Agriculture Reference
In-Depth Information
A
B
C
Figure 6.4
Model for the role of polar auxin transport in phyllotaxis. The shoot apical meristem with
an expanding primordium (P1) is shown in (A) and (B). The region between the dashed lines depicts
the peripheral organogenic zone of the meristem, and I1 marks the site of organ initiation. Arrows
indicate polar auxin transport through the epidermis and sub-epidermis, and arrowheads indicate the
flow of auxin from surrounding tissue into the developing organ. Shading represents the likely
distribution of auxin (darker - more auxin, lighter - less auxin). (A) Following organ initiation, the
primordium (P1) becomes an auxin sink, diverting both the flow of auxin away from the apex and
draining auxin from the surrounding meristem tissue. As a consequence, a zone of auxin depletion
forms around the expanding organ, with the lowest concentration of auxin being in region above the
primordium. Auxin accumulates in the region of the meristem farthest from P1, which in this case is
180
◦
from P1 (distichous). (B) Once the concentration of auxin passes a threshold within organogenic
region, organ formation is initiated. As the organ forms it becomes a new sink for auxin, resulting in the
local depletion of auxin from surrounding tissue. This pattern of auxin depletion and accumulation is
self-perpetuating and can largely account for the maintenance of phyllotaxis once it has been
established. Redrawn from Reinhardt
et al.
(2003b). (C) The type of phyllotaxis displayed by a plant is
largely determined by factors that influence the distribution of auxin, such as relative sink strengths,
and size and growth rates of the central and peripheral zones. This panel, which illustrates how
different types of phyllotaxis arise, shows transverse sections through the meristems of plants
displaying distichous (left), spiral (middle) and decussate (right) phyllotaxis. In distichous phyllotaxis,
the youngest primordium (P1) absorbs auxin whereas the older primordium (P2, not shown) has either
stopped absorbing auxin or is too far from the meristem to affect subsequent organ initiation. As a
result the newly initiating organ (I1) forms opposite P1. In spiral phyllotaxis, P1 and P2 both compete
for auxin, with P1 being a large sink than P2. As a consequence I1 forms between P1 and P2, but closer
to P2. In decussate phyllotaxis the two opposing primordia are weaker sinks for auxin than in the
previous examples, which allows organ initiation to occur at two opposing positions equidistant from
the P1 organs. Redrawn from Reinhardt and Kuhlemeier (2002).
it must somehow generate a self-perpetuating pattern. The above studies suggest
how this might be achieved. Initiating organs generate a zone of auxin depletion
by both deflecting the flow of auxin away from the apex and draining auxin from
surrounding tissue (Fig. 6.4A). As a consequence, auxin accumulates in regions
farthest from developing organs (Fig. 6.4B). Organ formation is initiated only when
the concentration of auxin has passed a certain threshold in the organogenic zone.
This in turn is likely to promote the expression of
PIN1
, further enhancing the flow
