Detection of Root Physiological Parameters and Potassium and Calcium Currents in the Rhizoplane of the Apple Rootstock Superior Line 12-2 With Improved Apple Replant Disease Resistance

Effect of solution pH on root architecture of four apple rootstocks grown in an aeroponics nutrient misting system

The pH of the solution in the rhizosphere is an important factor that determines the availability and mobility of nutrients for plant uptake. Solution pH may also affect the root distribution and architecture of apple rootstocks. In this study, we evaluated the effect of solution pH on root system development of apple rootstocks using an aeroponics system designed and developed at Cornell AgriTech Geneva, USA. Four Geneva® apple rootstocks (G.210, G.214, G.41, and G.890) were grown in an aeroponic system under nutrient solution misting featuring continuously adjusted pH levels to three pH treatments (5.5, 6.5, and 8.0). Root development was monitored for 30 days and evaluated regularly for distribution and root mass. Images of the developed roots grown in the aeroponic system were collected at the end of the experiment using a high-resolution camera and analyzed using GiA Roots® software, which generates root architecture parameter values in a semi-automated fashion. The resulting root architecture analysis showed that the Geneva® rootstocks were significantly different for two architecture parameters. The length-to-width ratio analysis represented by two GiA Roots parameters (minor-to-major ellipse ratio and network width-to-depth ratio) showed that G.210 was flatter than G.890, which had a greater tendency to grow downward. Rootstocks G.214 and G.41 displayed similar growth values. The solution pH affected most root architecture parameter measurements where overall root growth was higher at pH 8 than at pH 5.5 and 6.5, which showed similar growth. In general, the average root width tended to decrease at higher pH values. While there were no significant differences in the leaf nutrient concentrations of P, K, Ca, Mg, S, B, Zn, Cu, and Fe within the four rootstocks, the pH level of the solution had a significant effect on P, Ca, and Mn. This study is the first of its kind to investigate the effect of pH on root architecture in a soil-free (aeroponic) environment and may have implications for apple root behavior under field conditions where pH levels are different.

Multi-omics analysis reveals the biosynthesis of flavonoids during the browning process of Malus sieversii explants

Malus sieversii is a precious apple germplasm resource. Browning of explants is one of the most important factors limiting the survival rate of plant tissue culture. In order to explore the molecular mechanism of the browning degree of different strains of Malus sieversii, we compared the dynamic changes of Malus sieversii and Malus robusta Rehd. during the whole browning process using a multi-group method. A total of 44 048 differentially expressed genes (DEGs) were identified by transcriptome analysis on the DNBSEQ-T7 sequencing platform. KEGG enrichment analysis showed that the DEGs were significantly enriched in the flavonoid biosynthesis pathway. In addition, metabonomic analysis showed that (-)-epicatechin, astragalin, chrysin, irigenin, isoquercitrin, naringenin, neobavaisoflavone and prunin exhibited different degrees of free radical scavenging ability in the tissue culture browning process, and their accumulation in different varieties led to differences in the browning degree among varieties. Comprehensive transcriptome and metabonomics analysis of the data related to flavonoid biosynthesis showed that PAL, 4CL, F3H, CYP73A, CHS, CHI, ANS, DFR and PGT1 were the key genes for flavonoid accumulation during browning. In addition, WGCNA analysis revealed a strong correlation between the known flavonoid structure genes and the selected transcriptional genes. Protein interaction predictions demonstrated that 19 transcription factors (7 MYBs and 12 bHLHs) and 8 flavonoid structural genes had targeted relationships. The results show that the interspecific differential expression of flavonoid genes is the key influencing factor of the difference in browning degree between Malus sieversii and Malus robusta Rehd., providing a theoretical basis for further study on the regulation of flavonoid biosynthesis.

Study of the grafting compatibility of the apple rootstock 12-2, resistant to apple replant diseases (ARD)

BACKGROUND

Cultivation of resistant rootstocks can effectively prevent apple replant disease (ARD), and grafting tests are an important means of evaluating the compatibility of rootstocks with scions.

METHODS

The apple rootstocks 12-2 (self-named) and Malus hupehensis Rehd. (PYTC) were planted in a replanted 20-year-old apple orchard. The two rootstocks were grafted with scions of 13 apple varieties. Multiple aboveground physiological parameters of the grafted combinations were measured and evaluated to verify the grafting affinity of 12-2 with the scions as compared to Malus hupehensis Rehd. (PYTC).

RESULTS

The graft survival rate and graft interface healing of 12-2 did not differ significantly from those of PYTC. Mechanical strength tests of the grafted interfaces showed that some mechanical strength indices of Redchief, Jonagold, Starking, Goldspur and Yinv apple varieties were significantly higher when they were grafted onto 12-2 compared to the PYTC control. The height and diameter of shoots and the relative chlorophyll content, photosynthetic and fluorescence parameters, antioxidant enzyme activities and malondialdehyde content of leaves showed that Fuji 2001, Tengmu No.1, RedChief, Gala, USA8, and Shoufu1 grew similarly on the two rootstocks, but Tianhong 2, Lvguang, Jonagold, Starking, Goldspur, Yinv and Luli grew better when grafted onto 12-2 than onto the PYTC control. The rootstock 12-2, therefore, showed good grafting affinity.

CONCLUSION

These results provide experimental materials and theoretical guidance for the cultivation of a new grafting compatible rootstock to the 13 studied apple cultivars.

Homologous Cloning of Potassium Channel Genes From the Superior Apple Rootstock Line 12-2, Which is Tolerant to Apple Replant Disease

Potassium channels are important ion channels that are responsible for the absorption of potassium in the plant nutrient uptake system. In this study, we used homologous molecular cloning to obtain 8 K⁺ channel genes from the superior apple rootstock line 12-2 (self-named): MsAKT1-1, MsKAT3-2, MsKAT1-3, MsK2P3-4, MsK2P3-5, MsK2P5-6, MsK2P3-7, and MsK2P3-8. Their lengths varied from 942 bp (MsK2P5-6) to 2625 bp (MsAKT1-1), and the number of encoded amino acids varied from 314 (MsK2P5-6) to 874 (MsAKT1-1). Subcellular localization predictions showed that MsAKT1-1, MsKAT3-2, and MsKAT1-3 were localized on the plasma membrane, and MsK2P3-4, MsK2P3-5, MsK2P5-6, MsK2P3-7, and MsK2P3-8 were localized on the vacuole and plasma membrane. The 8 K⁺ channel proteins contained α helices, extended strands, β turns, and random coils. MsKAT1-3 had four transmembrane structures, MsKAT3-2 had six, and the other six K⁺ channel genes had five. Protein structure domain analysis showed that MsAKT1-1 contained nine protein domains, followed by MsKAT3-2 with four, MsKAT1-3 with three, and the other five two-pore domain K⁺ channel proteins with two. Semi-quantitative RT-PCR detection of the K⁺ channel genes showed that their expression levels were high in roots. qRT-PCR analysis showed that the relative expression levels of the 8 genes changed after exposure to ARD stress. The above results provide a theoretical basis for further research on the functions of potassium channel genes in 12-2 and a scientific basis for the breeding of ARD-resistant rootstock.