Ethylene regulation of root growth and phytosiderophore biosynthesis determines iron deficiency tolerance in wheat (Triticum spp)

Two separate experiments were conducted with four bread and two durum wheat cultivars, to test a hypothesis that high ethylene production will alter phytosiderophore production and release dynamics either directly, by competitive partitioning of SAM, or indirectly by causing changes in the root system architecture under iron deficiency. A significant decline in the leaf greenness index confirmed the Fe deficiency response across the wheat cultivars. Further, a significantly higher root to shoot ratio was measured for the durum wheat than the bread wheat cultivars under the iron deficiency treatment. Root attributes, although, were affected under iron deficiency, the variation between the bread and durum wheat cultivars were not too obvious. Durum wheat maintained a higher shoot, root and the whole plant iron levels than the bread wheat under Fe sufficient treatment while a role reversal was observed under Fe deficiency. Phytosiderophore production in the root tips and their release were induced under Fe deficiency and were higher for the bread than the durum wheat. A higher mRNA expression of ACS but a lower expression of NAS were identified with the durum wheat while the reverse was observed for the bread wheat. These results point towards a higher ethylene production and a lower phytosiderophore production in durum wheat as was also evidenced from observations on the ethylene release from the roots which was bettered in the presence of ethrel, an ethylene releasing chemical, but was completely inhibited in the presence of cobalt (Co), an ethylene synthesis inhibitor. An inhibited ethylene did improve the phytosiderophore production and phytosiderophore release for both the bread and the durum wheat cultivars under the iron deficiency treatment in the presence of cobalt. Since phytosiderophore production and release were not significantly altered over control, under Fe deficiency, in the presence of silver thiosulfate (STS), an ethylene action inhibitor, it could be concluded that ethylene does not affect NAS and that the effect of ethylene on the phytosiderophore production is direct and is caused by the dynamics of partitioning of S-adenosyl methionine (SAM) between the ethylene and the phytosiderophore biosynthetic pathways.