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Seed Germination and Gibberellic Acid: The Mobilisation of Nutrients

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BEFORE YOU PROCEED TO READING ABOUT SEED GERMINATION, WATCH THIS FIRST!

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BEFORE YOU PROCEED TO READING ABOUT SEED GERMINATION, WATCH THIS FIRST!

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Seed Germination | #aumsum #kids #science #education #children

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Exploring the Arctic's Global Seed Vault

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Beyond the Gardens: Millennium Seed Bank Partnership

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Seed dormancy is recognized as an innate physical or physiological property in the seeds of plants wherein its capacity to germinate over a specified time period is inhibited regardless of any combination of environmental conditions (adequate water, temperature, oxygen, and light) that will support the germination process, and other abiotic factors such as solar irradiation, moisture content, nutrient status, and altitude (Baskin and Baskin, 2004; Bhadouria, 2017). In addition, genetic factors are also claimed to control dormancy (Nakabayashi et al., 2017). However, this phenomenon is not universal (Nadella et al., 2003). In most plants, it is considered as an important phenomenon for their survival as this evolution helps them adapt to the prevailing environments to avoid unfavorable weather for subsequent plant establishment and reproductive growth (Srivastava, 2002; Graeber et al., 2017).

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TYPES OF SEED DORMANCY AND HOW TO BREAK IT

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TYPES OF SEED DORMANCY AND HOW TO BREAK IT

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There two basic types of seed dormancy: primary dormancy (innate dormancy) which occurs during seed development/maturation meaning that a seed is already dormant as it is released from a plant, and secondary dormancy (induced dormancy) which results from factors after the seed is shed, for example, exposing seeds (that are hydrated and ready to germinate) to supraoptimal temperature or salinity conditions, may prevent germination which would cause them to become "secondarily" dormant. Primary dormancy may occur due to several reasons wherein in some seeds, the seed coat or other structures surrounding the embryo prevent the embryo from completing germination. When these barriers are removed mechanically, another type of dormancy known as coat-related or coat-enhanced dormancy is able to occur which allows the embryo to emerge. However, this technique may not be applicable to all seeds, bearing no effect, when dormancy is inherent in the embryo which may be due to the reason that this embryo is still very immature after it was shed from the mother plant. This is now referred to as the embryo-related dormancy. In order for these seeds acquiring coat-related or embryo-related dormancies to germinate, the perception of some environmental signal such as chilling or exposure to light are of great importance. For instance, the small and rudimentary embryos of Panax quinquefolius, commonly called ginseng, and most members of the family Orchidaceae, require a period of growth and tissue organ differentiation before they can germinate. Before the seeds of ginseng germinate, they must be subjected to alternating cooling and warm temperature regimes for several months (Figure 19-18). One specific example of a coat-related dormancy, on the other hand, may be observed in the conifer Chamaecyparis nootkanensis (yellow cedar), where the cuticle on the surface of its gametophyte serves as a barrier that inhibits water uptake (Figure 19-19) (Srivastava, 2002).

There are still other techniques to break seed dormancy aside from the perception of some environmental signal. This includes after-ripening where seeds are dried at room temperature before they can germinate, scarification which can also be done with hot water, dry heat, fire, acid and other chemicals such as respiratory inhibitors, sulfhydryl compounds, oxidants, and nitrogenous compounds (Taiz et al., 2015; Emery, 1988).

