Cytokinin responses counterpoint auxin signaling during rhizobial infection

Abstract

The transcriptomics approach to study gene expression in root hairs from M. truncatula has shed light on the developmental events during rhizobial infection and the underlying hormone responses. This approach revealed the induction of several cyclins and an aurora kinase which suggests that the cell-division machinery plays a role in rhizobial infection. Changes in the cell cycle in plants are governed by hormones, in particular auxin and cytokinin. Through gene expression and genetic analyses, we have shown auxin plays a role during rhizobial infection. Here we provide further analysis of the data showing the induction of a set of cytokinin signaling components. These include genes encoding 2 cytokinin-activating enzymes, the cytokinin receptor CRE1, and 5 type-A cytokinin response regulators. We discuss the possible interactions between auxin and cytokinin signaling during the infection process. We also consider a potential role for cytokinin signaling in rhizobial attachment.

Keywords: Auxin; auxin response factor; auxin responses; cytokinin; cytokinin response regulator; cytokinin signaling; ethylene; hormonal interactions; infection thread; plant microbe interactions; rhizobia; rhizobial infection; symbiosis.

Figure 1.

Auxin response and nodulation phenotype of M. truncatula arf16a-4 mutant. (A) and (B) The inhibition of primary root growth by 10µM indole acetic acid in the wild type (R108) and arf16a-4 (NF4811). The picture (A) and histograms (B) show plants 14 d after germination. (C) Quantification of different stages of infection and development of nodule primordia in the wild type and arf16a-4 mutants 7 dpi with S. meliloti. Infection events and nodule primordia were scored 7 dpi with S. meliloti 1021 carrying pXLGD4 (LacZ) after LacZ staining. IT, fully elongated infection thread in root hair; eIT, elongating infection thread in root hair; MC, microcolony; rIT, ramified infection thread in cortex; NP, nodule primordium. Bar = SE. Significant (Student's t-test) differences between the wild type and mutant are marked with asterisks (**P < 0.01).