Organ-specific hormonal cross-talk in phosphate deficiency

Plant interactions with the environment, including nutrient sensing and acquisition, are regulated by plant hormones. Responses to phosphate (Pi) deficiency are governed by strigolactones, in cross-talk with other hormones. Reaction of Arabidopsis thaliana plants to Pi deficiency has been characterized in shoot apices, leaves and roots at hormonal and selected transcript levels. Pi starvation was associated with elevation of abscisic acid and jasmonic acid in all followed organs, as well as of salicylic acid in roots. Stimulation of strigolactone signaling pathway was indicated by suppressed expression of strigolactone repressors SMXL6 and SMXL8. Simultaneously, Pi deficiency downregulated the levels of gibberellin GA4 and the most physiologically active cytokinin, trans-zeatin, in all studied organs, increasing less active cis-zeatin in roots. Data imply suppression of the plastid cytokinin biosynthetic pathway and promotion of the cytoplasmic one. Strigolactone function has been characterized comparing the effect of 5 μM synthetic strigolactone GR24 at Pi deficiency and full Pi supply (100 μM). GR24 promoted Pi uptake, including expression of Pi transporters PHT1;4 and PHT1;7. GR24 in Pi presence generally mimicked the impact of Pi deficiency in roots, with exception of jasmonic acid and salicylic acid, which were in full Pi medium, i.e. under conditions optimal for growth, decreased. Pi availability strongly affected expression of selected genes in apices, the major differences being found in the expression of Pi-transporters, auxin- and strigolactone-related genes. The data show highly organ-specific effects of Pi deficiency and decisive role of strigolactones, not only in roots but also in apices.