Functional characterization of LHT7's role in plant immunity

Plants are constantly exposed to microbes. To survive, they evolve mechanisms to receive microbes and prevent infection. Pattern Recognition Receptors (PRRs) localized at the plasma membrane detect Pathogen-Associated Molecular Patterns (PAMPs) produced by microbes. When PRRs detect PAMPs, they induce defense responses that suppress microbial growth, and hence, the infection. This type of defense is called PAMP-triggered immunity (PTI) and typically leads to changes in the composition of metabolites that inhibit microbial growth by suppressing the expression of microbial virulence factors (Zhang et al., 2022; Zhang et al., 2023).

Pathogenic bacteria usually colonize the leaf apoplast, the intercellular space surrounding photosynthetic cells within the leaf mesophyll. Once in the leaf apoplast, bacteria can take up and metabolize plant-made carbon and nitrogen-containing sugars and amino acids to support their growth (Farvardin et al., 2020). Recent studies show that when plants perceive PAMPs, the concentration of amino acids increases in the leaf apoplast, thus impacting microbial growth (Zhang et al., 2023). However, the mechanisms that regulate the amino acid concentration in apoplast during PTI are still poorly understood (Zhang et al., 2022).

Nitrogen is an essential nutrient for plants and is usually transported across tissues in its reduced form as amino acids. The transport of amino acids between different cells and tissues requires amino acids transporter protein localized at cell membranes (Tegeder et al., 2012; Somawala et al., 2018). The Lysine–Histidine-like Transporters (LHT) family is a class of amino acid transporter that belongs to the ATF (Amino acid Transporter Family) (Ortiz-Lopez et al., 2000). The ATF superfamily contains six subfamilies: AAP (Amino Acid Permease), ProT (Proline Transporter), LHT, ANT1-like (Aromatic and Neutral amino acid Transporter), AUX/LAX (Auxin influx carriers), and GAT (Gama-Aminobutyric acid Transporter) (Tegeder et al., 2012). Some members within these families are well studied, and most of them have been confirmed to transport amino acids against a concentration gradient using a proton (H+) couple transport mechanism. To identify transporters that may contribute to modulating the concentration of amino acids in the leaf apoplast of Arabidopsis thaliana, I searched for H+ coupled amino acid transporters upregulated in response to PAMPs treatment (http://bar.utoronto.ca/). I found that LHT1 and LHT7 are highly induced in response to the PAMP flagellin-22 amino acids peptide (flg22), a canonical PAMP derived from gram-negative bacterial flagellin protein. Preliminary data produced in the Danna lab showed that LHT7 loss-of-function mutant plants (lht7) are more susceptible to Pseudomonas syringae infections than wild-type plants. As LHT7 is induced by flg22 and the lht7 mutants are susceptible to infections, an in-depth characterization will advance our understanding of the role of amino acid transport in plant immunity. Therefore, the LHT7’s role in plant immunity during infection is the overarching question that drives my research. I hypothesize that the LHT7 transporter contributes to the flg22-elicited changes in the concentration of amino acids that are needed to suppress P. syringae infections.