[11-13] - Light Sensing + Seasons Flashcards
Explain why there are limits on improving crop yield by increasing density
Although at low densities increasing plants per unit area results in a proportional increase in yield, past a certain threshold, the marginal increment of yield with increased population density DECREASES - this marginal increment approaches zero and eventually becomes negative
This is not only because there are too many plants in a single area and they take too much nutrients, but also because plants can sense how close their neighbours are, and change their growth accordingly (Shade Avoidance Syndrome) which reduces yield
What is meant by SAS?
SHADE AVOIDANCE SYNDROME
This refers to the suite of phenotypic changes exhibited by many plants as a response to the shade of a neighbouring plant. These traits include:
- Elongation of internodes, petioles and hypocotyls
- Early flowering
- Hyponasty
- Apical dominance
The aim of these responses is to outgrow the shade of the neighbouring plants, thereby providing the young, photosynthesising leaves with a superior position for light harvesting and photosynthesis
Explain how SAS differs from Shade Tolerance
Shade Avoidance Syndrome refers specifically to the set of traits used by many plants to outgrow neighbouring plants and avoid their shade (common in grasslands, fields, etc.)
Shade Tolerance is common in forest understories, where plants are unable to outgrow the trees, making it more advantageous to optimise carbon gain and minimise damage than to avoid the shade altogether. Shade Tolerance traits include:
- Increased Specific Leaf Area (SLA)
- Reduced PSII:PSI ratio
- High physical defense
- Reduced Chl-a:Chl-b ratio
Although both types of plants perceive canopy-associated changes in light quality (e.g., reduced R:FR ratio or reduced blue) and quantity (reduced light intensity), and some traits are shared in both types of plants (e.g., increased SLA and increased PSII:PSI) plants exhibiting Shade Tolerance do NOT show Shade Avoidance Traits
Discuss the adaptive value of SAS and how this relates to agriculture
In habitats with a STEEP VERTICAL LIGHT GRADIENT in the canopy (e.g., dense grasslands), it is likely that a plant can benefit from enhanced light interception by elongation [as opposed to environments where reduction in light occurs at a greater height and SAS cannot enable small annual plants to escape this shade, such as forests]
As well as interspecific variation in SAS, there are also examples of SAS variation between ECOTYPES of the same species (e.g., two ecotypes of Stellaria longipes - the prairie-growing ecotype exhibits SAS, while the Alpine-growing ecotype does not)
SAS is vital for the survival of many wild plants, as the ability to sense the proximity of neighbours and induce responses to adapt and outcompete them is essential when competing for limited resources such as light [SAS plants have greater fitness than non-SAS plants at high density, but reduced fitness at low density]
HOWEVER, from an agricultural perspective, SAS is detrimental to crop yield, as internode elongation takes place at the expense of the desirable, harvestable organs - therefore, it may be possible to improve the harvest index of these plants by inhibiting SAS responses such as elongation, in order to increase the proportion of carbon allocated to harvestable organs
Explain how light composition acts as an indicator to induce SAS responses in plants
Plants use cues – mainly light intensity and composition – to sense crowing, or initiation thereof. A reduced ratio of Red (600-700 nm) to Far-Red (700-800 nm) light indicates that light has been reflected or filtered through a leaf, thus indicating proximate vegetation, and this induces a SAS response. Meanwhile, reduction of blue light photon fluence is also thought to induce SAS.
Since the R:FR ratio of light is altered by reflection as well as passing through a leaf, plants are able to sense and respond to the proximity of their neighbours even before mutual shading occurs.
This is demonstrated by growing two plants in front of mirrors selectively reflecting R or FR light -> the FR plant shows increased internode elongation
Explain the molecular mechanism by which plants respond to the changes in light composition discussed on the previous FC
A photoreceptor called Phytochrome is implicated R:FR ratio sensing
Plant photoreceptors can perceive light from a large part of the spectrum
-> UVR8 for UV
-> cryptochromes and phototropins + ZTL for blue light
-> PHYTOCHROMES for red and far-red light
These photoreceptors collaborate to fine-tune many plant responses for which light is a determining factor (e.g., germination, day-length measurement in flowering, tropisms, etiolation, etc.)