References:
[1] Baskin, J. M., & Baskin, C. C. (2004). A classification system for seed dormancy. Seed science research, 14(1), 1-16.
[2] Bhadouria, R., Singh, R., Singh, V. K., Borthakur, A., Ahamad, A., Kumar, G., & Singh, P. (2019). Agriculture in the era of climate Change: Consequences and effects. Climate Change and Agricultural Ecosystems, 1-23. doi:10.1016/b978-0-12-816483-9.00001-3
[3] Emery, D. E. (1988). Seed propagation of native California plants. Santa Barbara Botanic Garden.
[4] Graeber, K., Nakabayashi, K., & Leubner-Metzger, G. (2017). Development of dormancy. Encyclopedia of Applied Plant Sciences, 483-489. doi:10.1016/b978-0-12-394807-6.00209-4
[5] Nakabayashi, K., Graeber, K., & Leubner-Metzger, G. (2017). Genetics of dormancy. Encyclopedia of Applied Plant Sciences, 504-508. doi:10.1016/b978-0-12-394807-6.00210-0
[6] Srivastava, L. M. (2002). Seed germination, mobilization of food reserves, and seed dormancy. Plant Growth and Development, 447-471. doi:10.1016/b978-012660570-9/50161-1
[7] Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. S. (2015). Plant physiology and development (6th ed.). Sunderland, MA: Sinauer Associates.

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Figure 4. Bulbophyllum gnomoniferum

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Figure 3. Polygala venulose

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Figure 2. Astronia apoensis

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Figure 1. Decaisnina cumingii

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Mount Apo, also known as Apo Sandawa in the Philippines, is a massive inactive stratovolcano on the island of Mindanao. It is the highest peak in the Philippine Archipelago, rising 2,954 meters (9,692 feet) above sea level between Davao City and Davao del Sur province in Region XI and Cotabato in Region XII. Mount Apo is a three-peak stratovolcano with a flat top and a height of 2,954 meters (9,692 feet). The highest point on the mountain is the southwest peak, which is crowned with a 200 m (656 ft) broad crater with a small lake.

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Figure 4. Mechanism of phototropism
(Image retrieved from: https://www.sciencefacts.net/phototropism.html)

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[1] Fostering Education & Environment for Development, Inc. (FEED). 2021. Seeds for FEED – Nursery Establishment / Enhancement.
[2] Jvofi.org. 2021. 17 Community Seed Banks Turned Over in South Cotabato and Ifugao – Jaime V. Ongpin Foundation, Inc.
[3] Kew.org. 2021. Royal Botanic Gardens, Kew | Kew.
[4] Seedvault.nordgen.org. 2021. Svalbard Global Seed Vault.

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References:
[1] Taiz, L. and Zeiger, E., 2010. Plant Physiology. 5th ed. Sunderland, Massachusetts, USA: Sinauer Associates Inc.

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Figure 2. The apical–basal organization of plant tissues and organs & the radial tissue patterns are also established during embryogenesis

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III. Establishment of the Apical-Basal and Radial Axis of Arabidopis thaliana
Arabidopis have been extensively used in numerous studies as a model organism in elucidating certain underlying mechanisms of a plant. Arabidopis embryos undergo four (4) stages of development:

1. The globular stage embryo
The apical cell undergoes a series of highly regulated divisions after the first zygotic division, resulting in an eight-cell (octant) globular embryo.

2. The heart stage embryo
Rapid cell divisions in two zones on either side of the future shoot apex form this stage. Outgrowths from these two locations give rise to the cotyledons, which give the embryo bilateral symmetry.

3. The torpedo stage embryo
This stage develops as a result of cell elongation along the embryo axis and cotyledon formation.

4. The maturation stage embryo
The embryo and seed lose fluids and become physiologically inactive as they enter dormancy at the end of embryogenesis.

The axial polarity of a plant is established early in embryogenesis wherein  the zygote itself becomes polarized and elongates approximately threefold before its first division. Once the zygote divides, it creates two cells which are the apical and basal cells which have different fates. Almost all of the embryo's and, eventually, the adult plant's structures are produced from the apical cell. On the other hand, the basal cells divides horizontally at right angles to the long axis producing the suspensor. The suspensor is a filament of six to nine cells that connects the embryo to the plant's vascular system. The identities and roles of the cells in the apical and basal half of the sphere are distinct. The embryo's axial polarity becomes increasingly pronounced as it grows and reaches the heart stage, and three distinct axial areas can be identified: (1) apical region, (2) middle region, and (3) hypophysis. Two certain hormones drives the apical and basal polarities, which are cytokinin for the apical polarity and auxin for the basal polarity.

