Just Add Water-The Leaves Are Up
Just Add Water–The Leaves Are Up
A review on the involvement of leaves in Mentha L. sp propagation
Mint (Mentha L. sp.) classification (USDA Plants database, n.d):
Kingdom: Plantae- Plant
Subkingdom: Tracheobionta- Vascular plant
Superdivision: Spermatophyta- Seed plant
Division: Magnoliophyta- Flowering plant
Class: Magnoliopsida- Dicotyledon
Subclass: Asteridae
Order: Lamiale
Family: Lamiaceae Martinov- Mint family
Genus: Mentha L.
Welcome back to another instalment of the ‘Just Add Water’ series on MC Science Blog. This article will answer the questions posed in my last post. The review will explain the functionality of leaves, why they need light, why their light requirements are so particular; and the pathway of energy transfer after photon absorption.
To understand terrestrial plants we must truly appreciate how (frankly) weird and highly organised they are. Leaves, for instance, serve as energy harvesting sites on the plants; they are akin to solar panels on a home. Technical and organised systems that convert sunlight into energy that can be efficiently utilised by the encoded systems within. This interdisciplinary junction is where an appreciation of concepts in physics brings its usual enlightenment. Under the law of conservation of energy, where the universe is a “closed system”, and the sun and the plants are objects within this system possessing the capacity to form interactions with each other. Understanding that within the universe energy can neither be created nor destroyed, but converted from one form to the next, it can now be appreciated that energy expelled by the sun and then absorbed by the leaves of the (terrestrial) plant is only being converted into a form that is useful to terrestrial mechanisms. In the case of our experiment, the leaves of Mentha L. (indirectly) provide energy to the node cells for root formation to be facilitated; much like the solar panels on a building will provide energy to your device for you to be able to read this article. The idea is the same however, the mechanisms are different.
Nothing can occur in a cell (organism) without energy transfers; significant quantities of energy are required for the expression of a single gene. Energy is needed for: releasing the compact gene segment on the condensed chromosome; activating the transcription factors; initiating transcription; capping the transcript; elongating the transcript (these nitrogenous bases are in the form of energy rich triphosphates that are polymerized as monophosphates, releasing thousands of pyrophosphate bonds at 𝚫G°-33.4 kJ/mol (Buchanan et al., 2015) each!); terminating transcription with a tail of approximately two hundred adenosine triphosphate units (a reaction that releases an approximated 6,680 kJ/mol per cell for a single transcription cycle); transporting the transcript out of the nucleus to the cytoplasm for exon splicing, ribosomal translation, protein packaging etc.. It is safe to infer that a lot of energy is needed by cells to execute their basic functionings, especially when we consider that one transcript termination cycle in one hour requires the energy transfer of approximately 1789.82 watts. A single gene can be expressed in as little a time as a couple of seconds (Alpert et al. 2016), and this gene will be expressed several times for mass production of the required protein in several cells in the organism. For reference, the energy required for observable changes in a plant will be enough to power multiple houses for several hours.
The clarity of my previous tangent leads to the question ‘How exactly are these things able to capture sunlight and turn it into so much usable energy?!’. The light harnessing sites of leaves are strategically positioned on specialised membranes within these little plastids called chloroplasts. Plastids, to date, are believed to be endosymbionts (Buchanan et al., 2015) that higher plants smartly utilised for adaptation to increase the productivity of their endogenic mechanisms. These oxygenic photosynthetic plastids are packaged neatly with a double membrane enveloping ribosomes, genetic material, stacks (granum) of thylakoids containing many protein bound chlorophyll molecules.
The name ‘chloro’-‘plast’ now becomes self-explanatory; the chlorophyll molecules within these plastids are the driver for all light harvesting activities. Chlorophyll is a porphyrin ring molecule, with a magnesium core. Magnesium has an electronic configuration of 1s2 2s2 2p6 3s2; within the chlorophyll molecules magnesium exists as Mg2+.
