Phosphate-Deprived Roots Are Hypersensitive to Auxin

Low concentrations of phosphate (Pi) in soil trigger alterations to the architecture of the root system that increase a plant's capacity to absorb Pi. Many plants stimulate the uptake of this essential nutrient by promoting lateral root (LR) formation, increasing root hair length and density (see figure), and suppressing primary root growth. Auxin is a key player in LR development: exogenous auxins enhance LR formation (Blakely et al., 1988), and auxin transport inhibitors reduce it (Reed et al., 1998). At low concentrations of auxin, AUX/IAA repressors block the activity of AUXIN RESPONSE FACTOR (ARF) transcription factors. When auxin concentrations exceed a certain thresh- old, TIR1/AFB1-3 receptors trigger the de- struction of AUX/IAA proteins. ARFs are then free to regulate the expression of auxin- responsive genes (reviewed in Lau et al., 2008), such as those involved in LR formation. Mechanisms that modulate auxin signaling are thought to mediate the formation of LRs in response to a Pi deficiency. Perez-Torres et al. (pages 3258-3272) investigated the activity of various compo- nents of the auxin signaling pathway during Pi deprivation. When they reduced the amount of free auxin by growing Arabidopsis seedlings on medium supplemented with an auxin polar transport inhibitor, they found that the roots of Pi-deprived seedlings were able to form LR primordia, whereas those of seedlings with a sufficient supply of Pi were not. Supple- menting these auxin-starved seedlings with low concentrations of auxin produced both LR primordia and LRs in Pi-deprived seedlings, but only LR primordia in seedlings grown on sufficient concentrations of Pi. These results suggest that Pi deprivation heightens the root system's sensitivity to auxin. Since the TIR1 auxin receptor is a central player in auxin perception, the group tested whether TIR1 is responsible for the increased auxin sensitivity of Pi-deprived seedlings. Mutant tir1-1 seedlings, which are unable to mediate AUX/IAA degradation, failed to increase LR production in response to Pi deprivation. A GUS fluorometric assay (see figure) and quantitative RT-PCR analysis revealed that TIR1 is specifically induced in response to Pi deprivation. By examining a series of mutants, other members of the TIR1/AFB1-3 family were shown to have partially redundant roles in LR formation in response to Pi deprivation, and ARF19 was found to be essential for increased LR for- mation in Pi-deprived seedlings. An experi- ment using transgenic Arabidopsis seedlings expressing a heat-inducible AUX/IAA protein demonstrated that Pi deprivation accelerates the degradation of the AUX/IAA protein. Thus, auxin sensitivity appears to be en- hanced in Pi-deprived seedlings due to the increased expression ofTIR1. This is proposed to cause the degradation of AUX/IAA proteins, which liberates ARF transcription factors and modulates the expression of genes involved in LR formation.