The radial pattern
The octant embryo is the first to show a radial pattern of tissue development. Transverse divisions separate the lower tier of cells radially into three zones as cell division continues in the globular embryo. These areas will form the root and stem axis' radially organized tissues. The protoderm is a one-cell-thick surface layer formed by the outermost cells. The cortex is produced by the ground meristem, which also produces the endodermis in the root and hypocotyl. The procambium is the inner core of elongated cells that produce vascular tissues and the pericycle in the root.

Certain genes expressions are required for tissues to exhibit patterning during embryogenesis namely: (1) GNOM gene for the axial patterning, (2) MONOPTEROS gene for the primary root and vascular tissue, (3) SHORT ROOT & SCARECROW genes for the ground tissues, (4) HOBBIT gene for the root meristem, and lastly the (5) SHOOTMERISTEMLESS gene for the shoot promeristem. All in all these genes are essential and participate in tissue patterning during the course of embryogenesis.

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Figure 1. Radial patterning of tissues in a cross-section of a root.

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Figure 3. Food reserve mobilization in a young cereal (barley) seedling following germination (Nonogaki, 2008).

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EMBRYOGENESIS ESTABLISHES ESSENTIAL FEATURES THAT PERSIST IN THE ADULT PLANTS NAMELY: (1) THE APICAL-BASAL AXIAL DEVELOPMENTAL PATTERN AND (2) THE RADIAL PATTERN OF TISSUES FOUND IN STEMS AND ROOTS. THESE TWO  DEVELOPMENTAL PATTERNS ARE EXHIBITED  BY PLANTS WHICH SERVE AS BASIS FOR ITS ORGANIZATION OF ORGANS.

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EMBRYOGENESIS ESTABLISHES ESSENTIAL FEATURES THAT PERSIST IN THE ADULT PLANTS NAMELY: (1) THE APICAL-BASAL AXIAL DEVELOPMENTAL PATTERN AND (2) THE RADIAL PATTERN OF TISSUES FOUND IN STEMS AND ROOTS. THESE TWO  DEVELOPMENTAL PATTERNS ARE EXHIBITED  BY PLANTS WHICH SERVE AS BASIS FOR ITS ORGANIZATION OF ORGANS.

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I. Axial Patterning
Axial polarity is a condition in which tissues and organs are arranged in a specific order along a linear, or polarized, axis which are exhibited by almost all plants. At one end of the axis is the shoot apical meristem, and at the other end is the root apical meristem.

II. Radial Patterning
Within plant organs, different tissues are organized in a precise way. The tissues in stems and roots are arranged in a radial pattern that extends from the outside of the stem or root to the center.

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Figure 2. Different phases of water imbibition (Nonogaki, 2008)

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Figure 1. Water imbibition in Pisum sativum L., var Baccara (Rolletschek & Borisjuk, 2009)

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Seed germination is considered a crucial phase in plant growth, development, and survival, influencing crop yield and quality (Kathare & Huq, 2020; Tuan et al., 2019). According to Pritchard et al (2002), germination is made possible by the antagonistic relationship between the phytohormones abscisic acid (ABA) and gibberellins (GAs). It is characterized by a series of events that begin with water imbibition and end with the embryonic axis (e.g. radicle) protrusion from the seed coat (Srivastava, 2002).

I. Mechanisms and Pathways of Water Imbibition
The imbibition of water by the seeds are primarily divided into three phases: Phase I being the rapid uptake of water, Phase II where water uptake is at equilibrium, and Phase III which is the uptake of water along with the initiation of growth (Sen & Puthur, 2020). To further visualize the mechanism behind these phases, let us take a look at the results from the study conducted by Rolletschek and Borisjuk in 2009 which focused on the imbibition of water by the seeds of pea (Pisum sativum L., var Baccara) (Figure 1).