Phytochrome can exist in two conformations - Pr and Pfr - this change is PHOTOREVERSIBLE:
-> Upon activation by Red light, the D-ring of the molecule rotates, causing a conformational change to the Pfr form, exposing the NLS and allowing translocation to the nucleus and regulation of gene expression
-> When the Pfr form senses FR light, it reverts back to the Pr form (photoreversibility)
-> As a result, the ratio of R:FR light reaching the plant will determine the equilibrium of Pr and Pfr forms of phytochrome in the plant, which is the basis for SAS responses
Phytochromes are a multi-gene family, allowing specialisation for each member (A and B are the predominant forms, with A being specialised in responses to FR, and B in responses to R or white light)
Of the 5 PHY genes in Arabidopsis, phyB mutants in particular resemble the SAS phenotype (elongated and early flowering), implicating a role for PHYB as a red light detector in the process
[For WT plants] in the absence of shade, the high R:FR ratio converts PrB to the PfrB form, causing it to translocate to the nucleus, where it interacts with its molecular regulatory partners, most notably the PIF subfamily of bHLH transcription factors, such as PIFs 3, 4, 5 and 7, while PIL1 and HFR1 specifically are known to mediate SAS responses [note that PfrB INHIBITS these PIFs and targets them for degradation at the 26S proteasome
-> BUT when shade results in a lower R:FR ratio, PfrB is reverted to the PrB form, preventing it from translocating to the nucleus. Therefore, PIFs are free to bind the E-BOX sequence and activate key shade-induced genes, thereby promoting elongation, early flowering and other key SAS responses
HORMONAL REGULATION:
- It has also been shown that PIFs can interact with, and be inhibited by, DELLA proteins - a growth suppressing subfamily of GRAS domain family of transcriptional regulators
- This links SAS responses and phytochrome signalling to hormonal regulation, since DELLA protein stability is controlled by hormones such as gibberellin, auxin and ethylene
- This also suggests that DELLA proteins are an important integrator of phytochromes, PIFs and several hormonal signal transduction pathways
It is worth noting that many other proteins which are thought to be involved in mediating SAS responses - for example, COP1 is known to target some negative regulators of the pathway, such as the aforementioned HFR1 and DELLA, for degradation, promoting hypocotyl growth. However, PhyB and PIFs are of particular interest, as they are the only proteins known to be required for ALL the phenotypic changes that comprise SAS.
Explain how SAS responses could be a target for GM
There have been experiments aiming to enhance the harvest index in crop monocultures by inhibiting SAS responses (since, in theory, this would result in more carbon being allocated to harvestable organs, rather than non-harvestable stems)
-> Such an approach would ALSO reduce the negative effects of weeds on yield (since, as well as stealing water and nutrients, weeds also induce SAS responses in proximal plants)
Ectopic expression of the Arabidopsis PHYB gene in Desiree potatoes (with the transgenic lines referred to as “Dara”) successfully reduced stem elongation responses at high crop densities, thereby alleviating the negative effect of density on tuber yield
-> The results suggest that this avenue of GM research may be able to shift optimum crop densities to higher values
-> HOWEVER, there was a negative side effect in Dara-5 and Dara-12 potatoes, as they showed less efficient use of water, due to transpiration through the increased number of stomata
-> Thus, overexpression of PHYB may have a detrimental effect in certain conditions, such as water deficiency
-> A more positive balance may be achieved by GM targeting genes downstream of PhyB, such as PIFs, as some of these could control a smaller subset of PhyB-mediated responses, and induce the desired morphological effects that have a positive impact on yield, while avoiding those with negative impacts
-> This will require further research to improve understanding of the genes downstream of phytochrome
Summarise the introduction to plant flowering in terms of ecological, economic and agricultural importance, and the issues climate change will present in this area
Flowering is an important process for pollination, seed development and seed dispersal in plants, and plants have evolved sophisticated mechanisms to control WHEN flowering occurs to optimise these
This usually involves restricting flowering to a certain time of year which is optimal in terms of temperature for seeds, water availability, competition with other plants, etc.
From an economic and agricultural perspective, flowering is important as the seeds, fruit and cut flowers of many plants are harvestable and profitable
The duration from sowing to flowering (and to seed setting) is of critical importance for crop adaptation to a given climate - breeding for optimal flowering time is one of the major factors in optimising yield in a given environment (e.g., Barley in Europe)
Many crops have been bred to optimise their flowering time for yield. However, climate change will generally force wheat production to move north, which may disrupt flowering time by altering cues such as day length - this could prove detrimental for yield
What key (mentioned) morphological change occurs upon induction of flowering?
MERISTEM TRANSITION:
- Normally, the apical meristem is vegetative, and makes leaves indefinitely
- Upon flowering induction, the vegetative meristem transitions to a floral meristem, and stops making leaves
- Instead, the floral meristem starts making flower parts, and then stops.
What is a major (mentioned) parameter/cue that controls flowering time, and how universal is this among plants?
