• after fertilization, zygotes divide to form: the upper apical cell and the lower basal cell
• GLOBULAR STAGE
  • Spherical form that is formed as the proembryo (produced by apical cell division) continues to divide
  • Considered as the embryo proper
• HEART STAGE
  • where cotyledons arise from the embryo proper
  • Cotyledons - embryonic leaf-like structure that plays a role in food storage, absorption, and/or photosynthesis
• TORPEDO STAGE
  • torpedo results from the elongation of cotyledons and thickening of the base of the embryo
  • most of the suspensor deteriorates during this stage
• MATURE EMBRYO
  • Results in the final stage of embryogenesis
  • Includes a radicle (embryonic root)
  • At this point, embryo becomes dormant.
• Cell division is concentrated at the shoot apical meristem (at the shoot tip, between the cotyledons) and at the root apical meristem (at the bottom part of the embryo)

• the process is similar to the eudicots, the only differences are:
  • there is only a single cotyledon
  • there is no heart stage
    • embryo proper becomes cylindrical at this point
  • shoot apical meristem is present at the shoot tip, but not in between cotyledons since monocots only have one cotyledon

Náves ex Fern.-Vill. Retrieved May 25, 2021, from https://pfaf.org/User/Plant.aspx?LatinName=Xanthostemon+verdugonianus.

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• First phase of seed germination
• Physical process defined by rapid uptake of water, release of gas, and changes in temperature

• Water enters the seed, distributed in the cracks of the seed cover, and absorbed by the seed tissues
• Water uptake rates are temperature-dependent and are accompanied by increase in respiration rate and sensitivity in light (in some species)

• Complex molecules such as carbohydrates, proteins, and lipids are what consists the seed storage reserves
• In early post-germinative growth, storage reserves are broken down into soluble molecules (ike sucrose) which can be transported around the developing seedling.
• Phytohormones - known to control storage reserves mobilization in cereals
  • ABA and GA regulates the expression of genes encoding the enzymes required for storage carbohydrates and protein mobilization in rice.
  • It is possible that this reserve mobilization control by phytohormones seen in cereals is conserved in dicots.
  • In Arabidopsis, experiment of Pritchard et al. (2002) showed that ABA blocks germination, but reserve mobilization still occurs

• Seed reserve mobilization is usually thought to happen post-germination, but studies found that this also happens during germination
• Mobilization during germination varies with the species and their amount of reserves.

• asymmetrical redistribution of this plant hormone is the principle regulator of this directional growth response
• higher level of auxin in shoots promote elongation
• higher level of auxin in roots are inhibitory

• Water crosses all the surface area of the grain
• Hydration process takes a long period of time as starch is the main reserve component and endosperm is compact in cereal grains
• Hydration kinetics behavior has a downward concave shape (DCS)
  • Characterized by a reduction in the hydration rate from a max. value to zero when the grain absorbs the max. water that it can hold, called the equilibrium moisture content.
• On the Figure below, we can see that:
  • water enters the seed tip cap and fills space between the germ and endosperm
  • water can also cross the bran through diffusion
  • therefore grain hydration process can take place by capillary flow or diffusion

• response to the direction of light
• widespread among plants, from mosses, to ferns, and even algaes
• most easily observed in stems and leaves
• plants generally grow towards the blue-light
  • phototropins are the sensory receptor for this response
• phototropin system appear to interact with a red-light sensor phytochrome dependent pathway to enhance the blue-light response

• plant's response to gravity
• specialized sensory cells are present in the root tips, or near the vascular tissues of the shoot
• red-light represses shoot gravitropism
• gravity vector's directional information is perceived to be translated to a biochemical signaling cascade through the sedimentation of starch-filled amyloplasts present in the cytoplasm
• In some species, the prior activation of red-light signaling system is where gravitropic responses depend

• plant's response to touch/touch stimulus
• usually observed in twining plants or tendrils
• it is thought that a calcium ion-related signaling network triggers a cellular response
• however, the sensor and downstream components that respond to the calcium ion change are still undefined
• shown to downregulate gravitropism

• directional growth of roots towards water/water source
  • plant roots are able to detect a water potential gradient around them, and change the direction of the root tip accordingly
• like thigmotropism, it is also shown to downregulate gravitropism
  • gravitropic responsiveness needs to be reduced so hydrotropism will happen
• Species-specific mechanistic differences in hydrotropism exists
  • In Arabidopsis thaliana, cortex tissue appears to play a necessary role in hydrotropism, with proof that differential elongation in the tissue is what drives the bending response

• Seedling body organization can be seen as a superimposition of 2 patterns: apical-basal pattern along the main axis of polarity and a radial pattern across the axis.
• The relative position of embryonic and non-embryonic cells that normally have its origins from the apical and basal daughter cells of the zygote, respectively, is what may have caused the polarity of embryo.
• There is a possible maternal influence in orienting the embryo axis, as it is aligned with the axis of polarity of the ovule.

• The apical and basal zygote daughter cells differ in several ways:
  • Apical cell - small, cytoplasm-rich, divided into 8 proembryo cells by 2 rounds of vertical divisions, then 1 round of horizontal division
  • Basal cell - large, contains vacuole, repeatedly divides horizontally which gives a single file of 7-9 cells
  • The asymmetric division of the zygote establishes 2 cells of different of different fates, indicating the embryo's apical-basal polarity
• Octant stage proembryo has 2 tiers:
  • Upper tier - gives rise to the apical region of the embryo where the shoot meristem and cotyledons originate
  • Lower tier - produces the central region of the embryo that generates the remainder of cotyledons, root, hypocotyl, and upper tier of meristem stem cells of roots
• Basal region gives rise to the rest of the root meristem, the center of quiescence, and lower tier of stem cells
• All these shows that the apical-basal axis of young embryo is partitioned into 3 regions that develops differently: apical, central, and basal.
  • Example: inner cells within the apical region of 16-cell embryo express the homeobox gene WUSCHEL (WUS) that has a role in shoot meristem development.

• Consists of tissue layers that are concentrically arranged from the periphery to the center: epidermis, cortex and endodermis, pericycle, and vascular tissues.
• The radial pattern of tissue layers was brought about by the series of periclinal/tangential cell divisions on the central region of a young embryo