Phase I (Figure 1. 0h) begins when the water enters into the seed, producing visual changes in its morphology. The dry seed that contains a mitochondria with poorly differentiated inner membranes rapidly uptakes water until all of its matrices and cell contents are fully hydrated, leading to the resumption of energy metabolism. The mechanism behind this is that the water was able to transfer from an environment with a higher water potential to an environment with a lower water potential, until it reaches equilibrium (ψ = 0), where water uptake is subjected to stop (Figure 2). During Phase II, the swelling of the seed coat (Figure 1. 10h) and cotyledons (Figure 1. 20h) can already be observed as sufficient ATP from functional enzymes necessary for respiration (such as those of the Krebs cycle and terminal oxidases) are supplied after several hours since the start of imbibition (Hourmant & Pradet, 1981). At this phase, the mitochondria or pre-existing mitochondria starts to undergo repair, replication, and/or reactivation which are most evident in starch-storing seeds such as peas and mung beans. At Phase III, cell expansion begins which induces reserve mobilization in storage tissues and seedling growth. This phase is where the radicle starts to protrude, breaking the seed coat (Figure 1. 30h), and continues to outgrow (Figure 1. 40h).

Two other respiratory pathways besides the Krebs cycle are active in water imbibition. This includes glycolysis and the pentose phosphate pathway (PPP) (Nonogaki et al., 2010). Production of ethanol is evident in the course of temporary anaerobic conditions during or after imbibition (Kennedy et al., 1992). When mitochondrial ATP production is restricted by a lack of oxygen, which is common due to the embryo's reduced permeability, the glycolytic pathway takes over. On the other hand, the PPP comes into play when the mitochondria become active (Roberts, 1964).

II. How Food Reserves are Mobilized
According to Nonogaki et al. (2010), there is only limited mobilization of reserves (e.g. sugars and amino acids) during germination as compared to reserve utilization after germination. Food reserve mobilization during germination can occur in the radicle, plumule, and endosperm which is considered important for early axis growth. At the latest phase of germination, the high-molecular-weight reserves such as carbohydrates, oils, and proteins, found in the seed storage organs (e.g. cotyledons or endosperm) are quickly transported to the expanding regions. This is done by being converted into easily transportable low-molecular-weight metabolites in order to sustain the energy-producing and synthetic events that are occurring (Nonogaki et al., 2008). Let us take a look at one example of how food reserves are mobilized using this diagram by Nonogaki (2008):

In the diagram above, the plant hormone, gibberellic acid (GA) promotes the synthesis of several hydrolytic enzymes as it was released from the scutellum and diffuses to the living cells of the aleurone layer (Jones & Armstrong as cited in Nonogaki, 2008). These are secreted into the starchy endosperm's nonliving cells, where the starch and protein reserves are stored. Several mechanisms then take place such as the degradation of starch to glucose (Glc) by key enzymes such as the α-Amylase and maltase, and the hydrolysis of proteins to short peptides and amino acids by proteinases. These hydrolytic products are absorbed by the part of the scutellum, which is part of the growing embryo. These processes continue until glucose (Glc) is converted to sucrose, and the products of protein mobilization are converted to glutamine and asparagine (Asn) which will be transported throughout the seedling via the vascular system as a supply of nutrients to support growth.

In the mobilization of food reserves in dicots and conifers, on the other hand, large amounts of proteins, lipids, and starch are accumulated in their seeds during seed development and maturation. According to Srivasta (2002), these reserves are mobilized to produce products that may be used as building blocks for macromolecules and as fuel for energy during seedling growth, leading to the decrease in amounts of food stored. In dicots, lipids (triacylglycerols) are hydrolyzed by lipases yielding fatty acids and glycerol where fatty acids will be used for membrane synthesis and for glucogenesis via 3 oxidation and the glyoxylate cycle.

III. Tropisms
Curvatures of plants and sessile organisms usually in response to light or gravity are termed as tropisms (Mead, 2008). Numerous studies have claimed that the hormone that is primarily active in inducing this phenomenon is auxin, specifically the Indole-3-acetic acid or IAA. This phytohormone is also considered as an important factor in regulating vascular tissue differentiation, leaf blade expansion, and inhibition of lateral bud development (Fosket, 1994). Moreover, IAA can be found everywhere in higher plants, leading them to possess the aforementioned defining roles as well as responses to external stimuli (Napier, 2017).

