PLASMODESMATAL FREQUENCY IN RELATION TO SHORT-DISTANCE TRANSPORT AND PHLOEM LOADING IN LEAVES OF BARLEY (HORDEUM-VULGARE) - PHLOEM IS NOT LOADED DIRECTLY FROM THE SYMPLAST
Cej. Botha et Rhm. Cross, PLASMODESMATAL FREQUENCY IN RELATION TO SHORT-DISTANCE TRANSPORT AND PHLOEM LOADING IN LEAVES OF BARLEY (HORDEUM-VULGARE) - PHLOEM IS NOT LOADED DIRECTLY FROM THE SYMPLAST, Physiologia Plantarum, 99(3), 1997, pp. 355-362
We investigated the phloem loading pathway in barley, by determining p
lasmodes-matal frequencies at the electron microscope level for both i
ntermediate and small blade bundles of mature barley leaves. Lucifer y
ellow was injected intercellularly into bundle sheath, vascular parenc
hyma, and thin-walled sieve tubes. Passage of this symplastically tran
sported dye was monitored with an epifluorescence microscope under blu
e light. Low plasmodesmatal frequencies endarch to the bundle sheath c
ells are relatively low for most interfaces terminating at the thin- a
nd thick-walled sieve tubes within this C-3 species. Lack of connectio
ns between Vascular parenchyma and sieve tubes, and low frequencies (0
.5% plasmodesmata per mu m cell wall interface) of connections between
vascular parenchyma and companion cells, as well as the very low freq
uency of pore-plasmodesmatal connections between companion cells and s
ieve tubes in small bundles (0.2% plasmodesmata per mu m cell wall int
erface), suggest that the companion cell-sieve tube complex is symplas
tically isolated from other Vascular parenchyma cells in small bundles
. The degree of cellular connectivity and the potential isolation of t
he companion cell-sieve tube complex was determined electrophysiologic
ally, using an electrometer coupled to microcapillary electrodes. The
less negative cell potential (average -52 mV) from mesophyll to the va
scular parenchyma cells contrasted sharply with the more negative pote
ntial (-122.5 mV) recorded for the companion cell-thin-walled sieve tu
be complex. Although intercellular injection of lucifer yellow clearly
demonstrated rapid (0.75 mu m s(-1)) longitudinal and radial transpor
t in the bundle sheath-vascular parenchyma complex, as well as from th
e bundle sheath through transverse veins to adjacent longitudinal vein
s, we were neither able to detect nor present unequivocal evidence in
support of the symplastic connectivity of the sieve tubes to the vascu
lar parenchyma. Injection of the companion cell-sieve tube complex, di
d not demonstrate backward connectivity to the bundle sheath. We concl
ude that the low plasmodesmatal frequencies, coupled with a two-domain
electropotential zonation configuration, and the negative transport e
xperiments using lucifer yellow, precludes symplastic phloem loading i
n barley leaves.