Evaluation of Resistance and Biochemical Responses of Different Barley Cultivars in Interaction with Meloidogyne incognita

 Meloidogyne incognita is the most well-known root knot nematode, with more than 2000 host species. Integrated nematode management (INM) is recommended to manage the destructive plant parasitic nematode. Integrated management is generally performed by using the maximum available management methods (at least two methods) and the minimum use of chemical nematicides to bring the pathogen population below the economic threshold. The use of resistant cultivars is of particular importance in integrated management, due to environmental compatibility, economic efficiency, and sometimes the impossibility of implementing other methods, especially in developing countries. Therefore, it is necessary to evaluate the resistance of the important barley plant (Hordeum vulgare) to prevent damage and also to investigate nematode interactions with it.

 In the current study, the resistance of different barley cultivars (i.e., Nik, Nimrouz, and Zarjow) was evaluated based on plant growth factors (length, fresh, and dry weight of aerial part and roots) and nematode gall index at 60 days post inoculation. Then the activity of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) enzymes were measured on days 0, 1, 2, 3, and 4 days post inoculation.

 Regarding the mean number of galls, the Nik cultivar showed a significant difference compared to Nimroz and Zarjow cultivars (P ≤ 0.05). However, no significant difference was observed between Nimroz and Zarjow cultivars. Based on plant growth factors, M. incognita nematode was found to have a negative effect on the aerial part length and weight and a positive effect on root weight. Evaluation of the gall index showed Nik is moderately susceptible, and Nimrouz and Zarjow are moderately resistant cultivars. SOD enzyme in Nik, Nimrouz, and Zarjow showed maximum activity in 2.72, 1.91, and 2.15 U mg-1 protein on the 4, 4, and 3 days post inoculation, respectively. The enzyme in Nik was determined to be 1.42 and 1.25 times higher than Nimrouz and Zarjow. There was a significant difference between 0, 1, and 2 with the 3 and 4 days of the infected samples in Nik (P ≤ 0.05). In the other cultivars, enzyme activity increased with a slight slope. CAT enzyme peaked in Nik, Nimrouz, and Zarjow at 0.204, 0.09, and 0.11 μmol min-1 mg-1 protein on the fourth-day post inoculation. In the Nik cultivar, unlike the other two cultivars, the enzyme increased more and had a steep slope from the second to the fourth day. In infected plants of Nimrouz, despite the gradual increase of enzyme, no significant difference was found between any of the days. APX enzyme peaked at 0.26, 0.27, and 0.24 μmol min-1 mg-1 protein in Nik, Nimrouz, and Zarjow on the fourth day, respectively. The activity of the above enzyme had an increasing trend in three cultivars. The maximum activity of this enzyme was at Nimrouz, which was determined to be 1.03 and 1.1 times higher than Nik and Zarjow, respectively. In this cultivar, the upward trend was rapid, although there was a significant difference between all-time points at the level of 0.05. In the current research, it was found that the invasion of the root knot nematode M. incognita reduces the growth of length, fresh and dry weight in the aerial part, reduces the length of the root but increases the fresh weight of it.

The hallmark of inducing pathogenicity in the sedentary root knot nematodes is the formation of special feeding cells named giant cells, which require controlling the expression of host genes and manipulation of plant hormones like auxin and cytokinin hormones. It is obvious that during the invasion of root knot nematodes and the formation of giant cells in host roots, the plant is weakened due to impaired transport of water and nutrients, and the host growth factors, especially in the aerial part, are reduced. However, due to hormonal disorders and the formation of galls, the weight of the roots increases. The higher expression of antioxidant enzymes superoxide dismutase, catalase, and ascorbate peroxidase in Nik possibly has occurred due to the compatible interaction, as a result of lack of necrosis and programmed cell death and to tolerate stress (nematode invasion). Less expression of SOD, CAT, and APX enzymes in Nimrouz and Zarjow cultivars possibly have occurred due to their moderate resistance to M. incognita invasion.