Abstract
The COP9 signalosome (CSN) plays an important role in proteasome-mediated degradation by regulating CUL1 rubylation of the SCF ligase and is involved in many crucial biological processes. Here, we demonstrate a link between IDEF1 accumulation and the decline in COP9 derubylation activity in response to iron deficiency (-Fe) in rice (Oryza sativa). CSN6 expression is rapidly down-regulated during Fe depletion, contributing to reduced CSN activity, as judged by CSN5 and CUL1 expression, indicating CSN6 is involved in the early stage response of -Fe. In contrast to CSN6, the IDEF1 protein and expression of several iron uptake/utilisation-related genes are increased in response to -Fe. Thus, we constructed CSN6 transgenic sense and antisense lines and found that experimental depletion of CSN6 results in accumulation of the IDEF1 protein and up-regulation of several iron uptake/utilisation-related genes. Furthermore, IDEF1 can be decorated with K48-linked polyubiquitin and degraded via the 26S proteasome. Accumulated IDEF1 in antisense lines led to increased chlorophyll and Fe content in seedlings during -Fe. Collectively, the cellular CSN6 level is decreased during early stages of -Fe to ensure the rapid accumulation of IDEF1, which in turn up-regulates several iron uptake/utilisation-related genes to help overcome -Fe stress in rice.
Highlights
The COP9 signalosome (CSN) is a multisubunit protein complex conserved among diverged organisms, including fission yeast, fruit fly, Arabidopsis, human, and probably blue-green algae[1,2,3,4]
During the first day of -Fe, defined as the early stage of -Fe, a majority of known iron uptake/ utilisation-related genes, such as IRO2, YSL15, and IRT1 are positively regulated by IDEF1, and IDEF1 is essential for the early response to iron deficiency[24]
We present evidence that IDEF1 is regulated by proteasome-mediated degradation and CSN6 is involved in this regulation during the onset of -Fe via the derubylation function of the COP9 signalosome
Summary
The COP9 signalosome (CSN) is a multisubunit protein complex conserved among diverged organisms, including fission yeast, fruit fly, Arabidopsis, human, and probably blue-green algae[1,2,3,4]. Derubylation relies on COP9 signalosome stability and integrity, and loss of function of CSN5, CSN6, CSN7, or CSN8 destabilises several other CSNs in Arabidopsis[10] Every subunit has their special function to ensure COP9 activity. Because of low solubility in aerobic and alkaline conditions[14], plants have acquired different mechanisms to ensure iron absorption under low availability These strategies include strategy I and strategy II, found in non-graminaceous monocots and dicots and in graminaceous monocots, respectively[15]. During the first day of -Fe, defined as the early stage of -Fe, a majority of known iron uptake/ utilisation-related genes, such as IRO2, YSL15, and IRT1 are positively regulated by IDEF1, and IDEF1 is essential for the early response to iron deficiency[24]. We have identified a new mechanism of IDEF1 regulation under -Fe conditions
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