression. This analysis sheds light on main adaptation mechanisms used by germinating sugar beet seeds to withstand stress.