Hormonal crosstalk for root development: a combined experimental and modeling perspective

Abstract

Plants are sessile organisms and therefore they must adapt their growth and architecture to a changing environment. Understanding how hormones and genes interact to coordinate plant growth in a changing environment is a major challenge in developmental biology. Although a localized auxin concentration maximum in the root tip is important for root development, auxin concentration cannot change independently of multiple interacting hormones and genes. In this review, we discuss the experimental evidence showing that the POLARIS peptide of Arabidopsis plays an important role in hormonal crosstalk and root growth, and review the crosstalk between auxin and other hormones for root growth with and without osmotic stress. Moreover, we discuss that experimental evidence showing that, in root development, hormones and the associated regulatory and target genes form a network, in which relevant genes regulate hormone activities and hormones regulate gene expression. We further discuss how it is increasingly evident that mathematical modeling is a valuable tool for studying hormonal crosstalk. Therefore, a combined experimental and modeling study on hormonal crosstalk is important for elucidating the complexity of root development.

Keywords: POLARIS peptide; hormonal crosstalk; kinetic modeling; osmotic stress; root development.

Figure 1

DR5::GFP expression in wild type and pls mutant, showing difference in auxin gradients (upper panel), and PIN1 immunolocalization in wildtype and pls mutant, showing differences in PIN protein levels (lower panel). This figure is adapted with permission from the Figure 4 of Liu et al. (2010a) and the Figure 1 of Liu et al. (2013).

Figure 2

A hormonal crosstalk network of auxin, ethylene and cytokinin for root development, showing that change in one signaling component leads to change in other signaling components in the network (modified with permission from Liu et al., 2013). The reaction rates are: v1, total auxin influx from all neighboring; v2, auxin biosynthesis rate in the cell; v3, total auxin efflux from the cell; v4, rate for conversion of the inactive form of the auxin receptor, Ra, to its active form, Ra^*; v5, rate for conversion of the active form of the auxin receptor, Ra^*, to its inactive form, Ra; v6, transcription rate of the POLARIS (PLS)gene; v7, decay rate of PLS mRNA; v8, translation rate of the PLS protein; v9, decay rate of PLS protein; v10, rate for conversion of the inactive form of the ethylene receptor, Re, to its active form by PLS protein (PLSp), Re^*; v11, rate for conversion of the active form of ethylene receptor, Re^*, to its inactive form, Re; v12, ethylene biosynthesis rate; v13, rate for removal of ethylene; v14, rate for conversion of the inactive form of the CONSTITUTIVE TRIPLE RESPONSE 1 (CTR1) protein, CTR1, to its active form, CTR1^*; v15, rate for conversion of the active form of CTR1 protein, CTR1^*, to its inactive form, CTR1; v16, rate for activation of the ethylene signaling response; v17, rate for removal of the unknown ethylene signaling component, X; v18, rate for cytokinin biosynthesis; v19, rate for removal of cytokinin; v20, transcription rate of the PIN gene; v21, rare for the decay of PIN mRNA; v22, translation rate of PIN protein; v23, rate for decay of PIN protein in cytosol; v24, rate for transport of PIN protein from cytosol to plasma membrane; v25, rate for internalization of PIN protein. When exogenous hormones are applied: v26, rate for uptake of IAA when exogenous IAA is applied; v27, rate for uptake of ACC when exogenous ACC is applied; v28, rate for uptake of cytokinin when exogenous cytokinin is applied.

Figure 3

A schematic description of a methodology for a combined experimental and modeling study on hormonal crosstalk in root development. Upper panel: Hormonal crosstalk networks are constructed using existing experimental data. The networks are used to check the consistency of existing experimental data and they are also used to formulate mathematical models such as models described using ordinary differential equations. The mathematical models are validated using the existing experimental data. Then the mathematical models are used to predict novel experiments. The data acquired using the novel experiments are used to expand the hormonal crosstalk networks and to validate the mathematical models again. The expanded hormonal crosstalk networks are used to check the consistency of the novel experiments, and they are also used to develop novel mathematical models. Lower panel: the same as the upper panel, but root architecture is included.