The two valence electrons in the 3s orbital are unbound and excitable to higher energy states, making the energy harvesting capacity of the molecule (when bound to chlorophyll binding proteins in the thylakoid membrane) quite efficient for photons of wavelengths which align with the possible excitation energy states of the Mg2+ ion.
Chlorophyll binding proteins are expressed by chloroplast DNA, these proteins when combined into subunits make up the energy harvesting system (referred to as a Photosystem). These proteins are all endogenous to the plastid and NOT the plant’s chromosomal DNA. Several chloroplast (photosystem related) genes such as PsbO, CP43, and CP26 (Drop et al., 2013) have been identified as conserved across several oxygenic photosynthetic species including the common ancestor Clamydomonas reinhardtii. The light harvesting complexes of these organisms are composed of a core containing the reaction centre (RC) where primary light harvesting occurs for the hydrolysis of water, as well as chloroplast and carotenoid antenna complexes that capture photons of varied wavelengths and transfer them to the RC (Drop et al., 2013). The outer antenna composition is thus predetermined by the cpDNA of the plant which is inherited in a non-mendelian manner.
When a plant is exposed to photon wavelengths that are too high for their light harvesting apparatus these systems may get damaged and result in the halting of photosynthesis (Photoinhibition); which will kill the plant if repair is not possible, and the cells are not receiving sufficient energy. The antenna systems of plants, algae, and cyanobacteria (Ruban, 2016) expel the excess energy as heat. These organisms have adapted to escape the fate of damaged photosystems over time; however, climate change events (as well as incorrect planting techniques) challenge these evasive mechanisms. This characteristic significantly contributes to the light tolerance of a plant; whether it is a heliophyte (light loving) or a sciophyte (shade loving).
Following everything mentioned above, we can now acknowledge that in the chloroplast, photons from the sun are converted into primary metabolites that are compatible for exiting the plastid and participating in cellular reactions. Each plant has its own unique by-products, which infers that they have unique metabolic compositions and as such their energy requirements will vary.
Consequently, the photosynthetic capacity and photoperiod of a plant significantly influences its economic value; the cost of growing a plant increases if your cultivation space requires adjustment to lighting for flowering to successfully occur, and for a sufficient fruit set and maturation to occur (for fruiting plants) or for optimal phytochemical yield (for herbs and medicinal crop).
Questions to ponder until next time:
What is a primary metabolite and how does it contribute to observable changes in plants?
How do plants know when to express which genes?
How do these metabolites exit the chloroplast?
Take care,
Mpule Clarke.
References
Alpert, T., Herzel, L., & Neugebauer, K. M. (2016). Perfect timing: splicing and transcription rates in living cells. Wiley Interdisciplinary Reviews - RNA, 8(2). https://doi.org/10.1002/wrna.1401
Buchanan, B. B., Gruissem, W., & Jones, R. L. (2015). Biochemistry and molecular biology of plants. John Wiley & Sons.
Drop, B., Webber-Birungi, M., Yadav, S. K., Filipowicz-Szymanska, A., Fusetti, F., Boekema, E. J., & Croce, R. (2014). Light-harvesting complex II (LHCII) and its supramolecular organization in Chlamydomonas reinhardtii. Biochimica Et Biophysica Acta (BBA) - Bioenergetics, 1837(1), 63–72. https://doi.org/10.1016/j.bbabio.2013.07.012
Mandal, R., & Dutta, G. (2020). From photosynthesis to biosensing: Chlorophyll proves to be a versatile molecule. Sensors International, 1, 100058. https://doi.org/10.1016/j.sintl.2020.100058
Ruban, A. V. (2016). Nonphotochemical Chlorophyll Fluorescence Quenching: Mechanism and Effectiveness in Protecting Plants from Photodamage. PLANT PHYSIOLOGY, 170(4), 1903–1916. https://doi.org/10.1104/pp.15.01935
USDA Plants Database. (n.d.). https://plants.usda.gov/home/classification/45882