The present invention includes modified phytochrome A (PHYA) nucleic acid molecules
in which DNA sequences coding for "active site" amino acid residues have been mutated
to generate hyperactive phytochromes. In particular; a serine/threonine residue
at the hinge between the N- and C-terminal domains as well as at the N-terminal
serine/threonine cluster of phytochromes (e.g., serine-598 and serine-7 in oat
phytochrome A) for (a) Pr/Pfr-dependent phosphorylation and (b) dephosphorylation
by a phytochrome phosphatase (PP2A) was substituted with alanine. (c) In addition,
amino acid residues within the phytochrome chromophore pocket are mutated to generate
the bathchromic shift of the Pr-absorption band of both wild type and above-mentioned
mutant phytochromes. The plants with the bathchromically shifted absorption spectrum
are expected to respond to the canopy and shade conditions for growth and greening
responses to far-red light with greater efficiency than are the wild type plants
with normal absorption band maxima. These mutative modifications confer hyperactivity
to the far-red light responsive phytochromes A. Thus, the biological activity of
the modified oat PHYA was shown to be hyperactive compared to wild type PHYA, characterized
by its ability to reduce internode elongation of adult plants. Overexpression of
the phytochrome phosphatase exhibits a suppressed growth with shorter internodes
and belated flowering, qualitatively consistent with the phenotype of a ser598ala
mutant oat phytochrome. The invention also includes plants having at least one
cell expressing the modified PHYA, vectors comprising at least one portion of the
modified PHYA nucleic acids, and methods using such vectors for producing plants
with reduced stature.
