THE AERIAL ROOTS (AER) TOMATO (Solanum lycopersicum L.) MUTANT PHENOTYPE IS DETERMINED BY DIFFERENTIAL AUXIN TRANSPORT IN THE STEM

The aerial roots (aer) mutant of tomato is characterized by a profuse and precocious formation of adventitious root primordia along the stem, resulting in faster adaptation of the mutant to flooding conditions. We demonstrated that auxins are 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 (TIBA) 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, labeled IAA is transported more rapidly in the apices of 3-week-old aer plants, than in the hypocotyls. Moreover, 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 proportion of free and total IAA was found at the base of aer stems with respect to their apex. This auxin accumulation is likely the cause of the aer phenotype.