Phosphorus Scavenging and Remobilization from Root Cell Walls Under Combined Nitrogen and Phosphorus Stress is Regulated by Phytohormones and Nitric Oxide Cross-Talk in Wheat

The role of endogenous phytohormones, nitric oxide (NO), and nitrogen (N) starvation on scavenging and remobilization of phosphorous (P) from root cell wall pectin was studied on dual-nutrient stress (DNS) tolerant (HD-2781) and sensitive (C-306) bread wheat genotypes. Plants were grown in hydroponics with low concentrations of N and P, their combined stress, and recovery from stress conditions. Results revealed that P remobilization from roots was higher in HD-2781 under nutrient stress and recovery conditions than C-306. In general, the hydrolysis of pectin and remobilization of P from root cell walls increased under low P, DNS, and recovery treatments but no effect of N was noted. In the roots, endogenous ethylene and auxin concentrations were significantly higher indicating enhanced adaptive response to low P and DNS. However, an enhanced ABA concentration in C-306 was also observed which may have an inhibitory effect on P remobilization from pectin. The cross-talk between ethylene, auxin, and NO stimulated pectin methyl esterase enzyme activity thereby, leading to an increased cell wall P scavenging and its transport to shoot as evident from the upregulation of TaPT8 gene. The low P and DNS treatments also induced ethylene and NO biosynthetic genes, TaACS2 and TaNOS, respectively. Thus, NO, ethylene, and auxin-stimulated PME activity leading to enhanced release of P from the root cell wall pectin independent of N treatment. We provided sufficient evidence that the root cell wall P remobilization is one of the low P adaptive mechanisms in P stress-tolerant wheat genotype. However, the cross-talk between hormones and signalling molecules, and their regulatory mechanism on PME enzyme activity needs to be studied.