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Aerenchyma

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Aerenchyma is an air channel in the roots of some plants, which allows exchange of gases between the shoot and the root. The channel of large air-filled cavities provides a low-resistance internal pathway for the exchange of gases such as oxygen and ethylene between the plant above the water and the submerged tissues.

Aerenchyma form in roots subject to anoxia such as what occurs during flooding of plants and soil [1]. For example, Blom et al. (1994) investigated the adaptive responses of plants to flooding along the banks of the Rhine River, which included such morphological changes such as aerenchyma formation.

Aerenchyma formation

Aerenchyma of Schoenoplectus tabernaemontani

In maize, an aerenchyma is formed from highly selective cell death and dissolution in the root cortex during anoxia in the roots [2]. When plant roots are submerged or the surrounding soil flooded, hypoxia develops, as soil microorganisms consume oxygen faster than diffusion occurs. Nitrification is inhibited as low oxygen occurs and toxic compounds are formed, as anaerobic bacteria use nitrate, manganese, and sulfate as alternative electron acceptors [3]. The reduction-oxidation potential of the rhizhosphere decreases and metal ions such as iron and manganese precipitate [4].

In general, low oxygen stimulates trees and plants to produce ethylene [5]. Yet Visser et al.., in 1997, found that ethylene slows down primary and adventitious root elongation and formation. Thus, in addition to supplying root tissues with oxygen, aerenchymas assist in diffusing the accumulation of ethylene in order to prevent elongation inhibition (Visser et al. 1997).

Formation of Aerenchyma

Aerenchymas are formed by cell differentiation and collapse (lysigenous aerenchyma) or by cell separation without collapse (schizogenous aerenchyma). The differentiation or separation forms large continuous air spaces that allow diffusion of oxygen from shoot to root [6]. Different experiments defined how cell collapse occurs. 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 (two references He et al.. 1996). An enzyme that was linked to this process is cellulase, which assists in cell wall breakage. In maize, a protein that 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) was found.[7].

Advantages of aerenchyma

The large air-filled cavities provide a low-resistance internal pathway for the exchange of gases between the plant organs above the water and the submerged tissues. Some of the oxygen transported through the aerenchyma leaks through root pores into the surrounding soil. The resulting small rhizosphere of oxygenated soil around individual roots support microorganisms that prevent the influx of potentially toxic soil components [8] such as sulfide, iron, and manganese. Nitrifying bacteria provide the roots with a favourable nitrogen source [9].

During drought, aerenchymas allow plant roots to grow deeper for water, even through compacted layers; thick and tough roots are formed. As the roots dieback and decay, the resulting voids are paths in which new roots can grow and elongate when resources are available.

Disadvantages of Aerenchyma

Not all plants are able to develop aerenchymas.

Aerenchymous roots may experience the following problems

  • Water and nutrient uptake may be less efficient; large intercellular spaces decrease the tissue available to transport water and nutrients from the root surface to the root xylem (Visser et al.. 1996, 2000a).
  • Large root diameters reduce biomass-to-surface ratio, resulting in less uptake of water and nutrients and the reduced opportunity to explore all microzones for nutrients.
  • Some roots with aerenchymas are less likely to resist the physical strain of compacted soils. Those roots that penetrate and survive dense and compact drained soils have a higher bulk density and a strongly lignified layer of cells surrounding the aerenchyma, which strengthens the root. This dense, lignified layer prevents radial leakage of oxygen from the aerenchyma and may block some water and nutrient uptake (Colmer et al.. 1998; Visser et al.. 2000).
  • During drought, roots with aerenchyma may be less tolerant to water stress as the open structure of the cortex is probably a low-resistance pathway for water vapor, as well as for air, thereby increasing the susceptibility of the root to water loss.

References

  1. ^ Visser et al., 1997
  2. ^ He et al. 1994
  3. ^ Patrick and Mahapatra, 1968, Reddy et al. 1989
  4. ^ Kim et al., 1999
  5. ^ Visser et al., 1997
  6. ^ Justin and Armstrong 1987, Blom et al. 1996
  7. ^ Saab and Sachs, 1996
  8. ^ Armstrong and Armstrong 1988 in Blom et al. 1996
  9. ^ Barko et al. 1991
  • 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
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