Results and discussion

Greenhouse spring production
Spring production was evaluated in trials performed in two different locations: Monsampolo and Pontecagnano. Spring production was divided into early production, consisting of the first four harvests, and total production, consisting of sixteen harvests. Early spring production corresponds to the cultivation period from March to the first half of May, with temperatures somewhat low for fruit-set and growth. During this period, the average minimum and maximum temperatures ranged from 7° to 17°C in southern Italy (Pontecagnano) and from 14° to 17°C in central Italy (Monsampolo). The transgenic parthenocarpic hybrids gave a significantly higher yield, on the average a six-fold increase during early production, in comparison to their controls (Table 1). The increment in the number of fruits per plant and the higher average weight of the fruits were the main causes of the increased early spring production of the parthenocarpic hybrids. The suboptimal/adverse environmental conditions did not affect the growth of GM parthenocarpic fruits and the average fruit weight was significantly higher in the GM eggplants in comparison to untransformed controls. The traditional parthenocarpic cultivar Talina produced fruits with an average weight similar to those of GM eggplant fruits (Table 1). However, due to the higher number of fruits per plant, the productivity of the transgenic parthenocarpic hybrids was also increased, by an average of five-fold, when compared to cv Talina (Table 1). During the whole harvesting period, the GM fruits were always seedless (Fig. 1) and were normal in both size and shape.
Figure 1  Eggplant fruits of the transgenic parthenocarpic hybrid P1 from the greenhouse spring trial. Left: fruit at the stage of commercial ripeness; middle: an overripe eggplant fruit from a border plant; right: a longitudinally cut fruit showing the complete absence of seeds.
Table 1  Eggplant production of parthenocarpic hybrids and their respective controls at springtime.
Genotype  Early production  Total production
Yield/plant (g)  Fruits/plant (n)  Fruit weight (g)  Yield/plant (g)  Fruits/plant (n)  Fruit weight (g)
P1  488 b  1.9 a  268.3 a  2241 a  8.4 a  253.9 a
P2  695 a  2.6 a  290.8 a  2288 a  8.6 a  260.2 a
C1  75 c  0.4 b  227.0 b  1547 b  7.8 a  187.5 b
P5  522 b  2.0 a  281.8 a  2163 a  9.2 a  230.5 a
C2  116c  0.7 b  170.6 b  1574 b  9.3 a  167.3 b
Talina  114c  0.3 b  270.5 a  2360 a  9.4 a  229.9 a
For each trait at least one common letter indicates no significant difference according to the Duncan test (P = 0.05). Mean values of yields per plant, number of fruits per plant, and fruit weight of three transgenic parthenocarpic hybrids (P1, P2 and P5), two controls (C1 and C2) and the commercial cultivar Talina. C1 hybrid plants represent the controls of P1 and P2 transgenic hybrid plants. C2 hybrid plants are the controls of the P5 transgenic hybrids. Data are the average of the Monsampolo and Pontecagnano locations. The experiments were carried out in greenhouse at springtime. The increased productivity of GM hybrids characterised both the early spring production (i.e. the first four harvests) and the whole spring production cycle (i.e. sixteen harvests). During the whole spring production cycle, the hybrids P1 and P2 gave an average yield that was 46% higher with respect to the corresponding control C1 (Table 1). The hybrid P5 gave a 37% higher yield with respect to its control C2. The average total number of fruits produced per plant in both locations was similar in all the hybrids (8–9 fruits/plant). However, the higher average weight of the GM fruits led to a higher total yield of transgenic hybrids with respect to their controls. When considering the whole spring cultivation cycle, the parthenocarpic cultivar Talina gave a total production that was not significantly different from either of the three GM hybrids (Table 1).

