[20-21] - GM Environmental Impact Flashcards
State in one brief sentence what Strigolactones are
Strigolactones (SLs) are a group of relatively recently discovered plant growth regulators, which control diverse signals
Briefly summarise the discovery and identification of SLs
Strigolactones were first reported as GERMINATION STIMULANTS for the parasitic weed Striga lutea in 1966 - strigol and strigyl acetate were isolated from cotton root exudates
Later, various additional SLs were found to trigger seed germination in a range of Striga, Phelipanche and Oraobanche parasitic plant species
It was later shown that SLs are released by many plants that act as hosts for these parasites, including sorghum, maize and red clover
Given that SLs were first identified as germination stimulants for parasitic weeds, why do plants produce them?
Strigolactones also perform signalling roles within the plants that produce them:
-> In 2005 it was found that 5-deoxystrigol acts as a branching factor for arbuscular mycorrhizal (AM) fungi, thus aiding colonisation (these fungi are important for providing inorganic nutrients to many plant species in exchange for carbohydrates, and can improve resistance to stresses such as drought and pathogens)
-> Thus, it is beneficial for plants to exude SLs into the rhizosphere to promote branching of these fungi (germination of parasitic weeds is an unfortunate side effect)
SLs also perform a hormonal role within plants:
-> branching inhibitor
-> regulate root architecture
-> increase root hair length
-> inhibit adventitious rooting
-> promote secondary growth
Overall, SLs co-ordinate plant responses to nutrient deficiency, in particular phosphate and nitrate (P and N) deficiencies
Explain how SL mutants contributed to Green Revolution Rice
A partial LoF mutant allele of HTD1/D17 (HIGH TILLERING AND DWARF1/DWARF17) has been widely utilized and co-selected with Semidwarf1 (SD1) - both contributed to the improvement of rice architecture during and since the Green Revolution
HTD1/D17 encodes a carotenoid cleavage dioxygenase enzyme (CCD7) required for SL biosynthesis, and the partial LoF mutant allele is known to increase tiller number and improve grain yield in rice
What do grafting experiments reveal about the function of SLs?
Strigolactones, like GAs (but unlike BRs) are transported throughout the plant, and can move from root to shoot, and vice versa
This is shown by grafting shoots from max3 mutant rice (which lacks SL biosynthesis and has excess tillers) onto WT roots, or vice versa - in both cases, the WT root/shoot rescues the mutant phenotype
Fully explain how branching in plants is altered in response to nutrient availability
In general, plants in nutrient-deficient conditions (e.g., low P) suppress shoot growth and branching, and enhance root growth to aid nutrient uptake
NON-LEGUMINOUS AM host plants (such as sorghum) do this in low P AND low N conditions; LEGUMINOUS plants do not show this response in low N (as they instead enter symbiosis with N-fixing bacteria) but may respond to low P
Phosphorous deficiency limits plant growth, and limits yield in much of the world (hence the widespread use of fertilizer)
MECHANISM OF REDUCED BRANCHING RESPONSE:
-> In Low P, Strigolactone (SL) biosynthesis genes are induced, leading to high SL levels and suppression of shoot branching, hence fewer tillers
-> However, in SL biosynthesis mutants (e.g., d10 rice), no SLs are produced in response to low P, meaning excess tillers are still produced
SL biosynthesis is a complicated pathway involving many cleavage events, and many SL biosynthesis mutants have been discovered, helping to elucidate the different steps and enzymes
-> Many of the enzymes involved are expressed in response to low P
While shoot branching is inhibited, SLs (in combination with another important plant hormone, auxin) PROMOTE root branching to increase nutrient uptake
Describe and explain the mechanism of signalling downstream of SL
Headline: SLs signal via F-box/SCF E3-ligase-mediated degradation of a transcriptional repressor (somewhat like GA and DELLA)
When SLs are produced, they bind the protein D14, causing it to change its conformation and be recognised by the CUL1-RBX1-SKP1-F-box complex (a member of the SCF family of E3 ubiquitin ligases), leading to ubiquitination and subsequent proteasomal degradation of the nuclear-localized repressor, D53
->D53 normally forms a complex with a co-repressor, TOPLESS (TPR) and inhibits SPL-family transcription factors such as SPL14 (encoded by IPA1), preventing them from activating SL target genes
-> However, when D53 is degraded downstream of SL signalling, IPA1 (along with other SPL-family TFs) is no longer inhibited, and is free to activate SL target genes, leading to further downstream signalling
OUTPUTS:
-> One of the main targets of IPA1 is OsTB1 (the rice ortholog of the Arabidopsis gene BRANCHED1, or BRC1)
-> TB1/BRC1 is a negative regulator of bud outgrowth, so its upregulation by IPA1 reduces shoot branching
-> Meanwhile, BRC1 is repressed in d14 and max2 mutants (which are deficient in SL production), leading to reduced inhibition of bud outgrowth, and hence increased shoot branching
-> Note that BRC1 integrates a wide range of hormones and environmental signals, including light quantity and composition (via Cytokinins and PHYB), and nitrate levels (also via Cytokinins)
