Apoplast

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The apoplastic and symplastic pathways Apoplast and symplast pathways.svg
The apoplastic and symplastic pathways

The apoplast is the extracellular space outside of plant cell membranes, especially the fluid-filled cell walls of adjacent cells where water and dissolved material can flow and diffuse freely. Fluid and material flows occurring in any extracellular space are called apoplastic flow or apoplastic transport. The apoplastic pathway is one route by which water and solutes are transported and distributed to different places through tissues and organs, contrasting with the symplastic pathway.

Contents

To prevent uncontrolled leakage to unwanted places, in certain areas there are barriers to the apoplastic flow: in roots the Casparian strip has this function[ clarification needed ] Outside the plant epidermis of aerial plant parts is a protective waxy film called plant cuticle that protects against drying out, but also waterproofs the plant against external water.

The apoplast is important for all the plant's interaction with its environment: The main carbon source (carbon dioxide) needs to be solubilized, which happens in the apoplast, before it diffuses through the cell wall and across the plasma membrane, into the cell's inner content, the cytoplasm, where it diffuses in the symplast to the chloroplasts for photosynthesis. In the roots, ions diffuse into the apoplast of the epidermis before diffusing into the symplast, or in some cases being taken up by specific ion channels, and being pulled by the plant's transpiration stream, which also occurs completely within the boundaries of the apoplast.[ clarification needed ] Similarly, all gaseous molecules emitted and received by plants such as oxygen must pass through the apoplast.

In nitrate poor soils, acidification of the apoplast increases cell wall extensibility and root growth rate. This is believed to be caused by a decrease in nitrate uptake (due to deficit in the soil medium) and supplanted with an increase in chloride uptake. H+ATPase increases the efflux of H+, thus acidifying the apoplast.[ clarification needed ] [1]

The apoplast is a site for cell-to-cell communication. During local oxidative stress, hydrogen peroxide and superoxide anions can diffuse through the apoplast and transport a warning signal to neighbouring cells. In addition, a local alkalinization of the apoplast due to such stress can travel within minutes to the rest of the plant body via the xylem and trigger systemic acquired resistance. [2]

The apoplast also plays an important role in resistance to aluminium toxicity.

In addition to resistance to chemicals, the apoplast provides the rich environment for microorganisms endophytes which arises[??] the abiotic resistance of plants.[ clarification needed ] [3] Exclusion of aluminium ions in[ clarification needed ] the apoplast prevent toxic levels which inhibit shoot growth, reducing[?] crop yields. [4]

History

The term apoplast was coined in 1930 by Münch in order to separate the "living" symplast from the "dead" apoplast. [5] [6]

Apoplastic transport

The apoplastic pathway is one of the two main pathways for water transport in plants, the other being symplastic pathway. In the root via the apoplast water and minerals flow in an upward direction to the xylem. [7]

The concentration of solutes transported through the apoplast in aboveground organs is established through a combination of import from the xylem, absorption by cells, and export by the phloem. [8]

Transport velocity is higher (transport is faster) in the apoplast than in the symplast. [9] This method of transport also accounts for a higher proportion of water transport in plant tissues than does symplastic transport. [10]

The apoplastic pathway is also involved in passive exclusion.[ clarification needed ] Some of the ions that enter through the roots do not make it to the xylem. The ions are excluded by the cell walls (plasma membranes)[ clarification needed ] of the endodermal cells. [11]

Apoplastic colonization

It is well known that the apoplast in plants’ tissues contains rich mineral nutrients, and it becomes the main factor for microorganisms to thrive at the apoplast. Even though there are apoplastic immunity systems, but there are pathogens that have effectors that can modulate the host immunity or suppress the immunity responses as known as effector-triggered susceptibility. [12] Another factor that pathogens colonize the apoplastic space so frequent is because when they enter the plants from leaves, the first place they come across is the apoplastic space. [13] Therefore, the apoplast is a popular biotic interface and also a reservoir for microbes. One of the common apoplastic disease appear in plants without restricted habitat or climate is black rot, caused by the gram-negative bacteria Xanthomonas campestris.

