The expression pattern of the Pho1a genes encoding plastidic starch phosphorylase correlates with the degradation of starch during fruit ripening in green-fruited and red-fruited tomato species

Abstract

Genes encoding plastidic starch phosphorylase Pho1a were identified in 10 tomato species (Solanum section Lycopersicon). Pho1a genes showed higher variability in green-fruited than in red-fruited tomato species, but had an extremely low polymorphism level compared with other carbohydrate metabolism genes and an unusually low ratio of intron to exon single nucleotide polymorphisms (SNPs). In red-fruited species, Pho1a was expressed in all analysed tissues, including fruit at different developmental stages, with the highest level in mature green fruit, which is strong sink organ importing sucrose and accumulating starch. In green-fruited species Solanum peruvianum and Solanum arcanum, the Pho1a expression level was similar in mature green and ripe fruit, whereas in Solanum chmielewskii, it was higher in ripe fruit, and in Solanum habrochaites, the dynamics of fruit-specific Pho1a expression was similar to that in red-fruited tomatoes. During fruit development, in red-fruited Solanum lycopersicum, sucrose level was low, the monosaccharide content increased; in green-fruited S. peruvianum, the sucrose concentration increased and those of monosaccharides decreased. In both species, the starch content and Pho1a expression were downregulated. The evolutionary topology based on Pho1a sequences was consistent with the current division of tomatoes into red-fruited and green-fruited species, except for S. habrochaites.