Molecular approaches in plant resistance

1. Molecular approaches in plant
resistance
KARTHIKEYAN, S (2015 800503)
Ph.D., Scholar,
Agricultural Entomology,
TNAU, Coimbatore.

2. Molecular approaches
• Introgression of resistance genes from the
wild relatives of crops(=wide hybridization),
• Marker assisted selection
• Genetic engineering
• Gene pyramiding.

3. Resistance genes from the wild
relatives
• High levels of resistance have been identified in
the wild relatives of sorghum
• Sorghum dimidiatum,
• S. australiense,
• S. purpeosericeum,
• S. nitidum, and
• S. angustum
 shoot fly, stem
borer, and midge
resistant genes

4. Groundnut
• Arachis duranensis,
• A. kempff-mercadoi,
• A. monticola,
• A. stenosperma,
• A. paraguariensis,
• A. pusilla, and
• A. triseminata
• A. cardenasii - hoppers
 Leaf miner
and Spodoptera.

5. Helicoverpa
• Pigeonpea
• Cajanus scarabaeoides,
• C. sericeus and
• C. acutifolius
• chickpea
• Cicer reticulatum,
• C.bijugum,
• C. pinnatifidum and
• C. judaicum.

6. Marker assisted selection
• Sources resistance to insects identified long ago,
but not used in crop improvement .
• Difficulties in screening and selection
• Molecular markers can play an important role in
accelerating the introgression of genes.
• Understanding the nature of gene action,
reducing the deleterious effects & introgressing
unwanted genes done through linkage drag.

7. Genetic engineering
• Plants to express Bt Toxins
• Plants to express proteinase inhibitors
• Plants to express Amylase inhibitors
• Plants to express foreign lectins
• Possible strategies for producing transgenics

8. Plants to express Bt Toxins
(changes to protein sequence)
• Crystalline toxin has three domains
• Domain I (N-terminal)
• 250AA forms helical bundle with 6 α-helics
• Pore formation
• Capability to insert itself into lipid bilayers
• Domain II
• 200AA
• Three β sheets – binding to the receptor
• Domain III – no functional role(150AA, β-sandwich)
• Complete protoxin - 0.0001%. Only N-terminal - 0.01%

9. Plants to express Bt Toxins
(changes to gene sequence)
• Codon usage of bacterial gene is markedly
different to typical plant genes
- high A+T in bacteria, plant has high G+C
- high A+T content results in truncated mRNA
- unstable/couldn’t produce functional protein
• Through mutagenesis & oligonucleotide synthesis
genes are reconstructed
• 0.3% of total protein (100% mortality = pesticide)

10. Inhibitors of digestive proteinases
• Serine proteinase inhibitors
• First plant to plant gene transfer CpTI from cowpea
• Transgenic CpTI tobacco expressed CpTI >0.1%
• Plants expressing 1% CpTI clonally probagated shows
reduction in damage up to 50% over control.
• Lepidoptera, Diptera, Orthoptera and Hymenoptera
• Cysteine proteinase inhibitors
• Coleoptera
• Not inhibited by serine proteinase inhibitors
• Oryzacystatin to transgenic poplar tree -Chrysomela tremulae

11. Inhibitors of digestive Amylases
• Insecticidal activity against coleopterans –
storage pests
• LLP – α-Amylase inhibitor gene from Phaseolus
vulgaris
• Seed specific gene expression
• 100% mortality to Bruchids
• Inactive at alkaline pH of lepidopteran gut

12. Transgenic plants with foreign Lectins
• Defensive role of Lectins (Janzen et al. 1976)
• Insecticidal against Homopterans
• Two most effective lectins GNA and WGA
• WGA - Brevicornye brassicae, BPH & GLH
• GNA – Myzus persicae & Aulacorthum solanum
• Pea lectin (P-Lec) in tobacco H. Virescens
• GNA – potato – reduced fecundity of aphids

13. Possible strategies for transgenic plants
• Hydrolytic enzymes - e.g. chitinases
• Oxidative enzymes - e.g. PPO
• Lipid oxidases - e.g. lipoxygenase
• Manipulation of secondary metabolism
• Exploiting secondary metabolites – insect resistance
• Goal for plant biotechnologists
• Bt > PI> Lectin

14. Gene pyramiding
• Stacking two or more genes of interest into
plants to offer resistance.
• BPH resistance genes (Bph14 and Bph15) and
one RSD resistance gene (Stv-bi) transferred
into three japonica varieties.
• Durable
• More stability
• Less chance of resistance development to insects