Isoprenoid Biosynthesis in Fruits and Vegetables - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 290

accumulation at the breaker stage, even with its lower levels of synthesis. However, increased
PSY activity at the later stages of ripening would certainly account for the bulk of carotenoid
accumulation.

13.9 Photomorphogenesis

Light-mediated changes in plant growth and development are called photomorphogenesis.
Phytochromes are one of the three classes of photoreceptors, along with cryptochromes
and UV-B photoreceptors that are involved in photomorphogenesis. Phytochromes are red
light (R)/far-red light (FR) receptors composed of a protein covalently attached to a linear
tetrapyrrole chromophore. Phytochromes are synthesized in the biologically inactive, R-
absorbing form: Pr (

λ =

660 nm). Upon exposure to R, Pr is converted to the biologically
active, FR-absorbing form: Pfr (

max

730 nm) (Quail, 2002). Exposing Pfr to FR converts
the protein back to Pr, and subsequent exposure to R and FR will enable phytochrome
conversion between these two forms. Most phytochrome responses occur at low fluence
rates of 1-1,000

λ max =

mol/m 2 of light, and are commonly characterized by R/FR reversibility
and by reciprocity, which is the requirement for a total number of photons, irrespective of
the duration of the light exposure (Neff et al., 2000).

μ

13.9.1 Phytochrome gene family in Arabidopsis and tomato

Tomatoes have five phytochrome genes ( PHYA , PHYB1 , PHYB2 , PHYE , and PHYF ) that
fall into the same four families as in Arabidopsis . They also have a conserved photosensory
domain and share (88-98%) amino acid sequence identity to their Arabidopsis counterparts
(Hauser et al., 1997; Alba et al., 2000b). In dark-grown tissue, phyA is the most abundant of
the phytochromes; however, exposure to light causes degradation of phyA in the Pfr form,
as well as downregulation of PHYA gene expression. Consequently, phyA levels can drop
up to 100-fold from light exposure. In light-grown Arabidopsis plants, phyB becomes the
most abundant isoform, with lower levels of phyC-E (Clack et al., 1994; Hirschfeld et al.,
1998; Neff et al., 2000); nevertheless, all five phytochromes are expressed throughout the
plant with only minor differences in their expression patterns (Somers and Quail, 1995;
Goosey et al., 1997; Neff et al., 2000).

13.9.2 Tomato phytochrome mutants hp-1 and hp-2

A number of tomato mutants exhibit exaggerated photoresponses such as high pigment
(hp-1 and hp-2), atroviolacea (atv), and intense pigmentation (Ip) (Kendrick et al., 1994).
hp-1 was found in 1916 at the New Jersey farm of Campbell Soup Co. (Thompson,
1955; Reynard, 1956). A second gene, hp-2 , was described by Soressi (1975). The two
hp genes are nonallelic, and a recessive mutant at either locus produces tomato plants
with similar characteristics. Mature fruits exhibit high levels of carotenoids, particularly
lycopene, and increased vitamin C. The leaves and immature fruit are dark green because
of high chlorophyll levels. When grown under continuous R or yellow light, seedlings have
shorter hypocotyls and higher anthocyanin levels (Kerckhoffs et al., 1997). Phenotypically
stronger alleles hp-1w and hp-2j have been described (Peters et al., 1989; Van Tuinen et al.,
1997).