DAY LENGTH or PHOTOPERIOD:
- The photoperiod is the duration of light over 24 hours, and varies with time of year and latitude (varies most drastically near poles)
- CASE STUDY - MAMMOTH TOBACCO: when the mammoth tobacco was first discovered, it was brought further south for study, but no longer flowered due to the longer days; it had to be put in a shed for certain periods to create apparent short days in order to induce flowering (it is a SD plant)
- A similar principle is used in sugarcane breeding programmes (it requires less than 12:32 hours of light per day in order to flower)
- Photoperiodism helps promote cross-pollination and select the growing season
- It also explains why some plant species can only be grown at certain latitudes (e.g., spinach cannot flower in the tropics, as the days never get long enough [14 hours])
In LONG-DAY PLANTS, the day length must be LONGER than a minimum (e.g., Arabidopsis, sugar beets, spinach, wheat) -> this is common in European plants which flower in late spring/summer
In SHORT-DAY PLANTS, day length must be SHORTER than a maximum (e.g., rice, maize, coffee, cannabis, sugarcane) -> alien species may have a mismatch between requirement and photoperiod in Europe
Note: “short-day” doesn’t necessarily mean that the threshold itself is short
HOWEVER, in many equatorial species, tomatoes, some maize crops and fruit trees, day length has no effect on flowering -> DAY-NEUTRAL PLANTS (do not exhibit photoperiodism)
Explain what is meant by the ‘Florigen’
The site of light/dark perception is the leaf, but a stimulus must travel to the apex to induce the flowering response -> THIS STIMULUS IS THE FLORIGEN (was hypothesised but unknown until 2008)
This is demonstrated by experiment in which a single leaf of a SD plant is kept under short days [using foil], inducing flowering in the plant - this leaf is then cut and grafted onto LD plants, and induces flowering in these too -> this shows that the florigen molecule itself is the same between SD and LD plants
Meanwhile, non-induced leaves do NOT induce flowering when grafted, while flowering meristem ALSO does not induce flowering
What is the actual Florigen molecule in plants and what is its mechanism of action?
FLOWERING LOCUS T (FT) PROTEIN is the florigen
In the leaves, environmental signals including light sensing activate FT expression
FT protein then travels in the phloem to the Shoot Apical Meristem (SAM), where FT associates with FD
This FT-FD complex then promotes expression of AP1 -> AP1 protein induces flowering
What is the overall mechanism/model that explains how both LD and SD plants respond to changes in day length?
THE COINCIDENCE MODEL -> a circadian oscillator controls the timing of light-sensitive and light-insensitive phases
Plants are known to have a Circadian Clock, as plants which have been entrained to a particular cycle (e.g., 12h diurnal cycle) will continue to show responses to this cycle for several days, even if moved into continuous dim light, before the responses eventually dampen and stop
This is mediated by the gene CONSTANS (CO): the expression of CO (mRNA) oscillates over a 24-hour period, under the control of the plant biological clock
IN LD PLANTS:
This CO mRNA is only translated to protein (necessary to induce flowering) when high CO levels coincide with light, which only occurs when days are long
-> this explains why a brief flash of light during the night can induce flowering in LD plants
The genetic proof of this is that transgenic LD plants constitutively expressing CO under a 35S promoter express very high levels of FT mRNA and flower early compared to the WT
IN SD PLANTS:
- Like LD plants, Hd1/CO expression fluctuates in a diurnal pattern, with expression peaking at midnight
- Under short days, Hd1/CO upregulates Hd3/FT, whose expression peaks at the beginning of the light period
- Hd3/FT travels through the phloem to the meristem, where it binds the 14-3-3 protein GF14c and OsFD1 to form the FLORIGEN ACTIVATION COMPLEX, which then promotes flowering
- HOWEVER, under long days, early illumination converts Hd1 to a REPRESSOR of Hd3a expression via a pathway involving phytochrome
How is climate change likely to affect flowering time, and what can we do about it?
Climate change will force wheat production to move north in order to grow it in approximately the same climate
-> HOWEVER, this will result in a different photoperiod at the same season time, resulting in EARLIER FLOWERING, which could be detrimental to yield
-> Yield losses from flowering delays will generally be greater in hotter regions
Plants that use temperature, or both day-length AND temperature to determine flowering time are more likely to be able to adapt to climate change than those which use ONLY day-length
For any given location, there is an optimum cultivar based on factors such as flowering, weather and water, so research which cultivars are best suited to new locations
HOWEVER, changing the photoperiod response of a crop has been done before - selecting for natural variation over thousands of years. This time, it would need to be done in a much shorter period of time -> GM may play a role
GM EXPERIMENT:
- Constitutive expression of a heterologous FT gene (FTa1) was able to rescue the late flowering phenotype
- To allow flowering to be induced in a controlled manner, the FTa1 gene was expressed under an ethanol-inducible promoter -> upon ethanol vapour treatment, FTa1 was upregulated and induced flowering
- This paper argued that the universal florigenic nature of FT means this system should be applicable to all crops of economic value where flowering control is desired
Why is photoperiodism not always enough to determine the correct flowering time (i.e., why do some flowers need another mechanism of control and what is it)?
Perennials in temperate climates need to distinguish Autumn from Spring as the photoperiod is equally long in these periods
VERNALISATION is the requirement of a cold period (i.e., winter) before flowering, to ensure that flowering only occurs in the spring
