Abstract
The TIR1/AFB auxin co-receptors mediate diverse responses to the plant hormone auxin. The Arabidopsis genome encodes six TIR1/AFB proteins representing three of the four clades that were established prior to angiosperm radiation. To determine the role of these proteins in plant development we performed an extensive genetic analysis involving the generation and characterization of all possible multiply-mutant lines. We find that loss of all six TIR1/AFB proteins results in early embryo defects and eventually seed abortion, and yet a single wild-type allele of TIR1 or AFB2 is sufficient to support growth throughout development. Our analysis reveals extensive functional overlap between even the most distantly related TIR1/AFB genes except for AFB1. Surprisingly, AFB1 has a specialized function in rapid auxin-dependent inhibition of root growth and early phase of root gravitropism. This activity may be related to a difference in subcellular localization compared to the other members of the family.
Highlights
The phytohormone auxin regulates diverse processes throughout the entire plant life cycle
The gene duplication event establishing the distinct TRANSPORT INHIBITOR RESPONSE1 (TIR1) and AFB1 clades is coincident with the At-b whole genome duplication (WGD) event at the base of Brassicales, while both the AFB2/AFB3 and the AFB4/AFB5 duplication events coincide with the more recent Ata WGD prior to divergence of the Brassicaceae family (Figure 1—figure supplement 1A; Schranz and Mitchell-Olds, 2006)
The TIR1/AUXIN-SIGNALING F-BOX (AFB) protein family has expanded through a series of gene duplication events that began before fern–seed-plant divergence
Summary
The phytohormone auxin regulates diverse processes throughout the entire plant life cycle. Auxin acts as a signal to promote cell differentiation during morphogenetic events such as embryogenesis, root development, and shoot organ formation. Formation of the co-receptor complex leads to degradation of the Aux/IAA protein and activation of ARF-dependent transcription (Reviewed in Lavy and Estelle, 2016). In addition to this established pathway, recent studies demonstrate that the TIR1/AFB proteins are required for very rapid auxin responses in the root and in developing root hairs that are independent of transcription (Dindas et al, 2018; Fendrych et al, 2018). The details of TIR1/AFB function in these rapid responses are currently unknown, but in the root, the response is thought to be important for early events in gravitropism
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