Transcriptome analysis reveals new insight into iron homeostasis in Populus tomentosa

Iron (Fe) is an essential micronutrient for the survival and proliferation of plants. Plants have evolved complex mechanisms to maintain Fe homeostasis in response of Fe deficiency conditions. To explore the mechanisms of Populus tomentosa response to Fe deficiency, we evaluated the physiological, biochemical and transcriptome differences of P. tomentosa between Fe-sufficient and Fe-deficient conditions. The results showed that, under Fe-free conditions, the chlorophyll synthesis and photosynthesis pathways in shoots were extremely depressed. The inhibition of these pathways caused chlorosis and reduced shoot growth. Meanwhile, although both two photosynthetic systems (PSI and PSII) were inhibited under Fe limited conditions, PSI is affected more serious and earlier than PSII. In order to maintain Fe homeostasis, several genes involved in Fe regulation network were differentially expressed. At the late period of Fe deficiency response, some genes (BTS, bHLH38/39 and PYE) in PYE regulatory network were up-regulated in roots, while some root-specific ethylene-dependent FIT regulatory genes (EIN3, ERF and FIT) were down-regulated. Moreover, FRO2 was induced in P. tomentosa roots to reduce more Fe3+, which is similar with other strategy I plants. It is interest that we found another Fe2+ transporter gene (NRAMP1) was induced, instead of the well-known Fe2+ transporter gene (IRT1) for strategy I plants, to promote Fe2+ absorption at the Fe deficiency late stage. Overall design: The roots and shoots of two group plants (Fe-sufficient and Fe-deficient Populus tomentosa) were collected for RNA-seq, in triplicate, using Illumina Hiseq2500.