The Iron Deficiency Response Regulators IAA-LEUCINE RESISTANT3 and bHLH104 Possess Different Targets and Have Distinct Effects on Photosynthesis in Arabidopsis

In higher plants, iron (Fe) is an essential element for photosynthesis and growth. Two basic helix-loop-helix (bHLH) transcription factors, bHLH104 and IAA-LEUCINE RESISTANT3 (ILR3), positively regulate Fe uptake in Arabidopsis. Furthermore, an E3 ubiquitin ligase, BRUTUS (BTS) functions as a negative regulator upstream of bHLH104 and ILR3 in the Fe homeostasis pathway. Interestingly, we characterized a lesion of BRUTUS (BTS), which exhibited pale-green leaves and retarded in growth. Genetic analyses showed that only loss of ILR3 could entirely compromise the chlorosis in bts-2 mutants. To further investigate whether bHLH104 and ILR3 have different roles in functioning downstream of BTS other than Fe absorption, we obtained bHLH104-, and ILR3-overexpressing plants. Interestingly, similar to bts-2, overexpression of ILR3 displayed chlorotic leaves. Moreover, overexpressing ILR3 also caused the abundance of thylakoid proteins reduced, along with photosynthetic genes decreased. However, there was no visible difference between bHLH104-overexpressing plants and WT. Furthermore, we found that bHLH104 and ILR3 may act independently on different downstream targets. bHLH104, but not ILR3, could bind to the promoter of At- NEET, which acts as a Fe-S/Fe cluster donor in chloroplasts. Collectively, our data demonstrate that bHLH104 and ILR3 possess different downstream targets that may have distinct effects on photosynthesis, although they share a common function in Fe deficiency responses.