Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

PHOSPHOLIPASE D, MEMBRANE DETERIORATION, AND SENESCENCE 231
10-3-B and 10-6-C than in those of wild type and 8-2-E. Overall, the relative effects on
ethylene and CO 2 production in the fruit of antisense lines 10-3-B and 10-6-C were consistent
with their respective levels of LePLDα2 suppression at 40 DAP (Fig. 9.10a). However,
it is hard to reconcile the delays in ripening and maximum ethylene and CO 2 production
in fruit of the antisense line 8-2-E because they exhibit both overexpression of LePLDα2
and high levels of PLD alpha activity at 40 DAP (Figs. 9.10a and 9.19). Hypothetically,
delayed ripening could be a secondary effect of transformation. Unfortunately, the empty
vector lines (including E8 without the antisense LePLDα2 insert), which were intended as
transformation controls, did not yield enough viable seed for further propagation.
9.7.9 Quality attributes of ripened LePLDα2 antisense transgenic fruit
Evaluation of quality in fruit harvested at breaker plus 7 days was limited to three LePLDα2
antisense lines (10-3-B, 10-6-C, and 8-2-E) compared with wild type, and three simple
parameters, including fresh weight, red color, and whole fruit firmness (Fig. 9.20). Fruit
size and weight varied considerably within each line, but the mean fresh weight was not
markedly different among the four lines, ranging from about 122 to 175 g (Fig. 9.20a). Mean
fresh weight of fruit from all three transgenic lines was at least slightly greater than that for
wild-type fruit, and fruit weight for line 10-3-B was significantly greater than that for both
10-6-C and wild type at the 5% confidence level. The positive CIE a* values, which are a
measure of “redness,” were quite similar for fruit from all four lines (Fig. 9.20b). The values
were highest for 10-3-B and 8-2-E, and lowest for 10-6-C, but the small differences were not
significant. Mean whole fruit firmness, measured by flat plate compression analysis, was
closely similar for wild type and antisense suppressed lines 10-3-B and 10-6-C (Fig. 9.20c).
In contrast, fruits of the antisense line 8-2-E, with anomalous overexpression of LePLDα2
at 40 DAP (Fig. 9.10a), were significantly firmer (p > 0.05) than those from the other three
lines. This is in accord with the delayed, relatively low-level ethylene production, and much
delayed (ca. 5 days relative to wild type) respiratory maximum exhibited by fruit of this
line (Fig. 9.19).
9.8 PLD in signal transduction
Molecular events resulting from ethylene perception by the receptor and changes in gene
expression are still being elucidated. Ethylene receptors belong to the hybrid kinase type
(ETR1, ETR2, EIN4) or histidine kinase type (ERS1, ERS2) (Chang and Stadler, 2001). Signal
reception results in the phosphorylation of a histidine residue in the transmitter domain
of the two-component receptor system. The phosphate is further transferred to the receiver
domain through a phosphorelay mechanism (D’Agostino and Kieber, 1999). Downstream
events in the activation of gene expression proceed through MAP kinase cascade and activation
of transcription of ERF1 (EREBP—ethylene response element-binding protein). ERF1
binds to the GCC-box promoter elements to activate ethylene-responsive gene expression.
Several genes show increased expression in response to ethylene treatment of tomatoes
within 15 min and include those for intermediates of signal transduction pathways (CTR1)
and other transcription regulator proteins (Zegzouti et al., 1999). Several details of these
events are yet to be worked out (Chang and Stadler, 2001; Lohrmann and Harter, 2002).
A recent study shows that ethylene signal transduction can proceed in parallel pathways,