One common type of tropism in plants is known as phototropism. It is defined as the differential cell elongation exhibited by a plant organ in response to blue light which provides the plant the ability to optimize photosynthetic light capture in the aerial (shoot) portion, and water and nutrient uptake in the roots (Liscum et al., 2014).  This ability of plants to respond to light are also considered to be one of their coping mechanisms and adaptive responses to their changing environment, with the aid of photoreceptors and their signaling pathways (Kami et al., 2010; Pedmale et al., 2010; Chen and Chory, 2011 as cited in Liscum et al., 2014). There are two different types of phototropisms: positive phototropism or the ability of the plant to reorient its organ growth toward a directional light source, and negative phototropism, which is the ability of the plant to reorient away from a directional light source (Holland et al., 2009 as cited in Liscum et al., 2014). The mechanism behind phototropism is that the auxin concentration of the plant increases at the side of the plant that is away from the light, or simply, the shaded part of the plant (Figure 4). This accumulation of auxin causes the cells on the shaded part of the plant to elongate which leads the plant to bend towards the light source (towards the right side in the case of Figure 4) as there is now an uneven cell growth. As mentioned, photoreceptors (e.g. phototropins 1 and 2 [phot1 and phot2]), along with other components and regulatory factors, elicit signaling pathways that induce tropism (Goyal et al., 2013). However, according to Pedmale et al., (2010), these phototropins might also activate or modulate auxin transport through an indirect pathway that utilizes Ca2+ as a second messenger, by activating Ca2+-dependent protein kinase (CDPK) (Satterlee and Sussman, 1998; Harmon et al., 2000 as cited in Pedmale et al., 2010) or PID kinase which would, in turn, alter the activity of the phytohormone. Aside from phototropism, other types of tropisms have also been extensively studied. This includes gravitropism or geotropism (response to gravity), chemotropism (response to chemicals), hydrotropism (response to water), halotropism (response to high salinity), and thigmotropism (response to touch) (Muthert et al., 2020).

References:
[1] Fosket, D. E. (1994). Light, hormones, and cell signaling pathways. Plant Growth and Development, 271-340. doi:10.1016/b978-0-12-262430-8.50010-3
[2] Goyal, A., Szarzynska, B., & Fankhauser, C. (2013). Phototropism: at the crossroads of light-signaling pathways. Trends in Plant Science, 18(7), 393–401. doi:10.1016/j.tplants.2013.03.002
[3] Kathare, P. K., & Huq, E. (2020). Light signaling in plants. Reference Module in Life Sciences. doi:10.1016/b978-0-12-819460-7.00085-2
[4] Kennedy, R. A., Rumpho, M. E., & Fox, T. C. (1992). Anaerobic metabolism in plants. Plant Physiology, 100(1), 1-6. doi:10.1104/pp.100.1.1
[5] Liscum, E., Askinosie, S. K., Leuchtman, D. L., Morrow, J., Willenburg, K. T., & Coats, D. R. (2014). Phototropism: Growing towards an understanding of plant movement. The Plant Cell, 26(1), 38-55. doi:10.1105/tpc.113.119727
[6] Mead, K. (2008). Taxis. Encyclopedia of Ecology, 3483-3489. doi:10.1016/b978-008045405-4.00551-6
[7] Muthert, L. W., Izzo, L. G., Van Zanten, M., & Aronne, G. (2020). Root tropisms: Investigations on Earth and in space to unravel plant growth direction. Frontiers in Plant Science, 10. doi:10.3389/fpls.2019.01807
[8] Napier, R. (2017). Auxins. Encyclopedia of Applied Plant Sciences, 367-377. doi:10.1016/b978-0-12-394807-6.00099-x
[9] Nonogaki, H., Bassel, G. W., & Bewley, J. D. (2010). Germination—still a mystery. Plant Science, 179(6), 574-581. doi:10.1016/j.plantsci.2010.02.010
[10] Nonogaki, H. (2008). Seed Germination and Reserve Mobilization. Encyclopedia of Life Sciences. doi:10.1002/9780470015902.a0002047.pub2
[11] Roberts, E. H. (1964). A survey of the effects of chemical treatments on dormancy in rice seed. Physiologia Plantarum, 17(1), 30-43. doi:10.1111/j.1399-3054.1964.tb09014.x
[12] Roberts, E. H. (1964). The distribution OF Oxidation-reduction enzymes and the effects of RESPIRATORY inhibitors AND Oxidising agents on Dormancy in rice seed. Physiologia
[13] Pedmale, U. V., Celaya, R. B., & Liscum, E. (2010). Phototropism: Mechanism and outcomes. The Arabidopsis Book, 8. doi:10.1199/tab.0125
[14] Plantarum, 17(1), 14-29. doi:10.1111/j.1399-3054.1964.tb09013.x
[15] Rolletschek, H., Stangelmayer, A., & Borisjuk, L. (2009). Methodology and significance of Microsensor-based Oxygen mapping in plant seeds – an overview. Sensors, 9(5), 3218-3227. doi:10.3390/s90503218
[16] Srivastava, L. M. (2002). Seed germination, mobilization of food reserves, and seed dormancy. Plant Growth and Development, 447-471. doi:10.1016/b978-012660570-9/50161-1
[17] Tuan, P. A., Sun, M., Nguyen, T., Park, S., & Ayele, B. T. (2019). Molecular mechanisms of seed germination. Sprouted Grains, 1-24. doi:10.1016/b978-0-12-811525-1.00001-4

