Are Auxin and Gibberellin the Main Agents in the Temperature-Dependent Growth of Geophytes?
The experimental data suggest that the only currently known cold-induced trigger in bulbs is auxin
(Figure 2). In this case, temperature may act by changing the sensitivity of a plant to auxin signals and,
thus, represents a growth-regulating factor.
Figure 2. A proposed model of the influence of low temperature on growth in geophytes.
Prolonged cold increases sensitivity to auxin, which induces shoot growth. The induction
of growth triggers the remobilization of all reserves in bulbs, an increase in respiration and
water flux and the onset of gibberellin biosynthesis. Accumulation of gibberellins leads to
an enhanced expression of invertase genes, which provides the hexoses necessary for shoot
elongation. All these parameters lead to proper stalk elongation and flowering. Auxin
probably affects the onset of gibberellin synthesis (indicated by a dashed line).
Induction of growth
Gibberellin
production
Auxin
Increase in
respiration
Water
redistribution
Accumulation
of hexoses
Starch
hydrolysis
Stalk elongation and
flowering
BULB
Low temperature
(4−9 °C)
Plants 2013, 2 706
In different plant species, both auxin and GA have central roles in temperature-controlled
elongation responses [50]. Moreover, the concentration of auxin may regulate key genes encoding
enzymes for GA biosynthesis [51–55]. This joint action of two hormones guarantees the growth of
shoots and leaves: auxin promotes GA biosynthesis, thus acting as a mobile factor, whereas GA is the
actual effector of growth [51,52].
Since stalk elongation in bulbous plants has also been shown to be due to the parallel activity of
auxin and gibberellins, we suggest that auxin signals not only play a role in the induction of shoot
growth, but also probably affect the onset of GA biosynthesis. The accumulation of the latter in
growing shoots induces the remobilization of all reserves in bulbs in order to supply nutrients for
growth. For instance, accumulation of GA leads to an enhanced expression of invertase genes, which
establishes a large sink of sucrose transported from the bulb and provides hexoses necessary for shoot
elongation. A normal growing shoot also induces an increase in respiration and water flux.
Therefore, according to our hypothesis, the differences observed between pre-cooled and non-cooled
plants during storage and after planting (in carbohydrate content, enzyme activities, water content and
respiration rate, etc.) independently do not explain growth inhibition. These parameters are only
secondary effects of the absence of growth, which is regulated by temperature via auxin signals and
GA biosynthesis.