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Potential Enhancement of Plant Iron Assimilation by Microbial-Induced Root Exudation of Phenolic Compounds

Cheng Zhou, Zhongyou Ma, Xin Xiao, Yue Xie, Jian Zhu and Jianfei Wang

Iron (Fe) deficiency in crop plants is a modern agricultural problem worldwide. Although multiple strategies have been evolved to improve Fe assimilation, some plant species, especially dicots and nongraminaceous monocots (strategy I plants), cannot avoid Fe deficiency in low Fe-availability soils. It is well documented that graminaceous plants (strategy II plants) employ the chelationbased Fe acquisition, and the strategy I plants use the reduction-based strategy to take up Fe. Intriguingly, under Fe deficiency the strategy I plants have recently been found to acquire Fe via exudation of phenolic compounds to mobilize Fe, which is much similar to the chelation-based mechanism of strategy II plants. Hitherto, increasing evidence has shown that soil microbes play a cooperative role in plant Fe acquisition. Several beneficial rhizobacteria have been found to increase plant Fe accumulation via activation of the reduction-based strategy. Moreover, microbial-induced root exudation of phenolic compounds can also promote plant Fe absorption by efficient mobilization of Fe, which increases Fe bioavailability in calcareous soils. Here, we briefly review the recent progress on the Fe assimilation strategies of strategy I and II plants, and further discuss the possible mechanisms underlying soil microbes enhance plant Fe acquisition.