|Trebbi, Daniele - KEYGENE NV, NETHERLANDS|
|MCGRATH, J MITCHELL|
Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2008
Publication Date: January 1, 2009
Citation: Trebbi, D., McGrath, J.M. 2009. Functional differentiation of the sugar beet root system as indicator of developmental phase change. Physiologia Plantarum. 135(1):84-97. Interpretive Summary: Sugar yield in sugar beet is the product of the mass of beets harvested times their percentage of sucrose. At germination, the mass of the crop is very small and the percentage of sucrose is miniscule. Therefore, changes must take place during development of the crop that lead to the high yields of harvested sucrose obtained in modern hybrids, but the nature of these changes is virtually unknown. Reasoning that the development of sugarbeet, and the accumulation of sucrose, is not a linear progression during the growing season, sucrose content and plant weight were measured at weekly intervals during the first nine weeks after germination to ascertain the pattern of growth and sucrose accumulation capacity during this early stage of root development. Plant weight increase did show a linear progression of increases, but the rate of accumulation of sucrose was maximal at 4-6 weeks after emergence, in the greenhouse. Molecular profiles of beets before and after this phase of growth showed a very striking shift in gene expression, suggesting a phase of development were occurring, such as the well known transition from juvenile to adult growth of stems in some plants. The existence of such a transition has great potential impact for sugar beet improvement germplasm enhancement, including now having opportunities to select for earlier or greater duration of the maximal sucrose accumulation rate, and allowing scientists to better understand the role of plant development in the expression of different disease symptoms caused by the same pathogen depending on plant age, which co-incidentally occurs at or around the physiological change described here at 4-6 weeks of age.
Technical Abstract: Developmental phase transition in the plant root system has not been well characterized. In this study we compared the dynamics of sucrose accumulation with gene expression changes analyzed with cDNA-AFLP in the tap root system of sugar beet (Beta vulgaris L.) during the first nine weeks after emergence (WAE) in two lines characterized by different sucrose yields. Although differences between lines started to be evident after 9-WAE, sucrose mainly accumulated between 4- to 6-WAE and a remarkable shift in gene expression was observed between 5- to 6-WAE in both lines. These common physiological and gene expression changes represented the functional differentiation of the tap root and were considered as indicators of developmental changes in the sugar beet root system. To identify genes and metabolic pathways involved in this developmental shift, a root cDNA library was hybridized with probes enriched for 3- and 7-WAE transcripts and level of expression of differentially expressed transcripts were analyzed by cDNA microarray. Several genes involved in the regulation of tissue development were found to be differentially regulated. Genes involved in protein metabolism, disease-related and secretory system were up-regulated before the functional differentiation transition, while genes under hormonal control were up-regulated after the functional differentiation transition. The implications of studying developmental phase change of the root system to comprehend plant developmental regulation at the whole-plant level and the possibility to use these changes as early-selection parameters in breeding programs are discussed.