Auxin transporters--why so many?

Abstract

Interacting and coordinated auxin transporter actions in plants underlie a flexible network that mobilizes auxin in response to many developmental and environmental changes encountered by these sessile organisms. The independent but synergistic activity of individual transporters can be differentially regulated at various levels. This invests auxin transport mechanisms with robust functional redundancy and added auxin flow capacity when needed. An evolutionary perspective clarifies the roles of the different transporter groups in plant development. Mathematical and functional analysis of elements of auxin transport makes it possible to rationalize the relative contributions of members of the respective transporter classes to the localized auxin transport streams that then underlie both preprogrammed developmental changes and reactions to environmental stimuli.

Figure 1.

Cellular auxin transport. The scheme shows the organization of proteins involved in auxin transport. PIN efflux carrier proteins depicted in red represent “long” PINs (PIN1, 2, 3, 4, and 7), whereas PINs marked in pink represent “short” PINs (PIN5, 6, and 8). ER marks endoplasmic reticulum, pale gray structures represent ER and endosomes, curved bold full arrows show constitutive protein cycling, and dashed arrows symbolize the process of transcytosis. Possible collaboration between ABCBs and PINs is suggested by placing the symbols close to each other.