Abstract
The phytohormone auxin plays significant roles in regulating plant growth and development. In Arabidopsis, a subset of ATP-BINDING CASSETTE subfamily B (ABCB) transporters participate in polar movement of auxin by exclusion from and prevention of reuptake at the plasma membrane. A previous analysis identified ABCB21 as a conditional auxin uptake/efflux transporter that regulates cellular auxin levels, but clear physiological roles for ABCB21 in planta remain unknown. Here we show that ABCB21 maintains the acropetal auxin transport stream by regulating auxin levels in the pericycle. Loss of ABCB21 reduces rootward auxin transport and delays lateral root emergence. In seedling shoots, ABCB21 regulates mobilization of auxin from the photosynthetic cotyledons that is important for phototropic bending. In rosette leaves ABCB21 contributes to lateral auxin distribution. These results support a primary role for ABCB21 in regulating auxin distribution supplementary to the primary ABCB auxin transporters ABCB1 and 19.
Highlights
Optimization of light capture from sunlight and accumulation of water and nutrients from the soil during seedling establishment are major determinants of plant fitness
The results presented are consistent with a role for ABCB21 in regulating cellular auxin levels in a manner similar to ABCB4 in the root epidermis
ABCB21 expression in the pericycle up to 7 d coincides with the rootward pulse of shoot derived auxin that triggers lateral root outgrowth (Bhalerao et al, 2002). abcb21 mutants exhibit reduced rootward auxin transport and delays in lateral root emergence, which is consistent with the activity of ABCB19 (Wu et al, 2007)
Summary
Optimization of light capture from sunlight and accumulation of water and nutrients from the soil during seedling establishment are major determinants of plant fitness. In Arabidopsis seedlings, light promotes synthesis of the phytohormone auxin (indole-3-acetic acid, IAA) in the cotyledons and young developing leaves (Bhalerao et al, 2002). During early post-photomorphogenic seedling growth auxin regulates expansion of the photosynthetically active cotyledons (Ni et al, 2001; Lewis et al, 2009) and tropic bending toward the light source (Christie et al, 2011). During later stages of seedling growth auxin regulates lateral root development in a sequence of events that can be grouped into two distinct major phases: initiation and elongation/emergence (Péret et al, 2009). Excision and labeling experiments in Arabidopsis demonstrate that auxin involved in both lateral root initiation and emergence is initially shoot derived and transported in a rootward stream between 5 and 7 d after
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