Focus Issue on the Plant Physiology of Global Change: A Nitrogen-Regulated Glutamine Amidotransferase (GAT1_2.1) Represses Shoot Branching in Arabidopsis

Shoot branching in plants is regulated by many environmental cues and by specific hormones such as strigolactone (SL). We show that the GAT1_2.1 gene (At1g15040) is repressed over 50-fold by nitrogen stress, and is also involved in branching control. At1g15040 is predicted to encode a class I glutamine amidotransferase (GAT1), a superfamily for which Arabidopsis (Arabidopsis thaliana) has 30 potential members. Most members can be categorized into known biosynthetic pathways, for the amidation of known acceptor molecules (e.g. CTP synthesis). Some members, like GAT1_2.1, are of unknown function, likely involved in amidation of unknown acceptors. A gat1_2.1 mutant exhibits a significant increase in shoot branching, similar to mutants in SL biosynthesis. The results suggest that GAT1_2.1 is not involved in SL biosynthesis since exogenously applied GR24 (a synthetic SL) does not correct the mutant phenotype. The subfamily of GATs (GATase1_2), with At1g15040 as the founding member, appears to be present in all plants (including mosses), but not other organisms. This suggests a plant-specific function such as branching control. We discuss the possibility that the GAT1_2.1 enzyme may activate SLs (e.g. GR24) by amidation, or more likely could embody a new pathway for repression of branching.Shoot branching plays an important role in establishing plant body plans during development and growth, also conferring the flexibility for plants to respond to environmental stresses. The control of bud growth/branching has been studied for many decades with much interest stemming from its value in agriculture. Indeed, many of our domesticated crops have been bred for modified branching to optimize yields. In early studies, auxin synthesized in the shoot apex was proposed to act indirectly to inhibit bud outgrowth, while cytokinin (CK) synthesized in the roots promoted bud outgrowth (Domagalska and Leyser, 2011). Studies on auxin inhibition suggested there should be another signal mediating bud growth control (Hayward et al., 2009; Stirnberg et al., 2010; Domagalska and Leyser, 2011). In the past decade, studies in Arabidopsis (Arabidopsis thaliana) and other plants have addressed this signal. Identification and characterization of mutants with increased branching in garden pea (Pisum sativum), Arabidopsis, rice (Oryza sativa), and Petunia hybrida demonstrated the existence of a long-distance signaling pathway that regulates shoot branching (Beveridge et al., 1996, 1997; Napoli, 1996; Stirnberg et al., 2002, 2007; Sorefan et al., 2003; Booker et al., 2004; Arite et al., 2007; Gomez-Roldan et al., 2008; Umehara et al., 2008, 2010; Lin et al., 2009; Liu et al., 2009, 2011; Zhang et al., 2010). Later, studies on pea (Gomez-Roldan et al., 2008) and rice (Umehara et al., 2008) demonstrated unequivocally that this hormone (or its precursor) is strigolactone (SL). Currently, it is proposed that SL acts downstream of auxin to regulate bud outgrowth (Brewer et al., 2009). It is also likely that SL and auxin have the capacity to modulate each other’s levels and distribution in a dynamic feedback loop required for the branching control (Ferguson and Beveridge, 2009; Hayward et al., 2009; Stirnberg et al., 2010). The interaction between SL and CK during bud outgrowth is less understood, although recent studies in pea indicate that SL and CK act antagonistically on bud growth (Dun et al., 2012).Branching is also modulated in response to environmental conditions, including nutrient supply. Generally, nutrient deficiency in soil causes a reduction in shoot to root ratio, resulting in decreased shoot branching (Lafever, 1981). Under nitrogen or phosphate limitation, elevated levels of SL repress shoot branching in rice, tomato (Solanum lycopersicum), and Arabidopsis (Yoneyama et al., 2007; López-Ráez et al., 2008; Umehara et al., 2008, 2010; Kohlen et al., 2011), and possibly increase lateral root formation (Ruyter-Spira et al., 2011). This makes sense physiologically, diverting resources to roots from shoots to scavenge more nutrients. The basis for modulation of SL levels or nutrient-dependent branching control is not understood.Here, we report a novel gene, GAT1_2.1 (At1g15040), predicted to encode a class I Gln amidotransferase (GAT1) in Arabidopsis, is highly repressed by long-term nitrogen stress (down 57-fold), and that mutation of this gene leads to an enhanced branching phenotype. Thus, this gene may present a link between the nitrogen stress response and branching control.