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Efforts have already started for our country to conserve and sustain the agro-biodiversity of the Philippines. Although these are preliminary actions such as the SEEDs for FEED program and local communities in South Cotabato constructing community seed banks whose goals are on the same track, which are to conserve and sustain the agriculture of the country for the future.

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These are large facilities where copies of seeds from gene banks all over the world are stored to serve as a back-up system in any case humanity or any natural phenomena can wipe-out agriculture.

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Zygote denotes that embryogenesis had begun which will then develop through a series of development events. Embryogenesis in animals is different compared to plants, as embryogenesis in plants also varies  among eudicot and monocot plants.

I. Zygotic Embryogenesis on Eudicots and Monocots
The diversification of monocots and eudicots clades a long time ago (approx. 140 to 150 MYA) gave a window for both clades to evolve differently particularly on the process of embryogenesis  in terms of embryo development, cell differentiation and cell fate establishment of apical and basal cell lineages. De novo-synthesized zygotic factors play major roles in both the development of eudicot and monocot plants but varies on how these plants utilize it. De novo synthesized zygotic factors are known to be critical in the development of a plant although it remains difficult to clearly picture its specific function due to scarcity of information. In eudicots, zygotic division and elongation are affected by transcription factors which also effect de novo transcription (Zhao and Sun, 2015). On the other hand, on model organism such as Zea mays, has shown that de novo transcripts for ribosomal proteins (Dresselhaus, 1999).

As shown in the figure below, both eudicot and monocot plants in terms of the mechanism of developing a zygote differs. It is already evident from the start how the immature egg cells' transition to mature egg cell differ in terms of its development in both plants, wherein in eudicots a central vacuole is formed compared to monocots which develops high number of vacuoles. In contrast to eudicots, zygote elongation and increase in cell size do not take place in monocots which proves to be  a major deciding factor of how both plants develops its way of being a zygote. Take note also the difference in the number of  genes required for zygote and morphological development among the two types of plants (Zhao et al., 2017).

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WHAT SEEDS WOULD WE DONATE FROM OUR PLANT AREA (MT. APO) TO SEED BANKS?

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WHAT SEEDS WOULD WE DONATE FROM OUR PLANT AREA (MT. APO) TO SEED BANKS?

