ABI1 interacts with the C terminus of AHA2 and dephosphorylates Thr ⁹⁴⁷ to negatively regulate root growth and hydrotropism.

White lupin (Lupinus albus) is a legume crop that develops cluster roots and has high phosphorus (P)-use efficiency (PUE) in low-P soils. Here, we assemble the genome of white lupin and find that it has evolved from a whole-genome triplication (WGT) event. We then decipher its diploid ancestral genome and reconstruct the three sub-genomes. Based on the results, we further reveal the sub-genome dominance and the genic expression of the different sub-genomes varying in relation to their transposable element (TE) density. The PUE genes in white lupin have been expanded through WGT as well as tandem and dispersed duplications. Furthermore, we characterize four main pathways for high PUE, which include carbon fixation, cluster root formation, soil-P remobilization, and cellular-P reuse. Among these, auxin modulation may be important for cluster root formation through involvement of potential genes LaABCG36s and LaABCG37s. These findings provide insights into the genome evolution and low-P adaptation of white lupin.

Plant roots respond to soil moisture gradients and remodel their growth orientation toward water through hydrotropism, a process vital for acclimation to a changing soil environment. Mechanisms underlying the root hydrotropic response, however, remain poorly understood. Here, we examined hydrotropism in 31 Arabidopsis (Arabidopsis thaliana) ecotypes collected from different parts of the world and grown along moisture gradients in a specially designed soil-simulation system. Comparative transcriptome profiling and physiological analyses were carried out on three selected ecotypes, Wassilewskija (Ws), Columbia (Col-0) (strongly hydrotropic), Col-0 (moderately hydrotropic), and C24 (weakly hydrotropic), and in mutant lines with altered root hydrotropic responses. We show that H⁺ efflux, Ca²⁺ influx, redox homeostasis, epigenetic regulation, and phytohormone signaling may contribute to root hydrotropism. Among phytohormones, the role of brassinosteroids (BRs) was examined further. In the presence of an inhibitor of BR biosynthesis, the strong hydrotropic response observed in Ws was reduced. The root H⁺ efflux and primary root elongation also were inhibited when compared with C24, an ecotype that showed a weak hydrotropic response. The BR-insensitive mutant bri1-5 displayed higher rates of root growth inhibition and root curvature on moisture gradients in vertical or oblique orientation when compared with wild-type Ws. We also demonstrate that BRI1 (a BR receptor) interacts with AHA2 (a plasma membrane H⁺-ATPase) and that their expression patterns are highly coordinated. This synergistic action may contribute to the strong hydrotropism observed in Ws. Our results suggest that BR-associated H⁺ efflux is critical in the hydrotropic response of Arabidopsis roots.

Significance: Among all regular nucleotides, GTP is commonly regarded as the sole signaling ligand associated with G proteins. However, the ability of the YchF subfamily (an unconventional G protein) to bind and hydrolyze both GTP and ATP poses a major question on the role of ATP binding. Through X-ray crystallography, we showed the different specific conformational changes caused by the binding of OsYchF1 to ATP versus GTP. A mutation that precludes the binding/hydrolysis of ATP also prevents OsYchF1 from functioning as a negative regulator of plant defense responses, demonstrating the specific role of ATP-binding/hydrolysis in disease resistance. This discovery will have a significant impact on our understanding of the structure–function relationships of the YchF subfamily of G proteins in all kingdoms of life.

Lysophospholipids (LPLs) are important lipid-signaling molecules in plants, of which lysophosphatidylcholine (lysoPC) is one of the most well-characterized LPLs, having important roles in plant stress responses. It is broken down by lysophospholipases, but the molecular mechanism involved in lysoPC degradation is unclear. Recombinant Arabidopsis thaliana ACYL-COA-BINDING PROTEIN2 (AtACBP2) has been reported to bind lysoPC via its acyl-CoA–binding domain and also LYSOPHOSPHOLIPASE 2 (AtLYSOPL2) via its ankyrin repeats in vitro. To investigate the interactions of AtACBP2 with AtLYSOPL2 and lysoPC in more detail, we conducted isothermal titration calorimetry (ITC) with AtACBP2_70–354, an AtACBP2 derivative consisting of amino acids 70–354, containing both the acyl-CoA–binding domain and ankyrin repeats. We observed that the interactions of AtACBP2_70–354 with AtLYSOPL2 and with lysoPC were both endothermic, favored by solvation entropy and opposed by enthalpy, with dissociation constants in the micromolar range. Of note, three AtLYSOPL2 catalytic triad mutant proteins (S147A, D268A, and H298A) bound lysoPC only weakly, with an exothermic burst and dissociation constants in the millimolar range. Furthermore, the binding affinity of lysoPC-premixed AtACBP2_70–354 to AtLYSOPL2 was 10-fold higher than that of AtACBP2_70–354 alone to AtLYSOPL2. We conclude that AtACBP2 may play a role in facilitating a direct interaction between AtLYSOPL2 and lysoPC. Our results suggest that AtACBP2_70–354 probably binds to lysoPC through a hydrophobic interface that enhances a hydrotropic interaction of AtACBP2_70–354 with AtLYSOPL2 and thereby facilitates AtLYSOPL2’s lysophospholipase function.