Physiological And Molecular Mechanisms Of Potassium Nutrition To Improve Maize (Zea Mays L.) Resistance To Stalk Rot

Stalk rot is a serious and widespread soil-borne disease in maize, which reduces both yield and quality. Moreover, recently, because the changes of cultivation practices (conservation tillage, straw returning etc) and climatic conditions, stalk rot of maize aggravated year by year in China. Using chemicals to control the disease can not control it completely, and it often cause environment pollution. It has been found that application of potassium (K) fertilizer can result in reduction of maize stalk rot. However, the mechanism of maize resistance to this disease due to K fertilizer application has not been completely understood. Both field experiment and sand-culture experiment were conducted to study the effect of maize K nutrition status on dry matter and NPK accumulation and translocation at entire growth stage of maize; the effect of K nutrition status on phenolic metabolism products ( total phenol and lignin ), the activity of related enzyme and expression of related genes of inoculated stalk pith and young root of maize; the effect of K nutrition status on carbohydrate distribution and sugar metabolism products, the activity of related enzyme and expression of related genes of inoculated young root. Scanning electron microscope and transmission electron microscope were used to observe the effect of K on the ultrastructure of maize stalk pith tissue and young root tip cell influenced by K and pathogen. The main results are summarized as follows:1) Potassium application to maize could reduce the incidence of stalk rot significantly. According to the results from two years field experiments, the control efficiency of K to stalk rot reached to 35%-50%. The K content in vegetative organs, such as root, stalk, leaf, sheath had negative correlation with stalk rot incidence. Potassium application stimulated the accumulation, translocation and distribution of dry matter and NPK. Under K deficient condition, dry matter and NPK accumulation rate declined early, and the amount of nutrients accumulated reduced, resulting in shortage of nutrients supply at later growth stage. Therefore, early senescence occurred in roots and stems when photosynthate and nutrients were deficiency, which limited healthy growth and development of plant.2) Potassium application not only improved the inherent lignin content, but also increased induced lignin and phenol content of stalk pith. K could enhance the activities of phenolic metabolism related enzymes (phenylalanine ammonia-lyase, PAL; peroxidase, POD; polyphenoloxidase, PPO), especially had significant influence on the peak and its appearance time of the enzyme activity. In treatment with K application (+K), the activity of PAL in stalk pith reached maximum at 3 days after inoculation; and the activity of PAL in young root reached peak at 48 h after inoculation. Moreover, the inoculated root in +K treatment had higher pal transcripts as compared with no K treatment (-K). Potassium status in maize plants could also regulate pod gene expression. After inoculation, pod gene expressed later and lower in K deficient root. POD activity of stalk pith and young root induced rapidly to the maximum in K abundant treatment. Moreover, in +K treatment, the transcripts of ppo after inoculation increased 4-folds, which was higher than in–K treatment. When inoculated, K also enhanced PPO activity in both stalk pith and young root. Therefore, K could increase the resistant ability to maize stalk rot, through affecting the expression of pal, pod, ppo, promoting PAL, PPO, POD activities, and meliorating phenol metabolism. In addition, the ferulic acid and chlorogenic acid excreted by root increased significantly in +K treatment after inoculation, which reduced the identification probability between host and pathogen.3) Potassium deficiency caused sugar accumulation in leaf, and K application stimulated sugar translocation from sink organ to source organ. Potassium application also promoted photosynthate distributed to stalk at the later growth stage of maize, and increased sugar content in stalk. At grain filling and maturity stages, the sugar content in maize stalk had negative correlation to stalk rot incidence. Potassium application also increased ss and sps expression in young root after inoculation, enhanced SS and SPS activity, and kept sucrose and glucose content steadily. In–K treatment, ss and sps expressed lower and sps expressed later than that with +K treatment. After inoculation, SPS activity decreased, but SS had little change, resulting in sucrose deficiently in roots. Both sucrose and glucose content reduced in maize root and the reduced rate of sucrose was higher than glucose, so the glucose/sucrose ratio increased significantly. Therefore, K application could increase the resistant ability to maize stalk rot, through affecting the expression of ss and sps, regulating SS and SPS activity, influencing sucrose and glucose content, and maintaining normal sugar metabolism in infected position.4) Potassiun application helped to maintain an integrated cell structure with rectangle arrangement of stem pith. In K deficient treatment, parenchyma cells of stalk pith had abnormity structure and the cell wall between upper and lower adjacent cell was broken, resulting in the loss of connections between vascular cells and bad supporting capacity. In addition, improved K nutrition also helped in keeping a quite tight arrangement of root cell with thick cell wall, prevent the invading of pathogen effectively. Moreover, K treated root cell had abundant golgi apparatus, which could excrete large amount of secretions to degrade mycelium. Papillary and highly electronic intensity dot were accumulated in the invading point to prevent the development of the mycelium. Therefore, improved K nutrition could increase the resistant ability of maize plant to stalk rot, through keeping cell structure stability and preventing the expansion of intracellular space to reduce the chances of pathogen invasions; through accumulation of phenolic compounds to reinforce cell wall; through formation intercellular and intracellular material to restrict pathogen further development in host cell.In conclusion, K is not only a necessary nutrient element for plant growth, but also a resistance element. On the one hand, K helped in control in the expression of plant defense genes involved in phenolic metabolism, produce antibacterial substances, keep cell structure stable, and reinforce cell wall to prevent the invasion and expansion of pathogen; on the other hand, K could also enhance the translocation of dry matter and NPK, regulate the expression the sugar metabolism related genes in invaded site, participate in the conversion of carbohydrate, and maintain sugar metabolism normally, to provide adequate energy and raw materials for defensive reaction. Consequently, K can regulate both plant secondary metabolism and primary metabolism, playing an important role in disease resistance.