The role of auxin transporters in monocots development

Abstract

Auxin is a key regulator of plant growth and development, orchestrating cell division, elongation and differentiation, embryonic development, root and stem tropisms, apical dominance, and transition to flowering. Auxin levels are higher in undifferentiated cell populations and decrease following organ initiation and tissue differentiation. This differential auxin distribution is achieved by polar auxin transport (PAT) mediated by auxin transport proteins. There are four major families of auxin transporters in plants: PIN-FORMED (PIN), ATP-binding cassette family B (ABCB), AUXIN1/LIKE-AUX1s, and PIN-LIKES. These families include proteins located at the plasma membrane or at the endoplasmic reticulum (ER), which participate in auxin influx, efflux or both, from the apoplast into the cell or from the cytosol into the ER compartment. Auxin transporters have been largely studied in the dicotyledon model species Arabidopsis, but there is increasing evidence of their role in auxin regulated development in monocotyledon species. In monocots, families of auxin transporters are enlarged and often include duplicated genes and proteins with high sequence similarity. Some of these proteins underwent sub- and neo-functionalization with substantial modification to their structure and expression in organs such as adventitious roots, panicles, tassels, and ears. Most of the present information on monocot auxin transporters function derives from studies conducted in rice, maize, sorghum, and Brachypodium, using pharmacological applications (PAT inhibitors) or down-/up-regulation (over-expression and RNA interference) of candidate genes. Gene expression studies and comparison of predicted protein structures have also increased our knowledge of the role of PAT in monocots. However, knockout mutants and functional characterization of single genes are still scarce and the future availability of such resources will prove crucial to elucidate the role of auxin transporters in monocots development.

Keywords: ABCB; AUX/LAX; IAA; PAT; PILS; PIN.

FIGURE 1

Auxin transport proteins regulate intracellular and cell to cell auxin fluxes. Auxin (IAA) crosses the plasma membrane through passive diffusion, as protonated form, or through PM transporters, as deprotonated form. PINs are efflux carriers located at the PM and ER and can be re-inserted in the lipid bilayer by recycling via the endocytic pathway. AUX/LAXs and PILs are influx carriers located at PM and ER, respectively. ABCBs are located at the PM and use energy from ATP to traslocate IAA. The coordinated localization of the different transporters determines the overall directionality of the auxin flux and contributes to the regulation of intracellular auxin levels.

FIGURE 2

The br2 maize mutant shows dramatically impacted stalk architecture. The br2 adult plant shows altered stalk height due to reduction in internode length, which is caused by the disruption of IAA transport mediated by ZmABCB1. The same phenotype is present in the sorghum dw3 mutant, which carries a tandem duplication in the SbABCB1/Dw3 gene.

FIGURE 3

Neighbor-joining sequence similarity analysis of the PIN and PILS proteins from Arabidopsis, rice, maize, sorghum, and Brachypodium. The unrooted tree shows the degree of sequence similarity among PIN and PILS proteins from Arabidopsis, rice, maize, sorghum, and Brachypodium. ZmPINX and ZmPINY present higher overall similarity to PILS rather than PIN proteins. Bootstrap values higher than 60 are indicated at each node.