Inside a

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...

, the apoplast can mean the space outside of

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

s, 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 solvent ...

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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

) 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. ...

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

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

. 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 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 d ...

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

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

s 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 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 ...

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

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 t ...

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 ...

. 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 Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

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 walls and spaces between cells in which

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

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