Abstract
BackgroundPreviously, transgenic trichome-bearing (hairy leaf) Brassica napus lines expressing either the Arabidopsis thaliana GL3 gene (line AtGL3+) [1] or the AtGL3 gene in combination with an RNAi construct to down-regulate TTG1 (line K-5-8) [2] were developed. The leaves of these lines exhibited altered insect feeding (flea beetle) and oviposition (diamondback moth) behaviour compared to the non-transgenic semi-glabrous leaves of B. napus cv. Westar. Interestingly, the cotyledons of these lines remained glabrous, but also showed reduced feeding by flea beetles. Here we examine the composition and global transcriptome of the glabrous cotyledons from these transgenic lines to ascertain the mechanism(s) underlying this unexpected phenomenon.ResultsApproximately, 7500 genes were up-regulated in cotyledons of each hairy line, compared with < 30 that were down-regulated. The up-regulated genes included those involved in cell wall synthesis, secondary metabolite production, redox, stress and hormone-related responses that have the potential to impact host plant cues required to elicit defense responses toward insect pests. In particular, the expression of glucosinolate biosynthetic and degradation genes were substantially altered in the glabrous cotyledons of the two hairy leaf lines. The transcriptomic data was supported by glucosinolate and cell wall composition profiles of the cotyledons. Changes in gene expression were much more extreme in the AtGL3+ line compared with the K-5-8 line in terms of diversity and intensity.ConclusionsThe study provides a roadmap for the isolation and identification of insect resistance compounds and proteins in the glabrous cotyledons of these hairy leaf lines. It also confirms the impact of mis-expression of GL3 and TTG1 on types of metabolism other than those associated with trichomes. Finally, the large number of up-regulated genes encoding heat shock proteins, PR proteins, protease inhibitors, glucosinolate synthesis/breakdown factors, abiotic stress factors, redox proteins, transcription factors, and proteins required for auxin metabolism also suggest that these cotyledons are now primed for resistance to other forms of biotic and abiotic stress.
Highlights
Transgenic trichome-bearing Brassica napus lines expressing either the Arabidopsis thaliana GL3 gene [1] or the AtGL3 gene in combination with an RNAi construct to downregulate TTG1 [2] were developed
We demonstrate that the glabrous cotyledons of these lines exhibited altered secondary metabolite and lignin content, as well as altered expression of genes specifying secondary metabolite biosynthesis and degradation, cell wall biosynthesis, hormones, and redox proteins which may have contributed to changes in host plant cues for insect pests
Cotyledon composition As with true leaves, the glabrous AtGL3+ cotyledons were much smaller than Westar and K-5-8 cotyledons (Fig. 1 insert), reflecting the smaller stature and lower vigor of the AtGL3+ line, while the K-5-8 line produced much larger cotyledons and subsequently plants with higher vigour [2, 18]
Summary
Transgenic trichome-bearing (hairy leaf) Brassica napus lines expressing either the Arabidopsis thaliana GL3 gene (line AtGL3+) [1] or the AtGL3 gene in combination with an RNAi construct to downregulate TTG1 (line K-5-8) [2] were developed. The leaves of these lines exhibited altered insect feeding (flea beetle) and oviposition (diamondback moth) behaviour compared to the non-transgenic semi-glabrous leaves of B. napus cv. FB have developed resistance to insecticides used for their control (reviewed in [8, 9]) This highlights the importance of developing insect-resistant cultivars to reduce the use of chemical protection methods
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