The auxin transporter PIN1 and the cytokinin transporter AZG1 interact to regulate the root stress response

Root system development is crucial for optimal growth and yield in plants, especially in sub-optimal soil conditions. The architecture of a root system is environmentally responsive, enabling the plant to exploit regions of high nutrient density whilst simultaneously minimizing abiotic stress. Despite the vital contribution of root systems to the growth of both model and crop species, we know little of the mechanisms which regulate their architecture. One factor which is relatively well understood is the transport of auxin, a plant growth regulator which defines the frequency of lateral root (LR) initiation and the rate of LR outgrowth. Here we describe a search for proteins which regulate RSA by interacting directly with a key auxin transporter, PIN1. The native separation of PIN1 identified several co-purifying proteins. Among them, AZG1 was subsequently confirmed as a PIN1 interactor. AZG1-GFP fusions co-localized with PIN1 in procambium cells of the root meristem. Roots of azg1 plants contained less PIN1 and blocking proteolysis restored PIN1 levels, observations which are consistent with PIN1 being stabilized by AZG1 in the plasma membrane. Furthermore, we show that AZG1 is a cytokinin import protein; accordingly, azg1 plants are insensitive to exogenously applied cytokinin. In wild-type plants, the frequency of LRs falls with increasing salt concentration, a response which is not observed in azg1 x azg2 plants, although their drought response is unimpaired. This report therefore identifies a potential point for auxin:cytokinin crosstalk in the environmentally-responsive determination of root system architecture.