Maize

Question 1

Key facts about maize

Answer 1

• Most corn not for food eg. bioplastic, animal feed, cosmetics
• Mainly produced US, China and heavily exported -> main imports to EU for animal feed
• Can’t use as sole diet as lacks vitamins etc (-> Pallegra)
• Most important food crop in Sub-Saharan African & S. America (major calorie provider for 900 million people worldwide)
• 30% of annual food shipments are maize (blocked by some countries due to GM)
• grow in very diverse climates (C4)
• Effected by >100 diseases (e.g. Ostrinia nubilalis burrow into stems to shield from pesticides)
• Need 50% yield increase by 2050 for developing world

Question 2

Maize phylogeny and origin

Answer 2

Ancestor= Zea mays (Seeds encased in wooden box and only 4/8 seeds on each individual cob)

Domesticated from teosinte

Originated balsas valley, Mexico
- 9000 yr ago Hunter gatherers colonised West coast of South America
- Discarded rubbish in abck of caves and maize built up over centuries -> evidence for domestication

Varieties:
- most genetically diverse crop -> between two varieties there is more than chimp + human genetic difference
- World first GM crop (released 1996) -> GM maize ~90% of all maize in USA
- Worlds first CRISPR crop (released 2020)

Question 3

Early domestication

Answer 3

QTL analyses shows genetic background of traits by crossing ancestor with modern strains and then backcrossing with modern strain and seeing which traits are associated with which segments (initial research by John Doebly)
-> one chromome is modern strain and one chromsome is hybrid

Teosinte to maize

Branched -> unbranched
- Chr1 tb1 gene
- tb1 gene = expressed in maize, but NOT expressed in teosinte
- TE hopscotched into promotor leading to expression of gene encoding TCP transcription regulator (regulates growth by supressesing growth of axillary branches)
- Domestication -> mutations that permitted tb1 gene expression in axillary meristems

2 -> >4 Row of cupules

Single -> Paired spikelets

Hard -> soft glumes
- Chr4 Teosinte glume architecture 1 (TGA)
- Tga1 gene encodes SBP domain transcription factor expressed in inflorescences leading to hard causing
- Mutant tga1 caused loss of hard glume (single amino acid difference)

Male -> female lateral influrecence
- tb1 gene

Long -> short lateral branches
- tb1 gene

Also had reduced shattering by shattering 1 (as did rice)

Question 4

Recent maize domestication: Maize gm

Answer 4

Maize GM focuses on preventing yield loss, rather than increasing yield

1. Pest tolerance – European Corn Borer

• Corn borer (moth larva within corn plant) -> 2-5% yield loss per larva -> cost of ~$1bn in US 1996 for crop loss + management expenses
• No natural resistance genes to borer in maize
• Some produce DIMBOA which is an insecticide but levels are not sufficient

Solutions: Bt maize

Traditional: insecitides and biological methods (Trichogramma ecanescens- wasp)

Modern:
- > Bacillus thuringiensis (bacterium biological control) makes crystal delta endotoxins proteins which are toxic to ECB larvae
- >Transfer gene for this protein into maize -> Bt maize
- >compound gets into pores of insects, extracts nutrients and kills them
- >only work in butterflies + moths as needs high pH to make them into pores
-> However, Bt resistant moth pop’ns arising

2. Herbicide tolerance – glyphosate tolerance

• Herbicides inhibits ESPS gene (which is needed for amino acid formation)

solution: Ht maize
- Bacteria had single point mutation which conferred resistance to glyphosate
- Took gene from bacterium, added promoter and inserted into maize
- Means we can spray whole field w/out killing maize
- However, other plants also developing glyphosate resistance

Bt and Ht maize (and other crops) mean:
- decrease in pesticide, land use, farmer suicides, impacts on human health (as less pesticide exposure)
- Increase in yield, biodiversity (as less insecticide used)
- meta-analyses: 124 showed increased yields, 32 indicated no difference, and 13 were negative when using Bt/ Ht maize.

Risk: developement of resistance

Question 5

Maize domestication in the next 20 years: Genome editing

Answer 5

Requirements:
- increased herbicide tolerance, insect resistance, speciality production (eg.to make vaccines such as HepB or COVID vaccine done in maize), yield, nutritional quality, reduced inputs / increased efficiency of resource use

GE may be the solution
- It is not considered GM in USA, Canada, Most of South America, UK, Europe, China and many Asian countries -> viable
- GM Maize grown in 17 countries

Example: Waxy corn released in 2020
- CRISPR-Cas9 knock out GBSS1
- Increased level of amylopectin compared to amylose by knocking out gene in amylose pathway
- Amylopectin used in processed food, adhesives, gloss paper etc.

Example: Nigeria 2024
- Nigeria has approved the commercial release of 4 varieties of transgenic insect-resistant and drought-tolerant maize varieties, known as TELA maize
- 10 tonnes per hectare vs average of 6

Question 6

Overview

Answer 6

Early domestication
- controlled by few traits (overlap with rice-> tb1)

recent domestication
- GM crops used for insect and herbicide resistance

future domestication
- More GM needed to improve the crop further
- Concentration on Genetic engineering as less risk