Entophytic bacteria can cause severe problems in agriculture in a way of inhibiting plant growth by alkalizing the apoplast with their volatiles. In especially, the rhizobacteria has been found that its major component of the volatiles are phytotoxic, it is identified as 2-phenylethanol. 2-phenylethanol can influence the regulation of WRKY18 which is a transcription factor that engages in multiple plant hormones, one of them is abscisic acid (ABA) hormone. [14] 2-phyenlethanol modulates the sensitivity of ABA through WRKY18 and WRKY40, but WRKY18 is the central mediator of the pathway of triggering cell death and modulation of ABA sensitivity influenced by 2-phyenlethanol. [15] Therefore, it results in the inhibition of root growth, and the plants have no capacity to grow without having the roots absorb nutrients in soils.

However, the microbial colonization in the apoplast is not always harmful to the plants, indeed, it can be beneficial to establish a symbiotic relationship with the host. One of the examples is the endophytic and phyllosphere microbes can indirectly promote plant growth and protect the plant from other pathogens by inducing salicylic acid (SA)and jasmonic acid (JA) signaling pathways, and they are both parts of the pathogen associated molecular patterns triggered immunity (PTI). The productions of SA and JA hormones also modulate the ABA signaling to be the components on the defense gene expression, and there are a lot more responses with the involvement of other hormones to respond to different biotic and abiotic stress. In the experiment performed by Romero et al., they inoculated the known entophytic bacteria, Xanthomonas into Canola, a plant that grows in multiple habitats, and it is found its apoplastic fluids that are 99% identity to another bacteria, Pseudomonas viridiflava, by performing 16S rRNA sequences with the Genebank and reference strains. They further used the markers on the SA-responsive transcriptional factor and other specific genes such as lipoxygenase 3 as marker genes for JA signaling and ABA signaling to perform quantitative reverse-transcription PCR. It has shown Xanthomonas only activates the related gene of SA pathway, in comparison, Pseudomonas viridiflava is able to trigger the genes of both SA and JA pathway, which suggest Pseudomonas viridiflava originally in Canola can stimulate PTI by the accumulation of both signaling pathway to inhibit the growth of Xanthomonas [16] . In conclusion, the apoplast acts as a crucial role in plants, involving in all kinds of regulations of hormone and transportation of nutrients, so once it has been colonized, the effect it brings cannot be neglected.

See also

Notes

  1. Apoplast was previously defined as "everything but the symplast, consisting of cell walls and spaces between cells in which water and solutes can move freely". However, since solutes can neither freely move through the air spaces between plant cells nor through the cuticle, this definition has been changed. When referring to "everything outside the plasma membrane", the term "extracellular space" is in use.
  2. The word apoplasm is also in use with similar meaning as apoplast, although less common.

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<span class="mw-page-title-main">Endodermis</span> Inner layer of cortex in vascular plant roots

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<span class="mw-page-title-main">Symplast</span> Interconnected intracellular space of a plant

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<span class="mw-page-title-main">Transpiration stream</span>

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<span class="mw-page-title-main">Hartig net</span> Network of inward-growing hyphae

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The exodermis is a physiological barrier that has a role in root function and protection. The exodermis is a membrane of variable permeability responsible for the radial flow of water, ions, and nutrients. It is the outer layer of a plant's cortex. The exodermis serves a double function as it can protect the root from invasion by foreign pathogens and ensures that the plant does not lose too much water through diffusion through the root system and can properly replenish its stores at an appropriate rate.

