Inside a plant, the apoplast can mean the space outside of cell membranes, where material can diffuse freely; that is, the extracellular spaces. ''Apoplast '' can also refer especially to the continuum of cell walls of adjacent cells; fluid and material flows occurring there or in any extacellular space are called ''apoplastic'' flow or apoplastic transport. The apoplastic route is one way by which water and

solutes In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solven ...

are transported and distributed to different places through tissues and organs; another way is

symplast The symplast of a plant is the inner side of a cell membrane in which water and low-molecular-weight solutes can freely diffuse. Symplast cells have more than one nucleus. ''Symplast'' could also refer to the connection of the inner contents (c ...

ic flow. To prevent uncontrolled leakage to unwanted places, in certain areas there are barriers to the apoplastic flow: in roots the

Casparian strip The Casparian strip is a band-like thickening in the center of the root endodermis (radial and tangential walls of endodermal cells) of vascular plants (Pteridophytes and Spermatophytes). The composition of the region is mainly suberin, lignin and ...

has this function larification needed On the outside of the skin of certain plant parts is a protective waxy film called plant cuticle to achieve this (protection against e.g. drying out, but also waterproofing against soaking). Air bubbles occupying extracellular spaces can also hinder apoplastic transport. 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 (also called ''plasma membrane'') into the cell's inner content (

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...

) (thereby then entering the ''symplastic'' possibility of flow) to be used (by the

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

s) for photosynthesis. In the roots, ions diffuse into the apoplast of the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...

before diffusing into the symplast, or in some cases being taken up by specific

ion channels Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ...

, and being pulled by the plant's

transpiration stream In plants, the transpiration stream is the uninterrupted stream of water and solutes which is taken up by the roots and transported via the xylem to the leaves where it evaporates into the air/apoplast-interface of the substomatal cavity. It is dr ...

, which also occurs completely within the boundaries of the apoplast. Similarly, all gaseous molecules emitted and received by plants such as

plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, p ...

s and other pheromones must pass 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 ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...

increases the efflux of H+, thus acidifying the apoplast. larification needed - maybe better an extra chapter for this? The apoplast is a site for cell-to-cell communication. During local

oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal r ...

, hydrogen peroxide and

superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of th ...

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 Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

and trigger

systemic acquired resistance Systemic acquired resistance (SAR) is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and although there are many shared aspects be ...

. 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 An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; h ...

which arises ?the abiotic resistance of plants. larification needed Exclusion of aluminium ions in larification needed: "EXclusion IN"??the apoplast prevent toxic levels which inhibit shoot growth, reducing crop yields.

History

The term apoplast was coined in 1930 by Münch in order to separate the "living"

from the "dead" apoplast.

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. 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. Transport velocity is higher (transport is faster) in the apoplast than in the symplast. This method of transport also accounts for a higher proportion of water transport in plant tissues than does symplastic transport. The apoplastic pathway is also involved in passive exclusion. larification neededSome 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'') larification needed: why would that hinder APOplastic flow?of the endodermal cells.

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. 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. 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. 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. 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''''.'' 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.

Notes

#Apoplast was previously defined as "everything but the

, consisting of

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering me ...

s and spaces between

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

in which water and

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. #The word apoplasm is also in use with similar meaning as apoplast, although less common.

References

Footnotes