Identification of a novel iron regulated basic helix-loop-helix protein involved in Fe homeostasis in Oryza sativa

Transcript abundance of one nuclear located basic helix-loop-helix family transcription factor, OsIRO3, is up-regulated from 25- to 90-fold under Fe deficiency in both root and shoot respectively. The expression of OsIRO3 is specifically induced by Fe deficiency, and not by other micronutrient deficiencies. Transgenic rice plants over-expressing OsIRO3 were hypersensitive to Fe deficiency, indicating that the Fe deficiency response was compromised. Furthermore, the Fe concentration in shoots of transgenic rice plants over-expressing OsIRO3 was less than that in wild-type plants. Analysis of the transcript abundances of genes normally induced by Fe deficiency in OsIRO3 over-expressing plants indicated their induction was markedly suppressed.A novel Fe regulated bHLH transcription factor (OsIRO3) that plays an important role for Fe homeostasis in rice was identified. The inhibitory effect of OsIRO3 over-expression on Fe deficiency response gene expression combined with hypersensitivity of OsIRO3 over-expression lines to low Fe suggest that OsIRO3 is a negative regulator of the Fe deficiency response in rice.Iron (Fe) is an essential micronutrient for plant growth and production. This is due to the fact that it is an essential co-factor in a variety of enzymes that play critical roles in photosynthesis, respiration and nitrogen fixation [1]. Although Fe is the second most abundant metal element in the earth crust, its bio-availability is limited, especially in alkaline soils where Fe largely exists as insoluble hydroxides or oxides [2]. While the optimal Fe concentration for plant growth is in the range of 10-9 to 10-4M , the bio-available Fe in most soils is approximately 10-17 M [2,3]. Plants have two distinct uptake strategies to increase the efficiency of Fe uptake from soil [4]. The reduction strategy employed by non-grass plant species uses a Fe deficiency induced reductase to convert insoluble Fe(III) to Fe(II), the latter being transported into plant cells by t