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Carbohydrate Reserves in Plants - Synthesis and Regulation A.K. Gupta and N. Kaur (Editors) © 2000 Elsevier Science B. V. All rights reserved.

Sucrose accumulation and synthesis in sugar beet*

Universität zu Köln, Botanisches Institut, Gyrhofstr. 15, 50931 Köln, Germany

Sugar beet is a biennial plant that reaches its highest sucrose content (15 to 20 % of fresh weight) in the storage root at the end of the first, vegetative year of development. The actual values are the results of classical breeding and optimisation of cultivation methods. The application of molecular genetic methods promises further increase of sucrose yield. One of the most important objectives of "molecular breeding" is to generate transgenic plants expressing specific target genes in excess or in reduced amount, e.g. sucrose synthesising or degrading enzymes or transmembrane transporters. The rationale for such an approach is to test for their importance for final sugar yields. Some results have been obtained meanwhile for the source side. It is known for example that sucrose phosphate synthase plays one of the major roles concerning metabolic control of sucrose synthesis in leaves, whereas plasma membrane bound sucrose transporters are essential for phloem loading. Comparable clear-cut results for the sink side are to be expected for the years to come.

I. INTRODUCTION

Sugar beet is a subspecies of Beta vulgaris (L.) "created" by classical breeding about two hundred years ago for use in industrial production of crystalline sucrose as an alternative to cane sugar. Now a days, sugar beet taproots may contain between 15 and 20% (relative to fresh weight) of sucrose. One of the aims of breeding by classical or more modern molecular genetics based breeding is, among others, certainly still to increase yield. In order to achieve this goal, a profound knowledge of the physiology, biochemistry and regulation of sucrose biosynthesis and accumulation down to the molecular genetic bases is necessary including sound information about beet anatomy and fine structure. Targets for molecular breeding may be located at the source, the long distance transport pathway or within the sink (figure 1). Increasing photosynthesis may result in higher storage capacity if long distance transport and deposition within the storage organ are able to keep pace. Enhancement of sink strength may result in increased sucrose storage by itself and by improved photosynthesis as a consequence of potentially reduced feedback inhibition. In the following book chapter some of the newest results are summarised and discussed in comparison with results reported for other plant species without claiming to represent a complete review.

Dedicated to Professor Dr. Johannes Willenbrink on the occasion of his 70th birthday Present address: al-Biotech GmbH, Neckarstr. 8, 41540 Dormagen, Germany

SE/CC-complex Phloem parenchyma Mesophyll

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