Open field (summer) production
Summer production was evaluated in an open field trial carried out during the optimal period of eggplant cultivation. Plants were transplanted on May 20th and the last harvest took place on September 11th. The early production of the transgenic hybrids, consisting of the first three harvests, was significantly higher than that of the untransformed hybrids (Table 2): P1 and P10 hybrids yielded, respectively, 36% and 76% more than their corresponding controls, C1 and C10. The difference in productivity between P1 and C1 hybrids, which have long-shaped fruits, was caused by the higher average weight of GM fruits. When comparing P10 and C10 hybrids, which have sub-oval fruits, the higher yield obtained with GM plants during the early harvesting period was due to the increased number of fruits per plant.
Table 2  Eggplant production of parthenocarpic hybrids and their respective controls at summertime.
Genotypes  Early production  Total production
Yield/plant (g)  Fruits/plant (n)  Fruit weight (g)  Yield/plant (g)  Fruits/plant (n)  Fruit weight (g)
P1  1158a  2.7 a  433 b  3039 a  9.1 a  335 b
C1  846 b  2.4 a  344 c  2211 b  7.7 ab  288 b
P10  1287 a  2.4 a  553 a  2791 ab  6.8 bc  410 a
C10  731 b  1.3 b  541 a  2386 ab  5.7 c  415 a
For each trait, at least one common letter indicates no significant difference according to the Duncan test (P = 0.05). Mean value of yields per plant, number of fruits and fruit weight of two transgenic hybrids (P1 and P10) and their corresponding untransformed hybrids (C1 and C10), cultivated during summertime under open field conditions. Consisting of ten harvests, the total production of P1 hybrids was 37% higher than control C1 eggplants (Table 2). The difference in total yield between P1 and control C1 hybrids was statistically significant and due both to the higher number of fruits/plant and to the increased weight of GM fruits. It is noteworthy to point out that when considered individually, neither trait (number of fruits/plant or fruit weight) showed statistically significant differences between the GM and untransformed plants (Table 2). Although higher in P10 hybrids in comparison to its control C10, the total yield (the number and average weight of the fruits) was not significantly different between the two. During the whole harvesting period, fruits from both P1 and P10 parthenocarpic hybrids were always seedless (Fig. 2), whilst control fruits always had seeds. Therefore, under open field cultivation, the DefH9-iaaM gene had a positive influence on fruit quality, as GM DefH9-iaaM fruits were always seedless. Fruit quality affects the economic value of eggplant production.
Figure 2  Eggplant fruits from the open field summer trial. Left, uncut and cut fruit of the transgenic hybrid P10; Right, cut and uncut fruit of the C10 control hybrid. Although the environmental conditions were optimal and consequently did not affect negatively fruit-set, the DefH9-iaaM gene caused both faster development and growth of the fruits as indicated by the increased early-summer production (the first three harvests). In this regard, it is worthwhile to stress that expression of the DefH9-iaaM gene takes place in the placenta and ovules before pollen development. As a consequence, in GM parthenocarpic plants fruits are seedless and fruit development initiates well before non-GM controls [11].
In all trials we have never used homozygous lines because growers mostly employ F1 hybrids. The use of hemizygous primary transformants as pollinator plants allowed us to obtain in rather short time, by in vivo selection for kanamycin resistance, F1 plants transgenic for the DefH9-iaaM gene. Young, healthy and vigorously growing plants did not produce seeds. However, it is possible to obtain seeds from aged DefH9-iaaM transgenic plants both by selfing and crossing. By exploiting the delayed female fertility we have produced the homozygous plants needed as male parents for rapid seed multiplication of F1 eggplant hybrids. Therefore, the female sterility of young and mature plants is not an insuperable hindrance for mass propagation and commercial fruit production.

Expression of the DefH9-iaaM gene takes place during both flower and fruit development in transgenic parthenocarpic eggplants
The DefH9 gene is expressed specifically in the placenta and ovules during early phases of flower development [6]. To determine whether expression of the DefH9-iaaM gene also occurs during later stages of fruit growth and whether it is influenced by pollen fertilization, mRNA from transgenic flowers and fruits obtained either from emasculated or hand pollinated flowers was analyzed by RT-PCR at different stages of development, until the fruit attained a length of 28 cm. An amplicon of 161 bp corresponding to the spliced DefH9-iaaM mRNA was detected in all stages analyzed (Fig. 3, lanes 2–6). Thus, the presence of the mRNA of the DefH9-iaaM gene and consequently its action is most likely not limited to early stages of flower and fruit development. Pollination did not affect the steady state level of DefH9-iaaM mRNA (Fig. 3, compare lane 5 and lane 6).
Figure 3  Expression analysis of the DefH9-iaaM gene by competitive RT-PCR during flower and fruit development. Untransformed plant (lane 1); 5, 8, 12 mm long flower buds (lanes 2, 3, 4, respectively); 40 mm long hand-pollinated fruit (lane 5); 280 mm long emasculated fruit (lane 6). An internal standard of 351 bp is present in all lanes. Treatment with auxin often causes parthenocarpic development in several plant species [for review, see: [12]]. However, in some species and/or varieties, to efficiently sustain fruit growth, the hormonal treatment of the flowers must be repeated [13]. The finding that DefH9-iaaM mRNA is also present during later stages of fruit development is consistent with the interpretation that in DefH9-iaaM parthenocarpic plants, the placenta, the ovules, and the tissues derived therefrom are a source of auxin during the whole growth of the fruit. As a consequence, they efficiently sustain fruit growth.