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Our chosen PlantArea is one of the famous landmarks of the Philippines which is Mount Apo. Famous landmarks come with their advantages and disadvantages as these attract anthropogenic-related activities. Besides the prevalence of climate change causing drastic changes and disturbances in our environment, human activities also largely contribute to this problem. Our 10 chosen plants are endemic to Mount Apo, meaning they are naturally occurring there and are restricted to that place, so that’s why we’ve chosen these 10 plants/seeds to be submitted to seed banks due to their ecological importance and they are on a high-risk system particularly exposed to anthropogenic activities due to their location.

1. Gymnopachys pulcherrima
- is a small tree known previously by a single collection from Mt. Apo at about 1,365 meters
2. Decaisnina cumingii (Figure 1)
- belongs to the family of Loranthaceae and considered as an epiphyte. It is known previously by collections from Mount Apo (Davao side, about 1,800 meters)
3. Macrosolen angulatus
- belongs to the family of Loranthaceae and considered as an epiphyte. Can be found from Mount Apo (825 meters).
4. Astronia apoensis (Figure 2)
- belongs to the family of Melastomataceae and it is a small tree found in Mount Apo (1,500 meters). Variant: A. mearnsii)
5. Aglaia apoana
- belongs to the family of Meliaceae and it is a small tree of local distribution in Mount Apo (1,600 meters).
6. Ardisia apoensis
- belongs to the family of Myrsinaceae and it is a small tree found in Mount Apo (1,600 meters).
7. Xiphopteris apoensis
- belongs to the family of Grammitidaceae and considered as an epiphyte found in Mount Apo (2,000 meters).
8. Grammatis torricelliana
- belongs to the family of Grammitidaceae and considered as an epiphyte found in the western base of Mount Apo.
9. Polygala venulose (Figure 3)
- belongs to the family of Polygalaceae and it is considered as an herb.
10. Bulbophyllum gnomoniferum (Figure 4)
- a critically endangered species of orchid endemic to Mt. Apo

References:
[1] List of critically endangered species in Philippines. (n.d.). Retrieved May 26, 2021, from https://rainforests.mongabay.com/biodiversity/en/philippines/CR.html
[2] Zamora, P. M. (n.d.). Inventory Of Important Species. Retrieved May 26, 2021, from http://www.mafi.org.ph/information/featured_topics/ft_sub_04.html

Images retrieved from:
http://phytoimages.siu.edu/imgs/paraman1/r/Loranthaceae_Decaisnina_cumingii_52786.html
https://www.flickr.com/photos/filibot/8044095794/
http://www.freenatureimages.eu/plants/Flora%20O-R/Polygala%20venulosa/index.html
http://www.phytoimages.siu.edu/imgs/pelserpb/r/Orchidaceae_Bulbophyllum_gnomoniferum_108598.html

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Figure 1. Egg cell maturation and zygote development in eudicots and monocots (Zhao et al., 2017).

References:
[1] Chaw, S. M., Chang, C. C., Chen, H. L., & Li, W. H. (2004). Dating the monocot–dicot divergence and the origin of core eudicots using whole chloroplast genomes. Journal of molecular evolution, 58(4), 424-441.
[2] Dresselhaus, T., Cordts, S., Heuer, S., Sauter, M., Lörz, H., & Kranz, E. (1999). Novel ribosomal genes from maize are differentially expressed in the zygotic and somatic cell cycles. Molecular and General Genetics MGG, 261(2), 416-427.
[3] Zhao, P., Begcy, K., Dresselhaus, T., & Sun, M. X. (2017). Does early embryogenesis in eudicots and monocots involve the same mechanism and molecular players?. Plant Physiology, 173(1), 130-142.
[4] Zhao, P., & Sun, M. X. (2015). The maternal-to-zygotic transition in higher plants: available approaches, critical limitations, and technical requirements. Current topics in developmental biology, 113, 373-398.

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Images retrieved from:
https://www.tripadvisor.in/ShowUserReviews-g298459-d320862-r158435645-Mount_Apo-Davao_City_Davao_del_Sur_Province_Mindanao.html
https://www.researchgate.net/publication/320146559_Contributions_to_the_orchid_flora_of_Mindanao_Long-Term_Ecological_Research_Sites_Philippines