Agriculture Reference
In-Depth Information
CH
2
CH
2
CH
2
COOH
CH
2
COOH
N
H
N
H
IAA
IBA
CH
2
COOH
OCH
2
COOH
Cl
1-NAA
Cl
2,4-D
Figure 1.1
Chemical structures of important auxins. IAA: indole-3-acetic acid; IBA: indole-3-butyric
acid; 1-NAA: 1-naphthylacetic acid; 2,4-D: 2,4-dichlorophenoxyacetic acid.
1.2
Auxin transport - known pathways
Twophysiologically distinct and spatially separated pathways act together to trans-
port auxin over long distances through plants (Fig. 1.2). Firstly, auxin is translocated
rapidly by mass flow with other metabolites in the mature phloem. Secondly, auxin is
transported downwards toward the root tips from immature tissues close to the shoot
apex by a much slower, carrier-dependent, cell-to-cell polar transport (Goldsmith,
1977).
1.2.1 Polar auxin transport pathway
Uniquely amongst plant signaling molecules, auxins can be transported in a strictly
regulated, polar fashion in plant tissues. Unlike auxin translocation in phloem, the
polar auxin transport (PAT) is slower, specific for active free auxins and has a strictly
unidirectional character. The main (basipetal) PAT stream runs from the apex with
avelocity of 5-20 mm/h toward the base of the plant (Lomax
et al
., 1995). Using
radioactively labeled auxin, this kind of transport was mainly detected in the cam-
bium and adjacent, partially differentiated xylem vessels and xylem parenchyma. In
the shoot, in addition to the basipetal stream, the movement of auxin was detected
also in the lateral direction (Morris & Thomas, 1978). In the root, the auxin stream
continues toward the root tip (acropetally), where a part of the auxin is redirected
backwards from the root cap and transported basipetally through the root epidermis
to the elongation zone (Fig. 1.2) (Rashotte
et al
., 2000). Auxin transport assays
revealed that PAT requires energy, is saturable and sensitive to protein synthesis
inhibitors. These results together with a cell-to-cell character of PAT suggested the
