Auxin: Cytokinine Ratio May Determine Plant Stem Elongation
Bjarke Veierskov*
Department of Plant and Environmental Sciences, University of
Copenhagen, Denmark
*Corresponding
author:
Bjarke Veierskov, Department of Plant and Environmental Sciences, University
of Copenhagen, Denmark. Tel: + 45-35332616; Email: bv@plen.ku.dk
Received Date: 06
September, 2018; Accepted Date: 12
October, 2018; Published Date: 19 October,
2018
Citation: Veierskov B (2018) Auxin: Cytokinine Ratio May Determine Plant Stem Elongation. Arch Appl Sci Res: AASR-101. DOI:10.29011/AASR-101. 100001
1. Abstract
Endogenous levels of phytohormones seldom correlate to growth and development. However, in Abies nordmanniana, a IAA: cytokinin ratio below one corresponds to meristematic activity in the apical meristem, whereas a high ratio about 50 was observed in the elongating stem tissue.
2. Keywords: Auxin Cytokinin Ratio; Stem Elongation
3.
Introduction
Although auxin has been known to influence cell
elongation since the 1930’es, we are still lacking an understanding of how
auxin participates in maintaining controlled stem elongation during a growing
season. In a changing environment auxin initiates developmental adjustments of
processes such as tropisms, apical dominance and cell proliferation [1], and auxin
synthesis as well as transport are influenced by irradiance and temperature [2], all
factors that may explain the weak correlations between hormone levels and plant
development as well as unexplainable peaks [3-5]. Where auxin is able to
stimulate cell elongation by itself, it is the interaction between auxin and
cytokinin that is required to maintain cell division [6]. It is well documented that auxin and cytokinin regulates each other [7,8], and thereby developmental
processes such as embryogenesis, meristem development and shoot branching [9]. However,
auxin and cytokinin are today still being evaluated separately regarding stem
elongation.
A model system where the development of the apical
meristematic is separated from the process of stem elongation is idle to probe
into the possible regulatory function of the auxin-cytokinin ratio in determining
stem elongation. Such a model does exist in gymnosperms. In conifers such as
spruce, pine and abies, a
none elongating shoot initial is formed within the apical meristem each year,
but the elongation thereof occurs first the flowering year. A cellular barrier
exists between the two types of tissue. These plants are therefore unique by
having separated the process of forming a shoot initial from the process of
stem elongation. In these species of gymnosperms, top leader elongation is
concealed to at short well-defined time period as bud burst occur in late May,
and elongation of the preformed shoot initial occurs in June/July, with some
variation between years [10].
We have made year round hormone determinations in
several parts of Abies
nordmanniana [11]. Although we observed a
dramatic changes in the level of hormones during the growing season, as well as
between different types of tissue, is was not possible to make firm conclusions
to how the observed hormonal changes might regulate plant development. Furthermore,
although weekly samples were taken in the period of growth, unexplainable
variation in hormone levels was always determined (Figure 1).
During a reevaluating of the data obtained, it became
apparent, that if the auxin-cytokinin ratio was used as a determinator of stem
elongation, the unexplainable hormone fluctuationsdisappeared (Figure 1 vs Figure 2). The period of fast stem
elongation corresponded to the period where the auxin-cytokinin ratio was very
high, around 60, and as soon as theelongation process began to ceases, this
ratio dropped to below 2. If the same ratio was determined within theapical
bud, where the none-elongating shoot initial were formed, the ratio stayed
below 1 throughout the entire developmental period, except at the very early
phase of initiation, where a ratio of 2 was observed (Figure
2).
The literature contains only very few data comparing
the level of auxin to cytokinin. However, studies of hormonal levels in
elongating buds in Lupinus [3] are in agreement with the hypothesis that it is the auxin:cytokinin
ratio that determine elongation. A cytokinin: auxin ratio above 1 was observed
in elongating bud tissue independent of the actual hormone levels, whereas a
low ratio corresponded to a low elongation rate [3]. These
authors concluded that although the data from each hormone may be suggestive
they do not relate completely to observed growth pattern [3].
The ratio of auxin and cytokinin may thus control stem
elongation independent of the actual level of each hormone. The threshold value
seems to be around 1. Cell elongation occurs when the levels of auxin are above
the level of cytokinin making the auxin: cytokinin ratio exceeding 1 whereas a
lower ratio probably favors meristematic activity.
Figure
1: Year-round
hormone levels in fluctuations Abies nordmanniana. The level of IAA (A and B) and
cytokinins (C and D) in the apical shoot initial (A and C) and the middle of
the elongating top leader (B and D. Cytokinin values are the sum of the major
cytokinins identified (t-zeatin, t-zeatin riboside, t-zeatin riboside phosphate
and t-dihydrozeatin). Original data are presented in [11].
Figure 2: IAA: Cytokinin
ratioin Abies nordmanniana. IAA:Cytokinin ratio in the apical
shoot initial and the middle of the elongating top leader. Ratios
based on data presented in (Figure 1).