Light regulated and tissue specific expression of Sn

--Gabriella Consonni and Chiara Tonelli

We reported (MNL 62:93-94, 1988) that Sn shares molecular similarity to the R-nj gene and several Sn accessions have been characterized at the DNA level using a R-nj DNA probe (S. Dellaporta et al., Chrom. Struct. and Funct., Plenum, 263-281, 1988).

The expression of Sn is tissue specific and light dependent. Tissues generally devoid of pigment but developing colour in the presence of Sn are the scutellar node, mesocotyl, leaf base and midrib, and ovary integuments. To study the expression of this gene we compared two different Sn accessions: Sn-Coop, which is strictly light dependent, and Sn-Bol3, which is partially constitutive in its expression, conditioning a weak pigmentation in dark growing seedlings. Total RNA was extracted from scutellar nodes, mesocotyls and leaves of seedlings grown either in dark or in light, analyzed by Northern blot according to Koes et al. (EMBO J. 7:1257-1263, 1988) and probed with R-nj:1 fragment.

Figure 1. Expression of Sn-Bol3 and Sn-Coop analyzed by Northern blot. Each lane contains 10µg of total RNA extracted from: scutellar nodes and mesocotyls of light grown seedlings (a), scutellar nodes and mesocotyls of dark grown seedlings (b), and leaves from light grown seedlings (c). The relative positions of the maize 26s and 17s ribosomal subunits are indicated on the left.

The results (Fig. 1) can be summarized as follows:

1) A transcript of about 3.5Kb is found only in the colored seedling tissues (scutellar nodes and mesocotyls and not leaves).

2) The transcript is found only in genotypes that carry Sn and not in sn.

3) The expression of Sn is light regulated. In fact no transcript is found in scutellar nodes of Sn-Coop grown in darkness, while for the partially constitutive allele Sn-Bol3 the mRNA is found, at lower levels as expected, and also in the scutellar nodes of dark growing seedlings. These results confirm the tissue specificity and light dependency of Sn expression.

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