The structure of two CONSTANS-LIKE1 genes in potato and its wild relatives

Day length controls development in many plants. In Arabidopsis thaliana, the CONSTANS (CO) gene has been firmly established as a key component in the photoperiodic pathway of floral transition; less is known about CONSTANS-LIKE1 (COL1) orthologues of this gene in Arabidopsis and several other species. The CONSTANS protein comprises two B-box-type zinc fingers, CCT domain, and a variable middle region (MR) which corresponds to exon 2 in the COL1 genes of Solanum species. Solanum COL1 proteins are over 85% identical within the genus and about 50% similar to Arabidopsis CO. Comparative COL1 analysis in several cultivated and wild Solanum species discerned two gene variants, which differed in the structures of exon 2 and introns 1 and 2. In exon 2, two variants were primarily discerned by the numbers of AAC/AAT and CAA/CAG repeats coding for polyasparagine and polyglutamine tracts in MR; therefore two variants were dubbed short and long COL1 genes (sCOL1 and lCOL1). However, intron 1 in lCOL1 was shorter than in sCOL1 due to three indels, whereas intron 2 in available COL1 sequences was represented by three different variants. The temporal profiles of sCOL1 and lCOL1 expression in tuberosum potato dramatically differed under short and long day, and the level of sCOL1 expression exceeded that of lSOL1 by an order of magnitude. Both sCOL1 and lCOL1 were found in each Solanum genome under study and in each individual plant, and the ratio of their copy numbers was not related to plant ploidy and photoperiodic response. Evidently the evolution of two COL1 genes preceded Solanum speciation, and the day-length response of diverse Solanum genotypes does not stem from the primary COL1 structure.