Influence of Nitrogen Application Rate on the Importance of NO 3 - -N and NH 4 + -N Transfer via Extramycelia of Arbuscular Mycorrhiza to Tomato with Expression of LeNRT2.3 and LeAMT1.1 .

Arbuscular mycorrhizal fungi (AMF) form mutualistic symbiotic relationships with many land plants and play a key role in nitrogen (N) acquisition. NO ₃ ^- -N and NH ₄ ⁺ -N are the main sources of soil mineral N, but how extraradical mycelial transfer affects the different N forms and levels available to tomato plants is not clear. In the present study, we set up hyphal compartments (HCs) to study the efficiency of N transfer from the extramycelium to tomato plants treated with different N forms and levels of fertilization. Labeled ¹⁵ NO ₃ ^- -N or ¹⁵ NH ₄ ⁺ -N was placed in hyphal compartments under high and low N application levels. ¹⁵ N accumulation in shoots and the expression of LeNRT2.3 , LeAMT1.1 , and LeAMT1.2 in the roots of tomato were measured. According to our results, both ¹⁵ NO ₃ ^- -N and ¹⁵ NH ₄ ⁺ -N were transported via extraradical mycelia to the shoots of plants. ¹⁵ N accumulation in shoots was similar, regardless of the N form, while a higher ¹⁵ N concentration was found in shoots with low N application. Compared with the control, inoculation with AMF significantly increased the expression of LeAMT1.1 under high N and LeNRT2.3 under low N. The expression of LeAMT1.1 under high N was significantly increased when NO ₃ ^- N was added, while the expression of LeNRT2.3 was significantly increased when NH ₄ ⁺ -N was added under low N. Taken together, our results suggest that the N transfer by extraradical mycelia is crucial for the acquisition of both NO ₃ ^- -N and NH ₄ ⁺ -N by the tomato plant; however, partial N accumulation in plant tissue is more important with N deficiency compared with a higher N supply. The expression of N transporters was influenced by both the form and level of N supply.