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^[clarify][1]. For example, Blom et al (1994) researched adaptive responses of plants to flooding along the banks of the Rhine river, which included such morphological changes such as aerenchyma formation.

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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 ^[2].

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 ^[3]. Furthermore, the red-ox potential of the area around the roots decreases and metal ions such as iron and manganese become insoluble and contaminous ^[4].

It has also been found that in hypoxic conditions, the accumulation of the plant hormone Ethylene is necessary for aerenchyma formation ^[5]. 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 more so than anaerobic soil conditions. It is therefore 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).

How are 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 ^[6]. 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 which 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 ^[7].

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 ^[8].

. 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 ^[9].

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 aerenchymous tissue (a link has been found to the amount of flooding in research performed on plants living on the banks of the river Rhine).

Aerenchymous roots may experience the following problems

• Water and nutrient uptake may be less efficient;
• 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);
• 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
• 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).
• 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.

References

• 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 Environment. 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