Summary Transgenic lettuce plants expressing the nucleocapsid (N) protein gene of the lettuce isolate of tomato spotted wilt virus (TSWV‐BL) were protected against TSWV isolates via transgenic N protein when the protein accumulated at high levels or via an N transgene silencing mechanism activated by its overexpression. In a transgenic lettuce line, post‐transcriptional gene silencing was activated at a relatively earlier developmental stage in homozygous than in hemizygous progenies. As a result, the homozygous progenies generally showed a uniform suppression of N protein accumulation and consequently high levels of virus resistance in all leaves of the silenced plants. in contrast, N protein accumulated at high levels in the lower leaves of the hemizygous progenies and at much reduced levels (due to transgene silencing) in the successive leaves, resulting in moderate levels of virus resistance. It was also observed that the timing of the N transgene silencing in both homozygous and hemizygous plants was affected by environmental factors.
In unisexual flowers, sex is determined by the selective repression of growth or the abortion of either male or female reproductive organs. The mechanism by which this process is controlled in plants is still poorly understood. Because it is known that the identity of reproductive organs in plants is controlled by homeotic genes belonging to the MADS box gene family, we analyzed floral homeotic mutants from cucumber, a species that bears both male and female flowers on the same individual. To study the characteristics of sex determination in more detail, we produced mutants similar to class A and C homeotic mutants from well-characterized hermaphrodite species such as Arabidopsis by ectopically expressing and suppressing the cucumber gene CUCUMBER MADS1 (CUM1). The cucumber mutant green petals (gp) corresponds to the previously characterized B mutants from several species and appeared to be caused by a deletion of 15 amino acid residues in the coding region of the class B MADS box gene CUM26. These homeotic mutants reveal two important concepts that govern sex determination in cucumber. First, the arrest of either male or female organ development is dependent on their positions in the flower and is not associated with their sexual identity. Second, the data presented here strongly suggest that the class C homeotic function is required for the position-dependent arrest of reproductive organs.
In unisexual flowers, sex is determined by the selective repression of growth or the abortion of either male or female reproductive organs. The mechanism by which this process is controlled in plants is still poorly understood. Because it is known that the identity of reproductive organs in plants is controlled by homeotic genes belonging to the MADS box gene family, we analyzed floral homeotic mutants from cucumber, a species that bears both male and female flowers on the same individual. To study the characteristics of sex determination in more detail, we produced mutants similar to class A and C homeotic mutants from well-characterized hermaphrodite species such as Arabidopsis by ectopically expressing and suppressing the cucumber gene CUCUMBER MADS1 ( CUM1 ). The cucumber mutant green petals ( gp ) corresponds to the previously characterized B mutants from several species and appeared to be caused by a deletion of 15 amino acid residues in the coding region of the class B MADS box gene CUM26 . These homeotic mutants reveal two important concepts that govern sex determination in cucumber. First, the arrest of either male or female organ development is dependent on their positions in the flower and is not associated with their sexual identity. Second, the data presented here strongly suggest that the class C homeotic function is required for the position-dependent arrest of reproductive organs. INTRODUCTIONIn flowering plants, strategies for sex determination have evolved to prevent self-fertilization that may lead to loss of fitness due to inbreeding (Darwin, 1876). One of these strategies involves the production of unisexual flowers, in which male and female gametes are segregated on different flowers of the same plant (monoecious species) or on separate individuals (dioecious species). Differences occur in the manner in which the reproductive organs are arrested; in maize, for example, the sex organ primordia abort completely (Dellaporta and Calderon-Urrea, 1994), whereas in cucumber, these primordia arrest in their growth (Malepszy and Niemirowcz-Szcytt, 1991;.Although sex determination in animals is well studied, little is known about the molecular control of this process in plants. To date, only two genes have been cloned that influence sex in maize. The TASSELSEED2 ( TS2 ) gene, which encodes a predicted protein with significant homology with a steroid-specific dehydrogenase from bacteria, is involved in the abortion of pistil primordia in the tassel (DeLong et al., 1993). In contrast, the feminizing ANTHER EAR1 ( AN1 ) gene (Bensen et al., 1995), which encodes an enzyme in the gibberellin biosynthesis pathway, is required for stamen abortion in the maize ear.The basic architecture of well-studied hermaphrodite flowers such as those in Arabidopsis, snapdragon, and petunia consists of four concentric whorls with, starting from the outside, the sepals (whorl 1), petals (whorl 2), stamens (whorl 3), and finally the carpels (whorl 4). The identity of the floral organs is specified by three distinct classes ...
Five transgenic squash lines expressing coat protein (CP) genes from cucumber mosaic cucumovirus (CMV), zucchini yellow mosaic potyvirus (ZYMV), and watermelon mosaic virus 2 potyvirus (WMV 2) were analyzed in the field for their reaction to mixed infections by these three viruses and for fruit production. Test plants were exposed to natural inoculations via aphids in trials simulating the introduction of viruses by secondary spread from mechanically infected susceptible border row plants. Plants of transgenic line CZW-3 expressing the CP genes from CMV, ZYMV, and WMV 2 displayed the highest level of resistance with no systemic infection, although 64% exhibited localized chlorotic dots which were mainly confined to older leaves. CZW-3 plants had a 50-fold increase in marketable yield compared to controls and the highest predicted cash returns. Plants of transgenic line ZW-20 expressing the CP genes from ZYMV and WMV 2 displayed high levels of resistance to these two potyviruses, but 22% became infected by CMV. However, ZW-20 plants provided a 40-fold increase in marketable yield relative to controls and good estimated cash returns. Three transgenic lines expressing single CP genes from either ZYMV (line Z-33), WMV 2 (line W-164) or CMV (line C-14) developed systemic symptoms similar to those of controls but showed a delay of 2 to 4 weeks before the onset of disease. Plants of transgenic line Z-33 were highly resistant to ZYMV but not to WMV 2 and CMV. Interestingly, Z-33 plants had a 20-fold increase in marketable yield compared to controls and some predicted cash returns if market sale prices were high. This study indicates that virus-resistant transgenic lines are economically viable even if they are affected by viruses other than those to which they are resistant.
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