<span class="mw-page-title-main">Phloem loading</span>

Phloem loading is the process of loading carbon into the phloem for transport to different 'sinks' in a plant. Sinks include metabolism, growth, storage, and other processes or organs that need carbon solutes to persist. It can be a passive process, relying on a pressure gradient to generate diffusion of solutes through the symplast, or an active process, requiring energy to create membrane-bound transporter proteins that move solutes through the apoplast against a gradient. Passive phloem loading transports solutes freely through plasmodesma in the symplast of the minor veins of leaves. Active transport occurs apoplastically and does not use plasmodesmata. An intermediate type of loading exists that uses symplastic transport but utilizes a size-exclusion mechanism to ensure diffusion is a one-way process between the mesophyll and phloem cells. This process is referred to as polymer-trapping, in which simple solutes such as sucrose are synthesized into larger molecules such as stachyose or raffinose in intermediary cells. The larger molecules cannot diffuse back to the mesophyll but can move into the phloem's sieve cells. Therefore, the synthesis of larger compounds uses energy and is thus 'active' but this strategy does not require specialized proteins and can still move symplastically.

Plants are exposed to many stress factors such as disease, temperature changes, herbivory, injury and more. Therefore, in order to respond or be ready for any kind of physiological state, they need to develop some sort of system for their survival in the moment and/or for the future. Plant communication encompasses communication using volatile organic compounds, electrical signaling, and common mycorrhizal networks between plants and a host of other organisms such as soil microbes, other plants, animals, insects, and fungi. Plants communicate through a host of volatile organic compounds (VOCs) that can be separated into four broad categories, each the product of distinct chemical pathways: fatty acid derivatives, phenylpropanoids/benzenoids, amino acid derivatives, and terpenoids. Due to the physical/chemical constraints most VOCs are of low molecular mass, are hydrophobic, and have high vapor pressures. The responses of organisms to plant emitted VOCs varies from attracting the predator of a specific herbivore to reduce mechanical damage inflicted on the plant to the induction of chemical defenses of a neighboring plant before it is being attacked. In addition, the host of VOCs emitted varies from plant to plant, where for example, the Venus Fly Trap can emit VOCs to specifically target and attract starved prey. While these VOCs typically lead to increased resistance to herbivory in neighboring plants, there is no clear benefit to the emitting plant in helping nearby plants. As such, whether neighboring plants have evolved the capability to "eavesdrop" or whether there is an unknown tradeoff occurring is subject to much scientific debate. As related to the aspect of meaning-making, the field is also identified as phytosemiotics.

<span class="mw-page-title-main">Strigolactone</span> Group of chemical compounds

Strigolactones are a group of chemical compounds produced by roots of plants. Due to their mechanism of action, these molecules have been classified as plant hormones or phytohormones. So far, strigolactones have been identified to be responsible for three different physiological processes: First, they promote the germination of parasitic organisms that grow in the host plant's roots, such as Strigalutea and other plants of the genus Striga. Second, strigolactones are fundamental for the recognition of the plant by symbiotic fungi, especially arbuscular mycorrhizal fungi, because they establish a mutualistic association with these plants, and provide phosphate and other soil nutrients. Third, strigolactones have been identified as branching inhibition hormones in plants; when present, these compounds prevent excess bud growing in stem terminals, stopping the branching mechanism in plants.

The acid-growth hypothesis is a theory that explains the expansion dynamics of cells and organs in plants. It was originally proposed by Achim Hager and Robert Cleland in 1971. They hypothesized that the naturally occurring plant hormone, auxin (indole-3-acetic acid, IAA), induces H+ proton extrusion into the apoplast. Such derived apoplastic acidification then activates a range of enzymatic reactions which modifies the extensibility of plant cell walls. Since its formulation in 1971, the hypothesis has stimulated much research and debate. Most debates have concerned the signalling role of auxin and the molecular nature of cell wall modification. The current version holds that auxin activates small auxin-up RNA (SAUR) proteins, which in turn regulate protein phosphatases that modulate proton-pump activity. Acid growth is responsible for short-term (seconds to minutes) variation in growth rate, but many other mechanisms influence longer-term growth.

Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding. The cohesion and tension properties of water allow for these water potential changes to be transmitted throughout the plant.

References

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Footnotes