Characterization of the Arabidopsis thaliana RING-type Ubiquitin Ligase XBAT31.1 and its Role in Response to Iron Deficiency Stress

Abstract

Iron (Fe) is a very important micronutrient, specially for animals and plants. In humans, Fe-deficiency can cause anemia, which affects 43% of children, 38% of pregnant woman and 29% of non-pregnant women (World Health Organization et al., 2013). In plants, Fe-deficiency can impact nutritional quality growth and limit crop productivity. As a result, plants must sense Fe deprivation and be capable of balancing Fe-concentration in a homeostatic way, to be able to provide the necessary amounts of the required micronutrient to important process such as respiration and photosynthesis (Briat and Lobreaux, 1997; Hell and Stephan, 2003). The ubiquitin proteasome system (UPS) controls the abundance of important enzymes structural and regulatory proteins. Plants utilize UPS to facilitate changes in cellular protein content required for tolerance of adverse environments, such as micronutrient deficiency. Central to the UPS are the ubiquitin ligase enzymes that govern protein substrate selection. Importantly, a number of RING-type ubiquitin ligases have been shown to regulate proteins involved in iron uptake under iron sufficient (Moroishi et al., 2011) and iron deficient conditions (Kobayashi et al., 2013; Shin et al., 2013). In this study, we demonstrate the role of Arabidopsis thaliana XBAT31.1, a RING-type ubiquitin ligase, in response to Fe-deficiency stress. XBAT31.1 expression is induced under iron deficient conditions. xbat31-1 seedlings have elevated transcript levels of Fe-utilization related genes, such as FIT, FRO2 and AHA2 under Fe-deficiency compared to wild type. Unexpectedly, the increase in the transcript level of IRT1, which encodes for the major metal transporter responsible for uptake of iron and other metals, was significantly lower compared to wild type. The low levels of IRT1 transcripts are correlated with xbat31-1 accumulating less iron, manganese and cobalt in shoots under Fe-deficient condition. Despite the low iron content, xbat31-1 seedlings are more tolerant to Fe-deficient than wild type seedlings as shown by significant higher root length, fresh weight and chlorophyll content. Based on these results a model is proposed where XBAT31.1 functions as an iron sensor and ubiquitin ligase that indirectly regulates expression of IRT1 during plant response to Fe-deficiency stress.