Role of hormones in the induction of iron deficiency responses in Arabidopsis roots

In "strategy 1" plants, several alterations in root physiology and morpholo gy are induced by Fe deficiency, although the mechanisms by which low Fe le vels are translated into reactions aimed at alleviating Fe shortage are lar gely unknown. To prove whether changes in hormone concentration or sensitiv ity are involved in the adaptation to suboptimal Fe availability, we tested 45 mutants of Arabidopsis defective in hormone metabolism and/or root hair formation for their ability to increase Fe(lll) chelate reductase activity and to initiate the formation and enlargement of root hairs. Activity stai ning for ferric chelate reductase revealed that all mutants were responsive to Fe deficiency, suggesting that hormones are not necessary for the induc tion. Treatment of wild-type plants with the ethylene precursor 1-aminocycl opropane-1-carboxylic acid caused the development of root hairs in location s normally occupied by non-hair cells, but did not stimulate ferric reducta se activity. Ectopic root hairs were also formed in -Fe roots, suggesting a role for ethylene in the morphological responses to Fe deficiency. Ultrast ructural analysis of rhizodermal cells indicated that neither Fe deficiency nor 1-aminocyclopropane-1-carboxylic acid treatment caused transfer-cell-l ike alterations in Arabidopsis roots. Our data indicate that the morphologi cal and physiological components of the Fe stress syndrome are regulated se parately.