Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

294 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
complex multiple gene family in maize (Z. mays) and pea (Pisum sativum) (Weissenborn
et al., 1995).
The presence of multiple genes is consistent with the hypothesis that different isoforms
of HMGR are involved in separate subcellular pathways to produce specific isoprenoid end
products (Stermer et al., 1994; Rodríguez-Concepción and Gruissem, 1999). Plant HMGR
activity responds in vivo to a variety of developmental and environmental signals, such as
cell division, light, and pathogen infection (Stermer et al., 1994). Plants regulate HMGR
activity at the level of mRNA by differential induction of HMGR gene family members,
and posttranslationally by enzyme modification (Stermer et al., 1994) by a protein kinase
cascade in which phosphorylation inactivates the enzyme (McCaskill and Croteau, 1997).
Calcium, calmodulin, and proteolytic degradation may also have a role in regulation of
plant HMGR (Stermer et al., 1994).
It has been suggested that some of the HMGR isoforms may be involved in separate
subcellular pathways for specific isoprenoid biosynthesis through metabolic channels, or
“metabolons” (Stermer et al., 1994). A number of investigators have reported a correlation
between the induction of isoprenoid biosynthesis, particularly that of sesquiterpenes, and
HMGR enzyme activity (Chappell et al., 1995). In potato, the expression of specific HMGR
genes has been correlated with the accumulation of steroids or sesquiterpenes (Choi et al.,
1992). Chye et al. (1992) observed that only hmg1 was inducible by C 2 H 4 among other
HMGR genes, and also speculated that distinct isoprenoid pathways do occur for rubber
biosynthesis in H. brasiliensis. In cotton, hmg2 has been associated with the synthesis of
specific sesquiterpenes in developing embryos (Loguercio et al., 1999). These results also
support the concept of metabolic channels, or arrays of isoenzymes, independently regulated
and specifically dedicated to the production of particular isoprenoids (Chappell, 1995).
HMGR isoforms are expressed differentially in response to a variety of developmental
and environmental stimuli such as fruit development, phytohormone levels, endogenous
protein factors, light, and pathogen infection (Stermer et al., 1994). In tomato, hmg1 is highly
expressed during early stages of fruit development, when sterol biosynthesis is required for
membrane biogenesis during cell division and expansion (Narita and Gruissem, 1989),
whereas hmg2 expression, not detectable in young fruit, is activated during fruit maturation
and ripening (Rodríguez-Concepción and Gruissem, 1999). Cotton hmg2 encodes the largest
of all plant HMGR enzymes described to date, and contains several functional specialization
features that include a unique 42-amino acid sequence located in the region separating the
amino-terminal domain and carboxy-terminal catalytic domain, which is absent in hmg1
(Loguercio et al., 1999).
13.12 Regulation of α-farnesene biosynthesis in apples
In addition to several ester volatiles that impart the characteristic aroma, apple fruits also
produce large amounts of the acyclic sesquiterpene α-farnesene (C 15 H 24 ;[3E,6E]-3,7,11-
trimethyl-1,3,6,10-dodecatetraene), which accumulates in the skin of apple fruit after harvest
during low-temperature (0–1 ◦ C) storage (Paliyath et al., 1997; Rupasinghe et al.,
2000a). The extent of oxidation of α-farnesene to conjugated trienes has been shown to
be proportional to the development and severity of the postharvest physiological disorder
superficial scald (Huelin and Coggiola, 1970). Biosynthesis of α-farnesene in apple skin
is highly regulated by temperature (Rupasinghe et al., 2000a) and C 2 H 4 (Gong and Tian,