Plants count on a wide variety of metabolic, physiological, and developmental responses to adapt their growth to variations in mineral nutrient availability. To react to such variations plants have evolved complex sensing and signaling mechanisms that allow them to monitor the external and internal concentration of each of these nutrients, both in absolute terms and also relatively to the status of other nutrients. Recent evidence has shown that hormones participate in the control of these regulatory networks. Conversely, mineral nutrient conditions influence hormone biosynthesis, further supporting close interrelation between hormonal stimuli and nutritional homeostasis. In this review, we summarize these evidences and analyze possible transcriptional correlations between hormonal and nutritional responses, as a means to further characterize the role of hormones in the response of plants to limiting nutrients in soil.

ORCID IDs: 0000-0003-2780-4888 (C.R.); 0000-0002-8800-2400 (V.R.)Prior to the release of their cargoes into the vacuolar lumen, sorting endosomes mature into multivesicular bodies (MVBs) through the action of ENDOSOMAL COMPLEX REQUIRED FOR TRANSPORT (ESCRT) protein complexes. MVB-mediated sorting of high-affinity phosphate transporters (PHT1) to the vacuole limits their plasma membrane levels under phosphatesufficient conditions, a process that allows plants to maintain phosphate homeostasis. Here, we describe ALIX, a cytosolic protein that associates with MVB by interacting with ESCRT-III subunit SNF7 and mediates PHT1;1 trafficking to the vacuole in Arabidopsis thaliana. We show that the partial loss-of-function mutant alix-1 displays reduced vacuolar degradation of PHT1;1. ALIX derivatives containing the alix-1 mutation showed reduced interaction with SNF7, providing a simple molecular explanation for impaired cargo trafficking in alix-1 mutants. In fact, the alix-1 mutation also hampered vacuolar sorting of the brassinosteroid receptor BRI1. We also show that alix-1 displays altered vacuole morphogenesis, implying a new role for ALIX proteins in vacuolar biogenesis, likely acting as part of ESCRT-III complexes. In line with a presumed broad target spectrum, the alix-1 mutation is pleiotropic, leading to reduced plant growth and late flowering, with stronger alix mutations being lethal, indicating that ALIX participates in diverse processes in plants essential for their life. INTRODUCTIONTrafficking of cargo proteins coming from the plasma membrane (PM) or the Golgi apparatus (GA) to the vacuole occurs through multivesicular bodies (MVBs) (Winter and Hauser, 2006). These organelles, also termed late endosomes or prevacuolar compartments (PVCs), contain internal vesicles that will be delivered, together with their cargoes, into the lumen of vacuoles/lysosomes upon MVB fusion with the tonoplast (Winter and Hauser, 2006). This process plays a central role in controlling the reutilization, storage, or degradation of membrane components and thus regulates fundamental biological processes including membrane turnover, defense against pathogens, development, hormone transport, nutrient uptake, and cell signaling. In the case of membrane-associated regulatory proteins, the MVB route allows modulation of their function by regulating their abundance at the PM and in other vesicular compartments (e.g., endosomes).The sorting of most integral membrane proteins into intraluminal vesicles (ILVs) is dependent on the attachment of ubiquitin to their cytosolic domains, although ubiquitin-independent sorting mechanisms also exist (McNatt et al., 2007). Selective packaging of protein cargoes into ILVs of MVB is mediated by ESCRT (ENDOSOMAL SORTING COMPLEXES REQUIRED FOR TRANSPORT) protein complexes (Conibear, 2002;Winter and Hauser, 2006;Nickerson et al., 2007;Henne et al., 2011). The latter consist of several cytosolic proteins of the VPS-E (class E Vacuolar Protein Sorting) class that are organized into five complexes: ESCRT-0...