Isoprenoid Biosynthesis in Fruits and Vegetables - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 296

conserved in the carboxyl terminal region, highly divergent in the amino terminal region,
and possess two putative transmembrane domains. The presence of two transmembrane
domains in the amino terminal region is a common feature of all the HMGR genes cloned
to date from plants, but differ from the animal HMGR gene that has seven transmembrane
domains (Liscum et al., 1985). Northern blot analysis revealed that hmg1 and hmg2 genes
were expressed differentially in apples during cold storage. It is interesting to note that
hmg1 was constitutively expressed throughout the 16-week postharvest storage period.
The transcripts for hmg2 showed the highest levels in parallel with the accumulation of
α

-farnesene in the skin, which also increased during storage. The increase in abundance
of hmg2 transcript coincides with endogenous C 2 H 4 production in apples during storage.
However, the abundance of hmg2 mRNA was relatively low compared with that of hmg1 .
The differential regulation of hmg1 and hmg2 expression in apple is consistent with the
theory that levels of the different HMGR isozymes in plants are modulated in response to
specific developmental and stress signals (Stermer et al., 1994). The expression of hmg2
resembled the in vitro changes of HMGR activity during storage, more than that of hmg1 .
The poor correlation between total HMGR activity and farnesene accumulation could be due
to several factors such as posttranscriptional events such as mRNA processing, transcript
stability, nucleocytoplasmic transport, translation efficiency, and/or protein modification
and half-life. HMGR is regulated by a protein kinase cascade in which phosphorylation
inactivates the enzyme (McCaskill and Croteau, 1997).

13.14 Regulation of HMGR by ethylene

C 2 H 4 -mediated stimulation of -farnesene biosynthesis is partly due to the induction of

HMGR activity by C 2 H 4 (Rupasinghe et al., 2001). In apples, the C 2 H 4 action inhibitor 1-
MCP suppressed the expression of hmg2 completely and hmg1 partially. 1-MCP inhibited
respiratory CO 2 evolution by 50%, suggesting that inhibition of

α

-farnesene synthesis
in apple by 1-MCP could be also regulated through the available acetyl CoA pool that
is utilized by isoprenoid pathway (Rupasinghe et al., 2001). However, it is clear from
the literature that C 2 H 4 or other stimuli, which induce C 2 H 4 production, can influence
differential expression of the isogenes of HMGR. In rubber, hmg1 is induced by C 2 H 4 ,
while hmg3 expression remains stable, indicative of a housekeeping nature of the gene
(Chye et al., 1992). Furthermore, hmg1 is expressed predominantly in the laticifers, the cells
specific to rubber biosynthesis (Chye et al., 1992); thus, it is postulated that hmg1 of rubber
encodes the HMGR enzyme involved in rubber biosynthesis. In tomato, hmg1 expression
is very high at early stages of fruit development but declines during ripening (Narita and
Gruissem, 1989), but hmg2 is highly expressed during ripening (Rodrıguez-Concepcion
and Gruissem, 1999). In Camptotheca acuminata , hmg1 mRNA increased in response to
wounding, but hmg2 and hmg3 transcript levels remained unaffected (Maldonado-Mendoza
et al., 1997). In potato, hmg2 mRNA levels are elevated in response to wounding or fungal
elicitors suggesting that hmg2 is a defense-related gene or hmg2 is the major elicitor-induced
isogene (Yang et al., 1991).
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).
In C. acuminata apices, hmg1 is expressed at high levels, hmg3 is moderately expressed,
and hmg2 transcripts are absent (Maldonado-Mendoza et al., 1997). It is speculated that the

α