Plants experiencing salt-induced stress often reduce cytokinin levels during the early phases of stress-response. Interestingly, we found that the cytokinin content in the apple rootstock "robusta" was maintained at a high level under salt stress. Through screening genes involved in cytokinin biosynthesis and catabolism, we found that the high expression levels of IPT5b in robusta roots were involved in maintaining the high cytokinin content. We identified a 42 bp deletion in the promoter region of IPT5b, which elevated IPT5b expression levels, and this deletion was linked to salt tolerance in robusta×M.9 segregating population. The 42 bp deletion resulted in the deletion of a Proline Response Element (ProRE), and our results suggest that ProRE negatively regulates IPT5b expression in response to proline. Under salt stress, the robusta cultivar maintains high cytokinin levels as IPT5b expression cannot be inhibited by proline due to the deletion of ProRE, leading to improve salt tolerance.
Iron (Fe) is a trace element necessary for plant growth. Many land plants have evolved a set of mechanisms associated with the Fe absorption process to deal with the problem of insufficient Fe supply in the soil. During Fe absorption, reactive oxygen species (ROS) can be used as a signal to initiate a response to stress caused by Fe deficiency. However, the molecular mechanisms underlying the involvement of ROS in the Fe deficiency stress response remains unclear. In this study, we have identified a kinase, MxMPK6-2, from Malus xiaojinensis, an apple rootstock that is highly efficient at Fe absorption. MxMPK6-2 has been shown to be responsive to ROS signals during Fe deficiency, and MxMPK6-2 overexpression in apple calli enhanced its tolerance to Fe deficiency. We further screened for proteins in the Fe absorption pathway and identified MxbHLH104, a transcription factor which interacts with MxMPK6-2. MxbHLH104 can be phosphorylated by MxMPK6-2 in vivo, and we confirmed that its phosphorylation increased Fe absorption in apple calli under Fe deficiency, with the presence of ROS promoting this process. Overall, we have demonstrated that MxMPK6-2 is responsive to ROS signaling during Fe deficiency, and is able to control its response by regulating MxbHLH104.
Reactive oxygen species (ROS) are important signaling molecules in plants that contribute to stress acclimation. This study demonstrated that ROS play a critical role in Fe deficiency-induced signaling at an early stage in Malus xiaojinensis. Once ROS production has been initiated, prolonged Fe starvation leads to activation of ROS scavenging mechanisms. Further, we demonstrated that ROS scavengers are involved in maintaining the cellular redox homeostasis during prolonged Fe deficiency treatment. Taken together, our results describe a feedback repression loop for ROS to preserve redox homeostasis and maintain a continuous Fe deficiency response in the Fe-efficient woody plant M. xiaojinensis. More broadly, this study reveals a new mechanism in which ROS mediate both positive and negative regulation of plant responses to Fe deficiency stress.
Understanding the mechanism of iron (Fe)‐deficiency responses is crucial for improving plant Fe bioavailability. Here, we found that the Arabidopsis Rho‐like GTPase 6 mutant (rop6) is less sensitive to Fe‐deficiency responses and has reduced levels of reactive oxygen species (ROS) compared to wild‐type (WT), while AtROP6‐overexpressing seedlings exhibit more sensitivity to Fe‐deficiency responses and has higher levels of ROS compared to WT. Moreover, treatment with H2O2 improves the sensitivity to Fe‐deficiency responses in rop6 mutants. By using the yeast two‐hybrid system, we further demonstrate the direct interaction between AtROP6 and Arabidopsis respiratory burst oxidase homolog D (AtRBOHD), which controls the generation of ROS. Overall, we suggest that AtROP6 is involved in AtRBOHD‐mediated ROS signaling to modulate Fe‐deficiency responses in Arabidopsis thaliana.
Iron (Fe) deficiency limits the yield of fruit trees. When subjected to Fe deficiency, H+ secretion increases in the rhizosphere of dicotyledonous plants and pH decreases. This leads to the acidification of the soil and promotes Fe3+ to Fe2+ conversion, which plants can better uptake. This study investigated the relationship between two inhibitory transcription factors (ethylene response factors MbERF4 and MbERF72) and the H+-ATPase gene MbHA2. Two species of apple woody plants were studied: the Fe-inefficient Malus baccata and the Fe-efficient Malus xiaojinensis. Yeast one-hybrid and electrophoretic mobility shift assays showed that both MbERF4 and MbERF72 bind to the GCC cassette (AGCCGCC) of the MbHA2 promoter. Moreover, yeast two-hybrid and bimolecular fluorescence complementation assays showed that MbERF4 interacts with MbERF72. Furthermore, β-glucuronidase and luciferase reporter assays showed that the MbERF4- and MbERF72-induced repression of MbHA2 expression is synergistic. Virus-induced gene silencing of MbERF4 or MbERF72 increased MbHA2 expression, and thus lowered the rhizosphere pH in M. baccata. Consequently, the high expressions of MbERF4 and MbERF72 induced by Fe deficiency contributed to the Fe sensitivity of M. baccata. Moreover, the low expressions of MxERF4 and MxERF72 contributed to the Fe-deficiency tolerance of M. xiaojinensis via different binding conditions to the HA2 promoter. In summary, this study identified the relationship of two inhibitory transcription factors with the H+-ATPase gene and proposed a model in which ERF4 and ERF72 affect the rhizosphere pH in response to Fe deficiency.
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