Plasmodesmata and symplastic transport in onion (Allium cepa L.) roots

Abstract

The structure of onion (Allium cepa L. cv. Ebeneezer) roots was studied by light and electron microscopy, with special references to the plasmodesmata and their functions in symplastic transport. In young zones (<50 mm from the root tip), all living cells were linked by plasmodesmata. In old zones (>50 mm from the root tip), all plasmodesmata in the exodermal long cells were severed by the developing suberin lamellae. Observations were focused on zones 100 mm and farther from the root tip where all long cells of the exodermis had formed suberin lamellae and the endodermal cells were at various stages of wall modification. Hydrophobic substances were also incorporated into epidermal cell walls. All these three cell layers had a low permeability, and this represented an enormous challenge for preserving cellular structures for transmission electron microscopy. One of the major contributions of the present study was the establishment of suitable techniques that made possible a series of investigations at the ultra-structural level.

A complete assessment of plasmodesmatal frequencies was performed for all cell types 100 mm from the root tip. Along the inward path (i.e. that taken by ions), the frequencies at the interfaces of epidermis-exodermal short cells-central cortex-central cortex-endodermis-pericycle-stelar parenchyma were 0.21, 1.03, 1.59, 0.58, 0.70, and 0.12 plasmodesmata per um^2 wall surface, respectively. When converted into numbers of plasmodesmata per mm root length, the numerical values for the interfaces of epidermis-exodermis-central cortex-endodermis-pericycle-stelar parenchyma were 8.96 x 10^4, 4.05 x 10^5, 5.13 x 10^5, 5.64 x 10^5, and 1.25 x 10^5, respectively. These numbers were believed to be the most instructive for assessing the degree of symplastic connection as all cell layers (except for those of the central cortex) were organized in concentric cylinders and the surface area traversed by ions was gradually decreasing. The data for the central cortex was not obtained because the cells were connected to each other in all directions, but the number of plasmodesmata available for radial transport was postulated to be high enough to support symplastic transport across this tissue. Therefore, significant symplastic transport was possible from the short cells of the exodermis up to the pericycle. From the epidermis to the short cells, much less symplastic transport was anticipated, and even less from the pericycle to the stelar parenchyma. Accordingly, a combination of symplastic and transmembrane transport was postulated to occur across the interface of the epidermis and short cells, and little transport from the pericycle to the stelar parenchyma.

In the phloem, the highest plasmodesmatal frequency was observed at the interface of metaphloem sieve elements and companions cells. The rest of the interfaces had much lower yet constant frequencies. This result suggested that photosynthates would be first transferred from the metaphloem sieve elements to companion cells and then distributed to the surrounding cells.

Ultrastructural observations and plasmodesmatal frequency analysis indicated that the pericycle could more intensively contribute to the transport process than previously envisioned. The high plasmodesmatal frequency on the radial walls of this layer implied that a significant transfer of solutes was occurring in the tangential direction. This would be beneficial to the overall transport in the root in which the xylem and phloem are arranged alternately.

By using an ultrastructural ion precipitation technique, chlorine movement in the root was monitored. It waws shown that all ranks of plasmodesmata were functional in transferring ions from the culture media to the stele. The precipitation pattern was approximately parallel to the plasmodesmatal frequencies. Under nutrient deficiency, the phloem was involved in reallocating chlorine from the shoot to the root tissues. Stelar parenchyma might play a role in transferring these ions from the companion cells to the pericycle. Also, epidermal cells sometimes developed wall ingrowths on their outer tangential side (i.e. transfer cells), which was regarded as a mechanism of enhancing transport across the boundary of the apoplast and symplast. When grown in a CaSO4 solution, the nutrient deficiency symptoms were remarkably alleviated. Calcium was postulated to have enhanced the reallocation of phloem-mobile ions (including chloride).

The development of wall modifications was investigated in the endodermis and exodermis. In the endodermis, Casparian bands, suberin lamellae, and tertiary walls formed in succession at increasing distances from the root tip. During band initiation, the tight binding of the plasma membrane to the anticlinical walls was established before the hydrophobic components could be detected in ultrathin sections. Casparian bands exhibited increased electron density with age. Mature bands never exceeded half the length of the radial walls. Suberin lamella formation started first along the outer tangential walls and then along the inner tangential walls. At the site of the Casparian band, the plasma membrane was detached from the wall, then hydrophobic substances were laid down. Tertiary wall formation commenced immediately afterwards. Large populations of dictyosomes and ER profiles were observed during suberin lamella and tertiary wall formation. These membrane systems were also found in association with the plasma membrane. This was suggestive of the involvement of membrane systems in the synthesis and delivery of wall materials. None of the wall modifications interrupted the symplastic connections of the endodermis. In the exodermis, Casparian bands were detected with fluorescence microscopy, but were not positively defined with the electron microscopy. The radial walls were extremely thin and alterations (if any) of their electron density were not apparent. Suberin lamellae were formed only in long cells, in which dictyosomes and ER were prominent. No tertiary walls had developed in the exodermis. All plasmodesmata in long cells were severed once suberin lamellae were laid down and the cells eventually died. Therefore, in a root with a mature exodermis, the epidermis was linked to the cortex only through the short cells without suberin lamellae.