Phytochrome A (phyA) is represented in plants by two species, phyA' and phyA'', with different properties and modes of action (Sineshchekov, Funct. Plant Biol., 2019, 46, 118-135). They differ by the modification of a serine(s) residue at the N-terminus, possibly, by phosphorylation. To verify if these serines could be the Ser8 and Ser18 (in Avena sativa phyA, AsphyA), whose autophosphorylation modulates AsphyA stability and sensitivity as shown with the use of the serine-to-alanine substitution AsphyA mutants (S8A, S18A and S8/18A) (Han et al., Plant Cell Physiol., 2010, 51, 596-609), we have undertaken low-temperature (85 K) fluorescence investigations of phyA in these transgenic lines. The content and proportion of phyA' and phyA'' were essentially the same in wild-type AsphyA and its mutants, and in endogenous Arabidopsis phyA. All the lines revealed a higher phyA'/phyA'' proportion upon longer germination-inducing preillumination (3 h vs. 15 min white light) supporting our earlier finding that the phyA differentiation into the subpools is light-regulated. These observations and our earlier data imply that this process involves N-terminal serine(s) different from the autophosphorylated Ser8 and Ser18 (in AsphyA) narrowing down the area of further search for the exact site(s) of the phyA modification.