Common Components of the Strigolactone and Karrikin Signaling Pathways Suppress Root Branching in Arabidopsis

Abstract

Adventitious and lateral root development is regulated by elements involved in both the strigolactone and karrikin perception pathways.

Figure 1.

KAI2 and MAX2 prevent JR development under normal growth conditions. A to D, Data are shown as the percentage of 9-d-old plants showing 0, 1, 2, or 3 JRs, for n > 47 plants per genotype combined from at least three experiments. In C, lines 12H and 10G are independent lines of the complemented kai2-2 mutant (Waters et al., 2015b). Plants were grown on 0.8% (w/v) agar, supplemented with one-half strength Murashige and Skoog salt, as reported in Swarbreck et al. (2019). Symbols indicate statistical significance in the distribution was tested using the Fisher’s exact test (·P < 0.1, *P < 0.05, and ***P < 0.001). n.s., No significance.

Figure 2.

KAI2 and MAX2 prevent lateral root development under normal growth conditions. A to D, Data for each genotype are displayed as a beanplot with the LRD for individual roots of 9-d-old plants shown as dark-brown horizontal lines, while the mean is represented by a thick-black horizontal line. The estimated density of the distribution is illustrated by the shaded color. The dashed line corresponds to the mean for the wild type. For each genotype, n > 50 in at least three separate experiments. Growth conditions as in Figure 1 and as reported in Swarbreck et al. (2019), except for C, where agar was supplied from Melford. Statistical significance was assessed using ANOVA and Tukey Test as post hoc test, and differences between mutants and wild type are indicated by asterisks (*P < 0.05 and ***P < 0.001). n.s., No significance.

Figure 3.

Hypothetical model of the complexes involved in SL and KAR/KL signaling and regulation of root system architecture, particularly suppression of JRs and LR formation. The perception of SL through D14/MAX2/SMXL6, SMXL7, and SMXL8 can lead to modification of the shoot architecture, secondary growth, and leaf morphogenesis. The perception of K (i.e. KAR or KL) regulates germination, photomorphogenesis, leaf morphogenesis, arbuscular mycorrhiza (AM) symbiosis, and root hair development. In addition, the careful analysis of root skewing (Swarbreck et al., 2019; Villaécija-Aguilar et al., 2019) suggest that an alternative complex may form that involves MAX2/KAI2 and SMXL6, SMXL7, and SMXL8, which could also regulate LR development (see Fig. 2; Villaécija-Aguilar et al., 2019) and JR (see Fig. 1). Further experiments are necessary to confirm that these interactions do occur at the protein level.