Aerenchyma
What is aerenchyma?
Aerenchyma is an airy tissue found in roots of plants, which allows exchange of gases between the shoot and the root. It contains large air-filled cavities, which provide a low-resistance internal pathway for the exchange of gases such as oxygen and ethylene between the plant parts above the water and the submerged tissues.
It is found in both roots that are submitted to anaerobic
conditions such as flooding
(Visser et al, 1997). For example, Blom et al (1994) researched adaptive
responses of plants to flooding along the banks of the Rhine river, wich included such morphological changes
such as aerenchyma fomation.
In 1995 Richard Zober discovered grasses in wetlands where
plants dug through clay layers for water and survived droughts. It founds the
roots to be spongy and tough. These roots were also found to create paths
through tough clay layers.
Another cool pic link:
What causes aerenchyma formation?
Aerenchyma is composed of airy compartments. In maize, for
example, it is formed as a result of highly selective cell death and
dissolution in the root cortex. It is formed as a response to anoxic conditions
in roots (He et al 1994).
When a plant’s roots get submerged underwater or the soil around them flooded, hypoxia tends to develop as soil microorganisms consume what little oxygen is left. Nitrification rate is inhibited (as nitrifying bacteria need oxygen), and toxic compounds are often formed as anaerobic bacteria use nitrate, manganese, and sulfate as alternative electron acceptors (Patrick and Mahapatra, 1968, Reddy et al. 1989). Futhermore, the red-ox potential of the area around the roots decreases and metal ions such as iron and manganese become insoluble and contaminous (Kim et al, 1999).
It has also been found that in hypoxic conditions, the
accumulation of the plant hormone Ethylene is necessary for aerenchyma
formation (Visser et al, 1997). This correlates with the fact that low oxygen
concentrations generally stimulate trees and plants to produce ethylene. In the
past, it was thought that high ethylene concentrations stimulate the formation
of adventitious roots. Now, some research indicates quite the opposite. It was
found that hypoxic soil does require ethylene for aerenchymatic growth and
adventitious root development but in anoxic areas, such as submergance in
water, Visser et al (1997) found ethylene to slow down adventitious root
elongation. Ethylene has also been implicated in slowing down primary root
elongation and adventitious root formation moreso than anaerobic soil conditions.
It is therfore possible that in addition to supplying root tissues with oxygen,
aerenchyma also assists in diffusing the accumulation of ethylene in order to
prevent elongation inhibition (Visser et al 1997).
Here is a picture of a “beefy”
adventitious root:
How is aerenchyma formed?
Aerenchymous tissue is
formed by cell differentiation and collapse (lysigenous
aerenchyma) or by cell separation without collapse (schizogenous aerenchyma). This forms large
continuous air spaces that allow diffusion of oxygen from shoot to root (Justin
and Armstrong 1987, Blom et al. 1996). Clues as to the mechanism of cell
collapse were gathered from different experiments. Cell death was blocked by
antagonists of phospholipid metabolism, of cytolsolic Ca2+ or
Ca-calmodulin and of protein kinases. By contrast, reagents that activate
G-proteins, raise cytolsolic Ca2+ or inhibit phosphatases promoted cell death (both
He et al 1996). An enzyme that was linked to this process was cellulase that
assists in cell wall breakage. In maize a protein was found wich is homologous
to the enzyme XET, a protein that breaks the β-1,4 links between glucans and xyulosyl – the cross-linking
molecule in plant cell walls (Saab and Sachs, 1996).
What are some
advantages of aerenchyma?
The first and obvious advantage is the large
air-filled cavities formed, which provide a low-resistance internal pathway for
the exchange of gases between the plant parts above the water and the submerged
tissues. Furthermore, some of the oxygen transported through the aerenchyma to
plant root tips leaks out of pores in the root and into the surrounding
soil. This can result in a small zone of oxygenated soil around
individual roots providing an aerobe environment for microorganisms that
can prevent the influx of potentially toxic soil components (Armstrong and Armstrong 1988 in Blom et al. 1996).
