A Gln alteration influences leaf morphogenesis by mediating gibberellin levels in tobacco

Nitrogen is a critical nutrient for plant growth and development. While numerous studies have investigated the mechanisms by which nitrate and/or ammonium regulate plant growth, little is known about whether and how amino acids regulate plant leaf development. This study demonstrates that tobacco plants with altered expression levels of an amino acid transporter (LYSINE HISTIDINE TRANSPORTER1, NtLHT1, Ntab0818090) exhibit significant differences in leaf morphology. Knock-out mutants exhibit elongated and narrower leaves compared to wild-type plants, whereas overexpression (OE) lines display orbicular leaves. Additionally, mutant plants exhibit decreased nitrogen use efficiency (NUE) under half MS medium and delayed development under nitrogen-depleted conditions. Moreover, overexpression lines demonstrate better performance. Although the mutant does not show significant lower level of nitrate or total amino acid content in the developing leaves, its amino acid profile, particularly glutamine (Gln), is significantly altered. Supplementation with Gln in the growth medium, rather than glutamate, can restore the morphological differences observed in mutant leaves, suggesting a pivotal role of Gln in regulating leaf shape. To further elucidate the mechanisms underlying leaf shape regulation, we analyzed endogenous hormone levels and applied exogenous hormones to these lines. Our findings suggest that amino acids transported from source organs, particularly glutamine (Gln), play a key role in controlling leaf development and morphology through the modulation of multiple phytohormones, such as gibberellic acid (GA).