MxRop1-MxrbohD1 interaction mediates ROS signaling in response to iron deficiency in the woody plant Malus xiaojinensis

Surviving a Double-Edged Sword: Response of Horticultural Crops to Multiple Abiotic Stressors

Climate change-induced weather events, such as extreme temperatures, prolonged drought spells, or flooding, pose an enormous risk to crop productivity. Studies on the implications of multiple stresses may vary from those on a single stress. Usually, these stresses coincide, amplifying the extent of collateral damage and contributing to significant financial losses. The breadth of investigations focusing on the response of horticultural crops to a single abiotic stress is immense. However, the tolerance mechanisms of horticultural crops to multiple abiotic stresses remain poorly understood. In this review, we described the most prevalent types of abiotic stresses that occur simultaneously and discussed them in in-depth detail regarding the physiological and molecular responses of horticultural crops. In particular, we discussed the transcriptional, posttranscriptional, and metabolic responses of horticultural crops to multiple abiotic stresses. Strategies to breed multi-stress-resilient lines have been presented. Our manuscript presents an interesting amount of proposed knowledge that could be valuable in generating resilient genotypes for multiple stressors.

PuNDH9, a subunit of ETC Complex I regulates plant defense by interacting with PuPR1

NAD+ and NADH play critical roles in energy metabolism, cell death, and gene expression. The NADH-ubiquinone oxidoreductase complex (Complex I) has been long known as a key enzyme in NAD+ and NADH metabolism. In the present study, we found and analyzed a new subunit of Complex I (NDH9), which was isolated from Pyrus ussuriensis combined with RT-PCR. Following infection with A. alternata, RT-qPCR analysis demonstrated an increase in the expression of PuNDH9. Genetic manipulation of PuNDH9 levels suggested that PuNDH9 plays key roles in NADH/NAD+ homeostasis, defense enzyme activities, ROS generation, cell death, gene expression, energy metabolism, and mitochondrial functions during the pear- A. alternata interaction. Furthermore, Y2H, GST-pull down, and a split-luciferase complementation imaging assays revealed that PuNDH9 interacts with PuPR1. We discover that PuNDH9 and PuPR1 synergistically activate defense enzyme activities, ROS accumulation, cell death, and plant defenses. Collectively, our findings reveal that PuNDH9 is likely important for plant defenses.

ZmRop1 participates in maize defense response to the damage of Spodoptera frugiperda larvae through mediating ROS and soluble phenol production

As plant-specific molecular switches, Rho-like GTPases (Rops) are vital for plant survival in response to biotic and abiotic stresses. However, their roles in plant defense response to phytophagous insect's damage are largely unknown. In this study, the expression levels of nine maize RAC family genes were analyzed after fall armyworm (FAW) larvae infestation. Among the analyzed genes, ZmRop1 was specifically and highly expressed, and its role in maize response to FAW larvae damage was studied. The results showed that upon FAW larvae infestation, salicylic acid and methyl jasmonate treatment ZmRop1 gene transcripts were all down-regulated. However, upon mechanical injury, the expression level of ZmRop1 was up-regulated. Overexpression of ZmRop1 gene in maize plants could improve maize plant resistance to FAW larvae damage. Conversely, silencing of ZmRop1 increased maize plant susceptibility to FAW larvae damage. The analysis of the potential anti-herbivore metabolites, showed that ZmRop1 promoted the enzyme activities of catalase, peroxidase and the expression levels of ZmCAT, ZmPOD, ZmRBOHA and ZmRBOHB, thereby enhancing the reactive oxygen species (ROS) production, including the content of O^2- and H₂O₂. In addition, overexpression or silencing of ZmRop1 could have influence on the content of the total soluble phenol through mediating the activity of polyphenol oxidase. In summary, the results illuminated our understanding of how ZmRop1 participate in maize defense response to FAW larvae damage as a positive regulator through mediating ROS production and can be used as a reference for the green prevention and control of FAW larvae.

Genome-wide identification of apple PPI genes and a functional analysis of the response of MxPPI1 to Fe deficiency stress

Iron (Fe) deficiency affects plant growth and development. The proton pump interactor (PPI) in plants responds to multiple abiotic stresses, although it has not been well characterized under Fe deficiency stress. In this study, we systematically identified and analyzed the PPI gene family in apple. Three PPI candidate genes were found, and they contained 318-1349 amino acids and 3-7 introns. Under Fe deficiency stress, we analyzed the expression of all the PPI genes in roots of apple rootstock Malus xiaojinensis. Expression of the gene MD11G1247800, designated PPI1, is obviously induced by Fe deficiency treatment in M. xiaojinensis. We first cloned MxPPI1 from M. xiaojinensis and determined its subcellular localization, which indicated that it is localized in the cell membrane and nucleus in tobacco. We found that the level of expression of the MxPPI1 protein increased significantly under Fe deficiency stress in apple calli. Moreover, overexpressing MxPPI1 in apple calli enhanced the activities of ferric chelate reductase and H⁺-ATPase, H⁺ secretion, MxHA2 gene expression and total Fe content when compared with the wild type calli. We further found that MxPPI1 interacted with MxHA2 using bimolecular fluorescence complementation and luciferase complementation assays. Overall, we demonstrated that MxPPI1 interacts with MxHA2 to enhance the activity of H⁺-ATPase to regulate Fe absorption in M. xiaojinensis.

MxMPK4-1 phosphorylates NADPH oxidase to trigger the MxMPK6-2-MxbHLH104 pathway mediated Fe deficiency responses in apple

Iron (Fe) deficiency is a nutritional stress in plants that commonly occurs in alkaline and calcareous soils. Mitogen-activated protein kinases (MPKs), the terminal player of MAPK cascade, are involved in distinct physiological processes. Once plants suffer from Fe deficiency stress, the mechanism of MPK function remains unclear owing to limited study on the MPK networks including substrate proteins and downstream pathways. Here, the MAP kinase MPK4-1 was induced in roots of Fe efficient apple rootstock Malus xiaojinensis but not in Fe inefficient rootstock Malus baccata under Fe deficiency conditions. Overexpression of MxMPK4-1 in apple calli and apple roots enhanced the responses to Fe deficiency. We found that MxMPK4-1 interacted with NADPH oxidases (NOX)-respiratory burst oxidase homologs MxRBOHD1 and MxRBOHD2, which positively regulated responses to Fe deficiency. Moreover, MxMPK4-1 phosphorylated the C terminus of MxRBOHD2 at Ser797 and Ser906 and positively and negatively regulated NOX activity through these phospho-sites, respectively. When compared with apple calli that overexpressed MxRBOHD2, the coexpression of MxMPK4-1 and MxRBOHD2 prominently enhanced the Fe deficiency responses. We also demonstrated that hydrogen peroxide derived from MxMPK4-1-MxRBOHD2 regulated the MxMPK6-2-MxbHLH104 pathway, illuminating a systematic network that involves different MPK proteins in M. xiaojinensis under Fe deficiency stress.