Differential auxin transport and accumulation in the stem base lead to profuse adventitious root primordia formation in the aerial roots (aer) mutant of tomato (Solanum lycopersicum L.)

The aerial roots (aer) mutant of tomato is characterized by a profuse and precocious formation of adventitious root primordia along the stem. We demonstrated that auxin is involved in the aer phenotype but ruled out higher auxin sensitivity of mutant plants. Interestingly, polar auxin transport was altered in aer, as young seedlings showed a reduced response to an auxin transport inhibitor and higher expression of auxin export carriers SlPIN1 and SlPIN3. An abrupt reduction in transcripts of auxin efflux and influx genes in older aer hypocotyls caused a marked deceleration of auxin transport in more mature tissues. Indeed, in 20 days old aer plants, the transport of labeled IAA was faster in apices than in hypocotyls, displaying an opposite trend in comparison to a wild type. In addition, auxin transport facilitators (SlPIN1, SlPIN4, SlLAX5) were more expressed in aer apices than in hypocotyls, suggesting that auxin moves faster from the upper to the lower part of the stem. Consequently, a significantly higher level of free and conjugated IAA was found at the base of aer stems with respect to their apices. This auxin accumulation is likely the cause of the aer phenotype.