. These
develop during anoxic conditions, and include such sulphide, iron and
manganese. Aerobic bacteria also provide the roots with a favourable nitrogen
source by converting ammonium into nitrate (Barko et al. 1991).
In dry drought conditions, aerenchyma allows the
plant’s roots to dig deeper for water sources, even through tough layres such
as clay. In cases like these a thick and tough root is formed. As the roots
decay, they leave paths in which new roots can grow and continue elongating the
path.
Any disadvantages?
Not all plants are able to develop aernchymous tissue (a link has been found to the amound of flooding in reseach perfomed on plants living on the banks of the river Rhine).
Aerenchymous roots may experience the following problems:
Î Water and nutrient uptake may be less efficient;
Î Large intercellular spaces decrease the diameter of the transport pathway for water and nutrients from the root surface to the vascular system of the root(Visser et al. 1996, 2000a);
Î Large root diameter reduces biomass-to-surface ratio,
resulting in a smaller uptake of water and/or nutrients and reduced
possibilities of exploration of small patches with nutrients
Î Some aerenchymatous roots are not likely to resist the
physical strain of compacted soils. Those that survive dense and compact
drained soils have a higher bulk density and a strongly lignified layer of cells
surrounding the aerenchyma, which makes the root structure more robust. This
prevents radial leakage of oxygen from the aerenchyma spaces and is likely to
block efficient nutrient uptake as well (Colmer et al. 1998; Visser et al. 2000).
Î Under drought conditions, aerenchymatous roots may be
less tolerant to water stress as the open structure of the cortex is probably a
low-resistance pathway for water vapor, as it is for air, thereby increasing
the susceptibility of the shoot to water loss.
(Eric Visser
at http://www-eco.sci.kun.nl/eco/expploec/project1.htm).
Continuing research today
As aerenchymous tissue provides an advantage in resistance
of plants to droughts, genetic analysis and crossbreeding have proved useful in
the creation of more resistant plants. Already, there has been successful
cross-breeding of gamagrass and corn. Aerenchyma is also very common in
sugarcane. More detail on specific research in cross-breading and fighting
drought, as well as email links to researchs in the field can be found at:
http://www.ars.usda.gov/is/pr/1997/aerenchyma0897.htm
Some of the leading researchers in the field of aerenchyma today
and their home-pages:
Dr. David E. Evans - Oxford University
|
|
|
|
Dr. Eric J.W. Visser -
University of Nijmegen
Prof. Malcolm Drew - Texas A&M University
The following
web pages and articles were used as information sources for this site:
http://www.rycomusa.com/aspp1997/12/1812.shtml
http://www-eco.sci.kun.nl/eco/expploec/project1.htm
http://sciences.aum.edu/bi/BI4523/student/cardwell/aernc.html
http://www.hort.iastate.edu/pages/news/turfrpt/2000/deyingreprt.html
http://www.bartleby.com/61/imagepages/A4aerenc.html
http://www2.kenyon.edu/people/marxl/tinwater.htm
http://www.ffp.csiro.au/research/mycorrhiza/root.html
Blom, C.W.P.M. ( et. al ). 1994. Annals
of Botany .74:253-263.
Visser, E.J.W., R.H.M. Nabben, C.W.P.M. Blom, A.C.J. Voesenek. 1997. Plant, Cell, and Enviroment. 20: 647-653.
Saab, IN and
Sachs, MM. 1996. Plant Physiol 112:385-391.
He, C.-J.,
Drew, M.C., Morgan, P.W. (1994), Plant Physiol. 105:861-865
He, C.-J.,
Morgan, P.W., Drew, M.C., Morgan, P.W. (1996) Plant Physiol. 112:463-472.
He, C.-J.,
Finlayson, S.A., Drew, M.C., Jordan, W.R., Morgan, P.W. (1996) Plant Physiol.
112:1679-1685.
Kim et al.(1999).
Bot. Bull. Acad. Sin. 40: 185-191.
Created by:
Dan Almagor
Roots –
Virtual Course
Life Sciences
Dept.
Tel-Aviv
University