Apple receptor-like kinase FERONIA regulates salt tolerance and ABA sensitivity in Malus domestica.

FERONIA (FER) is a membrane-localized receptor-like kinase that plays pivotal roles in male and female gametophyte recognition, hormone signaling crosstalk, and biotic and abiotic responses. Most reports focus on the functions of FER in model plant Arabidopsis thaliana. However, the functions of FER homologs have not been deeply investigated in apple (Malus domestica), an important economic fruit crop distributed worldwide, especially in China. In this study, we identified an apple homolog of Arabidopsis FER, named MdFER (MDP0000390677). The two proteins encoded by AtFER and MdFER share similar domains: an extracellular malectin-like domain, a transmembrane domain, and an intracellular kinase domain. MdFER was further proven to localize to the plasma membrane in the epidermal cells of Nicotiana benthamiana. MdFER was widely expressed in different apple tissues, but the highest expression was found in roots. In addition, expression of MdFER was significantly induced by treatment with abscisic acid (ABA) and salt (NaCl). Overexpressing MdFER dramatically improved the resistance to salt stress and reduced the sensitivity to ABA in apple callus, while suppressing MdFER expression showed contrary effects. Furthermore, ectopic expression of MdFER in Arabidopsis significantly increased the salt tolerance and reduced the sensitivity to ABA. In addition, under salt stress and ABA treatment, Arabidopsis with highly expressed MdFER accumulated less reactive oxygen species (ROS), and the enzymatic activity of two ROS scavengers, superoxide dismutase and catalase, was higher compared with that of wild type (WT). Our work proves that MdFER positively regulates salt tolerance and negatively regulates ABA sensitivity in apple, which enriched the functions of FER in different plant species.

Nitrate-inducible MdBT2 acts as a restriction factor to limit apple necrotic mosaic virus genome replication in Malus domestica.

Apple necrotic mosaic virus (ApNMV) is highly associated with the occurrence of apple mosaic disease in China. However, ApNMV-host interactions and defence mechanisms of host plants against this virus are poorly studied. Here, we report that nitrate treatment restrains ApNMV genomic RNA accumulation by destabilizing viral replication protein 1a through the MdBT2-mediated ubiquitin-proteasome pathway. MdBT2, a nitrate-responsive BTB/TAZ domain-containing protein, was identified in a yeast two-hybrid screen of an apple cDNA library using viral protein 1a as bait, and 1a was further confirmed to interact with MdBT2 both in vivo and in vitro. It was further verified that MdBT2 promoted the ubiquitination and degradation of viral protein 1a through the ubiquitin-proteasome pathway in an MdCUL3A-independent manner. Viral genomic RNA accumulation was reduced in MdBT2-overexpressing transgenic apple leaves but enhanced in MdBT2-antisense leaves compared to the wild type. Moreover, MdBT2 was found to interfere with the interaction between viral replication proteins 1a and 2apol by competitively interacting with 1a. Taken together, our results demonstrate that nitrate-inducible MdBT2 functions as a limiting factor in ApNMV viral RNA accumulation by promoting the ubiquitination and degradation of viral protein 1a and interfering with interactions between viral replication proteins.

The ankyrin repeat-containing protein MdANK2B regulates salt tolerance and ABA sensitivity in Malus domestica.

KEY MESSAGE: The ankyrin repeat-containing protein MdANK2B was identified to contribute to increasing resistance to salt stress and decreasing sensitivity to ABA in Malus domestica. Ankyrin (ANK) repeat-containing proteins occur widely in prokaryotes, eukaryotes, and even in some viruses and play a critical role in plant growth and development, as well as the response to biotic and abiotic stress. However, the function of ANK repeat-containing proteins in apple (Malus domestica) has not yet been investigated. Here, we identified apple MdANK2B based on homology analysis with the Arabidopsis ANK repeat-containing proteins AtAKR2A and AtAKR2B. MdANK2B was found to be localized in the cytoplasm, and its encoding gene was highly expressed in both apple leaves and fruits. In addition, MdANK2B gene expression was highly induced by salt stresses and abscisic acid (ABA). Overexpression of MdANK2B increased resistance to salt stress and decreased sensitivity to ABA in both transgenic apple calli and seedlings. In addition, overexpression of MdANK2B reduced the accumulation of reactive oxygen species (ROS) by enhancing the activity of antioxidant enzymes in response to salt stress. Our data revealed the role of MdANK2B in response to salt stress and ABA treatment in apple, which widens the known functions of ANK repeat-containing proteins in response to abiotic stress.

Determination of Protein Interactions among Replication Components of Apple Necrotic Mosaic Virus.

Apple mosaic disease is one of the most widely distributed and destructive diseases in apple cultivation worldwide, especially in China, whose apple yields account for more than 50% of the global total. Apple necrotic mosaic virus (ApNMV) is a newly identified ilarvirus that is closely associated with apple mosaic disease in China; however, basic viral protein interactions that play key roles in virus replication and the viral life cycle have not been determined in ApNMV. Here, we first identify an ApNMV-Lw isolate that belongs to subgroup 3 in the genus Ilarvirus. ApNMV-Lw was used to investigate interactions among viral components. ApNMV 1a and 2apol, encoded by RNA1 and RNA2, respectively, were co-localized in plant cell cytoplasm. ApNMV 1a interacted with itself at both the inter- and intramolecular levels, and its N-terminal portion played a key role in these interactions. 1a also interacted with 2apol, and 1a's C-terminal, together with 2apol's N-terminal, was required for this interaction. Moreover, the first 115 amino acids of 2apol were sufficient for permitting the 1a-2apol interaction. This study provides insight into the protein interactions among viral replication components of ApNMV, facilitating future investigations on its pathogenicity, as well as the development of strategies to control the virus and disease.