Control Systems in the Living World - Building the Most Complex Structure on Earth - page 44

Hormones in the Plant Control System
The bulk of information on the plant control mechanisms at the supercellular level is
related to the actions of plant hormones, known as phytohormones. Many phytohor-
mones that regulate physiological and developmental processes in plants are known.
In principle, their actions and mechanisms are similar to hormones in animals.
Especially relevant to understanding of the nature and mechanisms of hormonal
action in plants have been studies on the key plant hormone auxin or indoleacetic
acid (IAA), which among other things, is responsible for plant growth. Auxin moves
from one cell to another through plasma membranes not just with the help of auxin
transporters; in response to the perception of environmental stimuli, such as light and
gravity, plant cells also release auxin to neighboring cells via special vesicles in a
way reminiscent of the transport of neurotransmitters from the axon of one neuron to
the dendrites of another in animals. Indeed, auxin functions not only as a hormone,
but also as a morphogen and neurotransmitter. It is proposed that the quantal release
of auxin may have consequences for plant development. Hence, its release requires
tight control ( Baluška et al., 2008 ).
When a shoot is tipped off, auxin moves to the lower side of the shoot, caus-
ing it to bend upward (negative gravitropism). The polarity of transport of auxin in
the shoot is rootward ( Lewis et al., 2011 ). When a shoot is cut, auxin stimulates the
undifferentiated cells in the interior of the shoot to differentiate and form the apical
meristem of the root. This takes place in various plant species known to reproduce
vegetatively and is experimentally induced by dipping the cut surfaces into auxin
solutions. The stimulating effect of auxin on cell growth is based on the fact that the
hormone activates enzymes of the cell wall and causes acidification and loosening of
the rigid cellular wall.
Unlike any other hormone in plants, auxin transport between cells is polar. The
polarity of the movement of auxin from one cell to another is determined by the
influx and efflux carriers of auxin in the plasma membrane. Auxin is also involved
in its own transport through plant cells and tissues ( Petrášek and Friml, 2009 ). The
action of auxin depends not only on its synthesis and transport, but also on the cell's
competence to react to auxin, which “seems also to be controlled in time and space”
( Vernoux et al., 2010 ). Several auxin feedback loops acting at the cell level may con-
tribute to the elegant patterning of the plant tissues and organs. It is proposed that
auxin transport via the existing stem vasculature and auxin gradients set up through
a feedback loop auxin/PIN1 (PIN1 - an auxin efflux carrier) enable the shoot apical
meristem (SAM) to proceed with the initiation of leaves (Bayer et al., 2009; Leyser,
2010) ( Figure 1.26 ).
Auxin is essential for lateral root initiation. This process starts with the accumula-
tion of auxin in the central cells of the lateral root primordia, and later its concentra-
tion shifts to the tip of the primordia ( Péret et al., 2009 ). The new organ anlagen
(primordia) arises in the peripheral zone of the shoot meristem. Formation of organ
anlagen occurs at the sites of auxin maxima that are determined by the PIN1 gene,
which induces auxin efflux at the site of organ initiation at the shoot apical meristem
( Bohn-Courseau, 2010 ).