Describe the extent of the issues relating to Striga, the solutions currently applied and the limitations of these solutions
Parasitic plants such as Striga (promoted by SLs) currently infect around 60% of farmlands in sub-Saharan Africa, causing devastating loss of yield
These losses can be reduced with agricultural practices:
-> Before planting, apply germination stimulants to promote “suicidal germination” of parasites (i.e., germination with no host, resulting in death)
-> Apply sufficient fertilizer to prevent phosphorus deficiency and thereby reduce SL production by crop plants
HOWEVER, both of these methods are extremely expensive, and thus unviable in many parts of the world, especially those most affected by Striga
Explain how losses associated with Striga and Orobanche can be reduced without the need for expensive fertilizers or germination stimulants
The overall goal is to reduce SL exudation to prevent germination of these parasites
One simple method is intercropping of crops with beneficial plants, including nitrogen-fixing legumes such as Desmodium
-> These both reduce nutrient deficiency, AND produce allelopathic chemicals which interfere with Striga parasitism
However, there are also inherently resistant varieties of crops (including rice, tomato, faba bean and pea)
-> Mutations at the LOW GERMINATION STIMULANT 1 (LGS1) locus of Sorghum, for example, can confer resistance to Striga via a change in SL profile, from strigol-type to orobanchol-type
-> Fortunately, the mycorrhization status of these lgs1 mutants was similar to that of the WT, since the overall SL content was not affected, only the profile
Principle behind this:
-> There are many different naturally occurring SLs, with slight changes in structure resulting in considerable functional differences
-> In maize, there are two parallel SL biosynthesis pathways - modifying the flux between these two pathways can alter the SL profile, for example by shifting the balance between synthesis of ZEALACTONE (which strongly induces Striga germination) and ZEALACTOL/ZA (which both induce much less Striga germination)
How this modification can be carried out:
-> Li et al (2023) identified a single cytochrome P450 (ZmCYP706C37) which is responsible for catalysing several key steps of the SL biosynthesis pathway in maize
-> Lines with reduced activity in this key P450 enzyme, along with two other enzymes in the pathway (ZmMAX1b and ZmCLAMT1) shifted SL composition to give more Zealactol plus ZA in comparison to zealactone
-> As such, these lines (such as NP2222) were more resistant to Striga germination and infection
-> Mutation of ZmMAX1b using CRISPR had the same effect, and demonstrated that targeted GM has the potential to confer Striga resistance in crops
Fortunately, branching in these lines was normal - showing that the beneficial domestication traits of maize, such as reduced branching, are not affected by altered SL biosynthesis (possibly because the overall amount of SLs was not affected, only the types produced)
However, the effect of these changes on mycorrhization were not tested, so the authors concluded that further research onto this relationship would be beneficial
State some other potential benefits of SL modification (if we can understand their complicated signalling pathways well enough to avoid unfavourable effects)
In addition to Striga and Orobanche control:
- Branching architecture
- Lodging resistance
- Drought resistance
- Enhanced mycorrhization
- Root architecture
Summarise how vascular cell division is controlled in Arabidopsis
PXY (phloem induced with xylem) is an RTK which is essential in regulating vascular cell division
CLE41 gene (encoding TDIF peptide) is the corresponding ligand
When CLE41 is overexpressed, there is an increase in vascular cell division, leading to more xylem (but its structure is disorganised and disordered)
The current model is that CLE41 is synthesised at the phloem, and binds PXY in the membranes of procambium/cambium cells, thereby inhibiting the differentiation of xylem cells, and promoting polarised division of cambium cells
-> the GRADIENT of CLE41 is important for organising this (hence the disorganised phenotype when CLE41 is ubiquitously overexpressed)
Hence, expression of PXY under a constitutive 35S promoter, coupled with CLE41 expression under a phloem-specific promoter, successfully increased the size of the xylem while maintaining proper organisation
Explain the example of an attempt to increase growth in trees (end of lecture 20)
First, GUS reporter gene was used to identify tissue-specific promoters:
-> Poplar antegumenta (ANT) promoter gives cambium-specific expression)
-> Phloem-specific lectin (PP2) promoter gives high expression in the phloem
Tissue-specific overexpression of PttPXY and PttCLE41 successfully increased cell division in Hybrid Aspen
-> Significant increases in biomass
-> Increased diameter
-> Surprisingly, increase in height!
-> Also surprisingly, increase in leaf size!
Field trials are underway - not expected to match the 200% increase observed in the greenhouse, but likely to compete with the best current technologies (10-30% increase)
Open questions:
- Will this make plants more robust in the face of a more variable climate?
- Will manipulating PXY and CLE work in OTHER CROPS?
