Root Gravitropism Is Regulated by a Crosstalk between para-Aminobenzoic Acid, Ethylene, and Auxin

Abstract

Plants respond to gravitational force through directional growth along the gravity vector. Although auxin is the central component of the root graviresponse, it works in concert with other plant hormones. Here, we show that the folate precursor paraaminobenzoic acid (PABA) is a key modulator of the auxin-ethylene interplay during root gravitropism in Arabidopsis (Arabidopsis thaliana). In gravistimulated roots, PABA promotes an asymmetric auxin response, which causes the asymmetric growth responsible for root curvature. This activity requires the auxin response transcription factors AUXIN RESPONSE FACTOR7 (ARF7) and ARF19 as well as ethylene biosynthesis and signaling, indicating that PABA activity requires both auxin and ethylene pathways. Similar to ethylene, exogenous PABA reverses the agravitropic root growth of the auxin transport mutant pin-formed2 (pin2) and the auxin biosynthetic double mutant with loss of function of weak ethylene insensitive (wei) genes, wei8wei2, but not the pin2wei8wei2 triple mutant. This finding suggests that PABA regulates the ethylene-dependent reciprocal compensation between auxin transport and biosynthesis. Furthermore, manipulation of endogenous free PABA levels by modulating the expression of the gene encoding its glucosylation enzyme, UDP-GLYCOSYL TRANSFERASE75B1, impacts the root graviresponse, suggesting that endogenous free PABA levels may play a crucial role in modulating the auxin-ethylene cross talk necessary for root gravitropism.

Evolution of folate biosynthesis and metabolism across algae and land plant lineages

“…Interestingly, folate has previously been reported to influence auxin distribution during seedling development, together with sucrose 59 . Furthermore, it was recently shown that root gravitropism is regulated by a crosstalk between PABA (product of the ADCS + ADCL reaction), ethylene and auxin in Arabidopsis 60 .…”

A role for the auxin precursor anthranilic acid in root gravitropism via regulation of PIN‐FORMED protein polarity and relocalisation in Arabidopsis

“…It will be of great interest in future studies to investigate New Phytologist the distribution and dynamics of compound uptake, which could potentially be observed by their labelling, fluorescently or otherwise. Interestingly, the amino acid para-aminobenzoic acid, which has a similar structure to AA and is produced from the same precursor, has also recently been shown to play a role in root gravitropism, distinct from its better known role in folate biosynthesis (Nziengui et al, 2018). However, unlike AA, para-aminobenzoic acid promotes gravitropic root growth in WT plants as well as promoting gravistimulated root bending by enhancing the asymmetric auxin response between the two root sides (Nziengui et al, 2018).…”

AtDAT1 Is a Key Enzyme of D-Amino Acid Stimulated Ethylene Production in Arabidopsis thaliana

“…Consequently, the loss of this enzyme would lead to the inability to produce essential folate which would reduce the plant viability dramatically. Interestingly, pABA is also involved in the regulation of root gravitropism (Nziengui et al, 2018). This implies a modulatory role of AtDAT in differential root growth including gravitropism, which can be tested in future by the analysis of our dat1 mutants and accessions without or reduced AtDAT1 activity.…”

“…For tobacco leaf infiltration transformed Agrobacterium containing pUB10-GFP::DAT1 was mixed with a strain of transformed Agrobacterium for expression of the mCherry plastid marker (CD3-999 pt-rk; Nelson et al, 2007) and P19 A. tumefaciens cells into infiltration media [10 mM MES-KOH (pH 5.7), 10 mM, MgCl 2, 0.2 mM Acetosyringone]. Using a syringe 1 ml of infiltration media with the mix of the three types of cells was infiltrated in the abaxial side of Nicotiana benthamiana leaves.…”