Abstract
The Arabidopsis root is a dynamic system where the interaction between different plant hormones controls root meristem activity and, thus, organ growth. In the root, a characteristic graded distribution of the hormone auxin provides positional information, coordinating the proliferating and differentiating cell status. The hormone cytokinin shapes this gradient by positioning an auxin minimum in the last meristematic cells. This auxin minimum triggers a cell developmental switch necessary to start the differentiation program, thus, regulating the root meristem size. To position the auxin minimum, cytokinin promotes the expression of the IAA-amido synthase group II gene GH3.17, which conjugates auxin with amino acids, in the most external layer of the root, the lateral root cap tissue. Since additional GH3 genes are expressed in the root, we questioned whether cytokinin to position the auxin minimum also operates via different GH3 genes. Here, we show that cytokinin regulates meristem size by activating the expression of GH3.5 and GH3.6 genes, in addition to GH3.17. Thus, cytokinin activity provides a robust control of auxin activity in the entire organ necessary to regulate root growth.
Highlights
Organ growth in plants is supported by the meristems, regions providing a reservoir of undifferentiated cells whose activity depends on the stem cell niche [1]
In order to unveil cytokinin-dependent mechanisms acting in the lateral root cap (LRC) to control meristem size, we took advantage of the already published microarray data reporting genes differentially regulated in the LRC in response to ARABIDOPSIS RESPONSE REGULATOR 1 (ARR1) induction
It was demonstrated that GH3.5, GH3.6, and GH3.9 IAA-amido synthases participate in maintaining auxin homeostasis by conjugating amino acids to the hormone [18,19,21,22,23,24] and thereby affect the levels of free auxin molecules that are biologically active and suited for binding to their receptors
Summary
Organ growth in plants is supported by the meristems, regions providing a reservoir of undifferentiated cells whose activity depends on the stem cell niche [1]. The boundary between proliferating and differentiating cells is called transition zone (TZ) The position of this cell boundary depends on the coordinated activity of the stem cell niche, the division zone, and the differentiation zone. The activities of these zones are controlled by a dynamic equilibrium between cell division and cell differentiation. The regulation of this equilibrium results in a shoot-ward or a root-ward shift of the TZ position along the root longitudinal axis [2,3,4,6]. It has been demonstrated that cytokinin controls TZ localization by positioning an auxin minimum in Plants 2019, 8, 94; doi:10.3390/plants8040094 www.mdpi.com/journal/plants
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