FAIR-CT96-5003 Transcriptional regulation and tissue specificity of walnut shoot chalcone synthase genes

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Objectives:

The rooting of walnut shoots decreases as the trees grow older. This is an obstacle to mass propagation. Flavonoids are good markers of plant development and it has been shown that two flavonols, namely myricitrin and quercitrin, as well as condensed tannins accumulate during the growth period in aged walnut shoots. It has also been shown that the addition of myricitrin to in vitro culture medium diminishes rooting ability. Flavonoids accumulate mainly in leaves, buds and phloem. An enzymatic study has shown that phenylalanine ammonia lyase (PAL), the first enzyme of the phenylpropanoid pathway, does not control flavonoid accumulation but is linked to the lignification process. On the other hand, chalcone synthase (CHS), the first enzyme of the flavonoid pathway regulates the accumulation of flavonoid in phloem, buds and leaves. Consequently, this research aimed to improve knowledge of walnut ageing and related consequences on rooting ability through the regulation of the level of CHS expression in the various tissues of a walnut shoot.

Activities and Results:

Two full length chalcone synthase cDNAs, about 98% identical, at the nucleic acid level were characterised from leaves of the adult walnut tree. The open reading frame sequences coded for two 389 amino acid polypeptides, 99.5% identical. The two deduced amino acid sequences exhibited the typical CHS consensus in the middle of the sequences and the essential amino acids for CIIS activity. Additionally, they showed a high degree of similarity to CHS of Pyrus malus (91%) and Pinus sylvestris (85%). As with CHS from other trees, some parts of the predicted protein sequences from the walnut CHS are similar to the stilbene synthase protein sequences (between 70 and 76% identity). Southern blot analysis indicated that CHS in walnut is encoded by a small gene family consisting of maximum three members. In addition a CHS promoter of 1.7 kb length has been isolated, which still had to be sequenced.

The expression of CHS during growth of adult and rejuvenated walnut shoots was investigated. A 1.5 kb CHS transcript was detected in leaves, buds, fibre and bark, whereas no transcript was detected in wood and medulla. Growth of walnut shoots resulted in a transient increase in leaf and fibre CHS transcripts, which was much more pronounced in adult than in rejuvenated tree shoots

Conclusion:

The results, correlating with CHS enzyme activity and flavonoid accumulation, indicate a tissue specific influence of age on flavonoid biosynthesis at the CHS mRNA level.

Keywords: Walnut shoot, chalcone synthase

Contacts

Scientific Supervisor

• Heinrich SANDERMANN / GSF-Forschungszentrum fuer Umwelt und Gesundheit GmbH Institut fuer Biochemische Pflanzenpathologie / GERMANY