Suppression of NGB and NAB/ERabp1 in tomato modifies root responses to potato cyst nematode infestation.

Plant-parasitic nematodes cause significant damage to major crops throughout the world. The small number of genes conferring natural plant resistance and the limitations of chemical control require the development of new protective strategies. RNA interference or the inducible over-expression of nematicidal genes provides an environment-friendly approach to this problem. Candidate genes include NGB, which encodes a small GTP-binding protein, and NAB/ERabp1, which encodes an auxin-binding protein, which were identified as being up-regulated in tomato roots in a transcriptome screen of potato cyst nematode (Globodera rostochiensis) feeding sites. Real-time reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization confirmed the localized up-regulation of these genes in syncytia and surrounding cells following nematode infection. Gene-silencing constructs were introduced into tomato, resulting in a 20%-98% decrease in transcription levels. Nematode infection tests conducted on transgenic plants showed 57%-82% reduction in the number of G. rostochiensis females in vitro and 30%-46% reduction in pot trials. Transmission electron microscopy revealed a deterioration of cytoplasm, and degraded mitochondria and plastids, in syncytia induced in plants with reduced NAB/ERabp1 expression. Cytoplasm in syncytia induced in plants with low NGB expression was strongly electron translucent and contained very few ribosomes; however, mitochondria and plastids remained intact. Functional impairments in syncytial cytoplasm of silenced plants may result from NGB's role in ribosome biogenesis; this was confirmed by localization of yellow fluorescent protein (YFP)-labelled NGB protein in nucleoli and co-repression of NGB in plants with reduced NAB/ERabp1 expression. These results demonstrate that NGB and NAB/ERabp1 play important roles in the development of nematode-induced syncytia.

Metal response of transgenic tomato plants expressing P(1B) -ATPase.

Heterologous expression of HMA4 (P(1B) -ATPase) in plants is a useful strategy to engineer altered metal distribution in tissues for biofortification or phytoremediation purposes. This study contributes to understanding mechanisms underlying complex Zn-dependent phenotypes observed in transgenic plants and to better predict the consequences of transgene expression. Tomato was transformed with AhHMA4(p1) ::AhHMA4 from Arabidopsis halleri encoding the Zn export protein involved in xylem loading of Zn. Homozygous lines were tested for Zn tolerance, Zn and Fe concentrations in organs and in the apoplastic fluid, and for the expression of the transgene and tomato metal homeostasis endogenes. Expression of AhHMA4 facilitates root-to-shoot Zn translocation and induces Zn uptake in a Zn supply-dependent manner. Unexpectedly, it increases Zn excess-triggered Fe deficiency in leaves and transcriptional activation of Fe-uptake systems in roots. Moreover, AhHMA4 expression causes Zn overload of the apoplast, which may contribute to enhanced Zn sensitivity of transgenics and may lead to cell-wall remodeling. This study highlights that alteration of the apoplast/symplast Zn status through introduction of cellular Zn export activity via AhHMA4 may alter tomato metal homeostasis network, thus seems to be crucial in the generation of the phenotype of transgenic tomato.