SPX1 is a phosphate-dependent inhibitor of Phosphate Starvation Response 1 in Arabidopsis.

To cope with growth in low-phosphate (Pi) soils, plants have evolved adaptive responses that involve both developmental and metabolic changes. Phosphate Starvation Response 1 (PHR1) and related transcription factors play a central role in the control of Pi starvation responses (PSRs). How Pi levels control PHR1 activity, and thus PSRs, remains to be elucidated. Here, we identify a direct Pi-dependent inhibitor of PHR1 in Arabidopsis, SPX1, a nuclear protein that shares the SPX domain with yeast Pi sensors and with several Pi starvation signaling proteins from plants. Double mutation of SPX1 and of a related gene, SPX2, resulted in molecular and physiological changes indicative of increased PHR1 activity in plants grown in Pi-sufficient conditions or after Pi refeeding of Pi-starved plants but had only a limited effect on PHR1 activity in Pi-starved plants. These data indicate that SPX1 and SPX2 have a cellular Pi-dependent inhibitory effect on PHR1. Coimmunoprecipitation assays showed that the SPX1/PHR1 interaction in planta is highly Pi-dependent. DNA-binding and pull-down assays with bacterially expressed, affinity-purified tagged SPX1 and ΔPHR1 proteins showed that SPX1 is a competitive inhibitor of PHR1 binding to its recognition sequence, and that its efficiency is highly dependent on the presence of Pi or phosphite, a nonmetabolizable Pi analog that can repress PSRs. The relative strength of the SPX1/PHR1 interaction is thus directly influenced by Pi, providing a link between Pi perception and signaling.

An exopolysaccharide-producing novel Agrobacterium pusense strain JAS1 isolated from snake plant enhances plant growth and soil water retention.

A peculiar bacterial growth was very often noticed in leaf-initiated tissue cultures of Sansevieria trifasciata, a succulent belonging to the Asparagaceae family. The isolate left trails of some highly viscous material on the walls of the suspension vessels or developed a thick overlay on semisolid media without adversities in plant growth. FTIR identified this substance to be an extracellular polysaccharide. Various morphological, biochemical tests, and molecular analyses using 16S rRNA, atpD, and recA genes characterized this isolate JAS1 as a novel strain of Agrobacterium pusense. Its mucoidal growth over Murashige and Skoog media yielded enormous exopolysaccharide (7252 mg l-1), while in nutrient agar it only developed fast-growing swarms. As a qualifying plant growth-promoting bacteria, it produces significant indole-3-acetic acid (86.95 mg l-1), gibberellic acid (172.98 mg l-1), ammonia (42.66 µmol ml-1). Besides, it produces siderophores, 1-aminocyclopropane-1-carboxylic acid deaminase, fixes nitrogen, forms biofilms, and productively solubilizes soil inorganic phosphates, and zinc. Under various treatments with JAS1, wheat and chickpea resulted in significantly enhanced shoot and root growth parameters. PGP effects of JAS1 positively enhanced plants' physiological growth parameters reflecting significant increments in overall chlorophyll, carotenoids, proline, phenols, flavonoids, and sugar contents. In addition, the isolated strain maintained both plant and soil health under an intermittent soil drying regime, probably by both its PGP and EPS production attributes, respectively.