Potassium-induced plant resistance against soybean cyst nematode via root exudation of phenolic acids and plant pathogen-related genes

<div><p>Soybean cyst nematode (SCN) is a severe soil borne disease. The control of this disease is still a worldwide problem in agriculture. In this study, we found that application of potassium (K) fertilizer could decrease the occurrence of SCN at two field sites. Furthermore, the application of K could suppress <i>Heterodera glycines</i> with the activation of Phenylalanine Ammonia Lyase (<i>PAL)</i> and Polyphenol Oxidase (<i>PPO)</i> expression via pot experiments in a greenhouse. The release of cinnamic, ferulic and salicylic acids was significantly enhanced by K application of 3 mM, and each of three acids can dramatically constrain <i>Heterodera glycines</i> in vitro. This research indicated that K induce multiple mechanisms to improve the resistance of soybean against SCN and provide a new strategy to control SCN in fields with nutrient application.</p></div

Potassium-induced plant resistance against soybean cyst nematode via root exudation of phenolic acids and plant pathogen-related genes - Fig 5

<p>Effect of applied root exudates from soybean cultivated under different K concentration treatments (a) and of four different species of phenolic acids at four concentrations (b) on the mortality rate of <i>Heterodera glycines</i>. K0, No K application; K1, 1 mM K; K3, 3 mM K; K6, 6 mM K; VA is vanillic acid; FA is ferulic acid; SA is salicylic acid; CA is cinnamic acid. Each histogram represents the mean of four replicates ± SE. Different letters above the bars indicate significant differences (<i>P<0</i>.<i>05</i>) by Duncan’s t-tests.</p

Effect of different K concentrations on cyst nematode number, K concentration, total phenol content and biomass of soybean in the sand culture.

<p>K0, No K application; K1, 1 mM K; K3, 3 mM K; K6, 6 mM K; +<i>H</i>. <i>glycines</i> means inoculation with <i>Heterodera glycines</i>; -<i>H</i>. <i>glycines</i> means no inoculation with <i>Heterodera glycines</i>. Each histogram represents the mean of four replicates ± SE. Different upper-case letters (A, B, C) above the bars indicate significant differences (<i>P<0</i>.<i>05</i>) by Duncan’s t-tests on inoculation with <i>H</i>. <i>glycines</i> treatment. Lower-case letters (a, b, c) mean no inoculation with <i>H</i>. <i>glycines</i> treatment. x,y indicate the different inoculation treatments within the same K concentration treatment. The same designations are used below.</p

Expression of two defense-related genes and their enzyme activity in roots of soybean in response to <i>H</i>. <i>glycines</i> infection under the treatment of different K concentrations in the sand culture experiment.

<p>K0, No K application; K1, 1 mM K; K3, 3 mM K; K6, 6 mM K; +<i>H</i>. <i>glycines</i> means inoculation with <i>Heterodera glycines</i>; -<i>H</i>. <i>glycines</i> means no inoculation with <i>Heterodera glycines</i>.</p

Effect of different K application levels on the disease index (DI) of soybean cyst nematode and cyst nematode number (CNN) in the two field trials over two years.

<p>Effect of different K application levels on the disease index (DI) of soybean cyst nematode and cyst nematode number (CNN) in the two field trials over two years.</p

Effect of the different K application rates on the yield, K concentration and total phenol content of soybean plants in two field trials over two years.

<p>Effect of the different K application rates on the yield, K concentration and total phenol content of soybean plants in two field trials over two years.</p

The exudation rate of four phenolic acids from soybean roots under treatments of different K concentrations in the sandy culture at the flowering stage.

<p>K0, No K application; K1, 1 mM K; K3, 3 mM K; K6, 6 mM K; Each histogram represents the mean of four replicates ± SE. Different letters above the bars indicate significant differences (<i>P<0</i>.<i>05</i>) by Duncan’s t-tests.</p

The correlation coefficient analysis of the two field trials over two years.

<p>(a) between disease index of soybean cyst nematode and cyst nematode number in rhizosphere soil parameter, and (b) between K concentration and total phenol content. For correlation coefficient analysis: <i>R</i>² value *: significant at <i>P</i><0.05, n = 48.</p

Changes in the pH of the black soils under long-term fertilization at Gongzhuling and Harbin.

<p>Changes in the pH of the black soils under long-term fertilization at Gongzhuling and Harbin.</p

Relationships between the of PAC, soil organic matter and changes in the soil Olsen P in black soils under long-term fertilization.

<p>Note: To distinguish the no P and P addition treatments, the changes in soil Olsen P by each 100 kg ha^-1 P balance of no P treatments (CK, N and NK) were defined to the negative values in this figure.</p