The role of Sn in the light-regulated activity of enzymes of flavonoid biosynthesis

Sn is a light-regulated factor, closely linked to R, responsible for specific pigmentation of the mesocotyl of the maize seedling. One of the Sn accessions, Sn-bol3, appears to be present in two variants, one producing intense (Sn-s) and the other weak (Sn-w) pigmentation of the seedling tissues. We had previously shown (Gavazzi et al. in Plant Genetics, 1985) that the phenylalanine ammonia-lyase (PAL) and UDP glucose 3-0-glucosyl transferase (UFGT) are related to the Sn genetic constitution. To further analyze the role of Sn on flavonoid biosynthesis, we tested the activity of chalcone synthase (CHS) and chalcone isomerase (CHI), two enzymes involved respectively in the production of naringenin chalcone and in its isomerization to naringenin. The activity was determined on extracts of homozygous Sn-s mesocotyls following different time of irradiation (14000 Lux provided by Power stars -HQ1-T400 W/DV OSRAM lamps).

Increases of CHS and CHI were observed in the Sn-s tissues with a similar time course (Figure 1); the maximum was observed 48 hours after the onset of irradiation, which was not coincident with anthocyanin accumulation, still increasing after 72 hours. In the dark both enzymes and pigment do not show any increment. Data in Table 1 show the pigment content and the two enzyme activities determined at the onset of irradiation and after 48 hours of growth in continuous light or darkness. The results indicate that both pigment content and enzyme activities are regulated by light as well as by the Sn genetic constitution. In homozygous c2 sn seedlings (c2 is the gene responsible for CHS activity) there is a complete suppression of pigments and CHS activity while a low but significant level of CHI is still present. This low level of enzyme activity could be accounted for by a residual non-induced activity of CHI or by the existence of two enzymatic forms of CHI, only one of the two being Sn and light dependent.

These observations indicate that the presence of Sn promotes the enzymes tested in a coordinate, light-dependent manner suggesting a transactive regulatory role of this gene on the structural genes (or their products) involved in flavonoid biosynthesis in maize.

Table and Figure.

G. Consonni, M.L. Racchi, S. Shammah, G.A. Gavazzi
 
 


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