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

Abstract

The cultivation of resistant rootstocks is one of the more effective ways to mitigate apple replant disease (ARD). We performed an ion current test, a pot experiment, and a pathogen infection test on the apple rootstocks 12-2 (self-named), T337, and M26. The ion current test showed that exposure to ARD soil extract for 30 min had a significant effect on K+ ion currents at the meristem, elongation, and mature zones of the M26 rhizoplane and on Ca2+ currents in the meristem and elongation zones. ARD also had a significant effect on Ca2+ currents in the meristem, elongation, and mature zones of the T337 rhizoplane. Exposure to ARD soil extract for 5 min had a significant effect on K+ currents in the meristem, elongation, and mature zones of 12-2 and on the Ca2+ currents in the elongation and mature zones. Compared to a 5-min exposure, a 30-min exposure to ARD extract had a less pronounced effect on K+ and Ca2+ currents in the 12-2 rhizoplane. The pot experiment showed that ARD soil had no significant effect on any root architectural or physiological parameters of 12-2. By contrast, ARD soil significantly reduced some root growth indices and the dry and fresh weights of T337 and M26 compared with controls on sterilized soil. ARD also had a significant effect on root metabolic activity, root antioxidant enzyme activity (except superoxide dismutase for T337), and malondialdehyde content of T337 and M26. Pathogen infection tests showed that Fusarium proliferatum MR5 significantly affected the root structure and reduced the root metabolic activity of T337 and M26. It also reduced their root antioxidant enzyme activities (except catalase for T337) and significantly increased the root malondialdehyde content, reactive oxygen levels, and proline and soluble sugar contents. By contrast, MR5 had no such effects on 12-2. Based on these results, 12-2 has the potential to serve as an important ARD-resistant rootstock.

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

“…Semi-quantitative RT-PCR detection and qRT-PCR analysis showed that the 8 potassium channel genes may participate in rhizosphere K + homeostasis of 12-2 by responding to ARD stress. These results also help to explain our previous findings on ion currents in 12-2 roots (Mao et al, 2021b). A study by Fuchs et al (2005) showed that the K + channel gene OsAKT1 regulated the absorption of K + to improve rice salt tolerance, and a study by Jin et al (2017) showed that ion channels could increase plant drought resistance through K + efflux.…”

“…In mid-November 2021, one hundred 12-2 seedlings of similar size with 4-5 leaves were transplanted into black plastic pots (7.0 cm × 5.0 cm × 8.5 cm) filled with sterile substrate after seedling acclimatization. The specialized Fusarium proliferatum strain MR5 (MW600437.1) associated with ARD was recently screened and identified (in review); it is highly pathogenic to apple roots (Mao et al, 2021b) and was discovered by the research group of Professor Mao Zhiquan of Shandong Agricultural University. On November 24, a layer of pathogenic fungi was inoculated in liquid potato dextrose medium (PDB, Haibo, Qingdao, Shandong, China), cultured for 7 days, and then filtered through 8 layers of sterile gauze to obtain a spore suspension.…”

“…In early December, before irrigation with MR5 spore solution, the roots of the controls that had not been irrigated with PDB medium were sampled and recorded as MR5 spore solution treatment for 0 days. Samples were subsequently collected from both treatments at 1, 3, 5, 7, and 9 days (Mao et al, 2021b;Xiang et al, 2021), quick frozen in liquid nitrogen, and stored in a freezer at −80 °C for use in RNA extraction. The RNA extracted at this time was used for qRT-PCR analysis.…”

“…A variety of physiological indexes were measured under ARD, and preliminary results showed that 12-2 had excellent resistance (Mao et al, 2021a). The use of non-damaging and nondestructive testing technology (Xu et al, 2020) to measure changes in K + flow showed that ARD had little effect on K + absorption in 12-2 (Mao et al, 2021b). Although numerous studies have shown that maintaining cell K + balance is very important for plant resistance to various abiotic stresses (Assaha et al, 2017), there are few reports on how K + channels affect plant resistance to ARD.…”

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

“…It is a new line of Malus species that has not been identified previously. In our previous research, ARD had significant effects on several parameters of M.9T337 and M.26 rootstocks, but had no significant effect on 12-2 [16,17]. On this basis, we used 12-2 and PYTC as control, planted in an ARD orchard as experimental rootstock materials and grafted them with 13 apple varieties, including Fuji 2001 and Tianhong 2.…”