Preliminary Study Of Exogenous Jasmonic Acid Effects On Potato Resistance To Late Blight

Potato late blight is one of the most harmful diseases in the world. The P.infestans variation and disease epidemic occur frequently, bringing difficulty to its research and the development of integrated control technique. At present, less attention is focused on pathogenesis mechanism of P.infestans and its toxins, especially the induced resistance of current potato varieties. In this paper, three potato varieties(Mira, S88 and PB06) and three isolates(ZY15,LSX18 and XH05-5-4)of P.infestans from Yunnan province were employed as materials, the pathogenic factor of P.infestans and the effects of exogenous jasmonic acid on potato resistance to late blight have been systematically studied. The results are as follows:1. The P.infestans was cultured in the liquid rye medium for 30 d, crude toxin was obtained after filtration, refined toxin was obtained by (NH4)2SO4 precipitation. The refined toxin was further purified by glucan G-75 chromatography column and SDS polyacrylamide gel electrophoresis. And then, enzyme activity and bioactivity were tested, and the ultrastructure of potato leaf was observed. The results indicate:(1) Both P.infestans and its toxins can be toxic to damage potato. The toxins are some enzymes, its molecular weights is from 20.709kDa to 71.763kDa. It has non-specific poisoning effect on the host, the symptoms still appeared by toxin even after diluted for 200 times. The toxicity is determined by the physiological race of P.infestans, indicating different physiological race may have different toxicity. Different potato varieties have different resistance to toxins, vertical resistance variety is more resistant than horizontal resistant variety and susceptible variety. (2) The photosynthesis and respiration of potato leaf were affected by P.infestans and its toxins, membrane and chloroplast of potato leaf were destroyed, causing eventual leaf death.2. The inhibitory effect of JA to P.infestans was tested, including the resistance evaluation of potato under JA treatment, the activity measurement of PAL, CAT and POD. The results show:(1) Naturally occurring JA was found in potato leaf, the JA content in leaf were different in different potato varieties. The JA content of PB06 was higher than S88 or Mira. While the exogenous JA changed the JA content in potato leaf, but the change range varied with potato varieties and JA treatment concentrations. Meanwhile, JA was found to have immediate inhibitory effect on P.infestans. (2) Resistance of potato to late blight was induced by exogenous JA, while the resistant ability was affected by JA concentrations, potato variety and the organ of the plants. In PB06, JA treatment at 100μmol·L-1 and 200μmol·L-1 were found to be the most effective concentrations for upper part and the tuber respectively. In Mira, the most effective concentrations for upper part and the tuber were 150μmol·L-1 and 200μmol·L-1 respectively. The optimal JA treatment concentration for both the upper part and the tuber of S88 is 150μmol·L-1. The JA content was not correlated to resistance of potato, only the upper part of plant showed increased resistance while the tuber did not show any resistance improvement. Therefore, different JA content will be used to induce resistance for different potato varieties. JA can be used as an inducer when sprayed JA on potato plants in field, inducing resistance of potato to late blight. (3) Activities of resistance related enzymes in potato leaf were enhanced by exogenous JA. The change in enzyme activity varied with potato varieties. There was a negative correlation between enzyme activity and degree of the late blight occurrence. The PAL activity of Mira leaf was enhanced by exogenous JA, both the POD activities of PB06 and S88 were also increased by JA. It was speculated that exogenous JA served as a signal molecule activating the defense enzymes against P.infestans invasion. (4) P.infestansw faced resistance somehow when it invaded the host, depending on potato resistance ability. The resistance ability of PB06 was better than S88 and Mira, because it has less stomatic numbers. No special resistance structure in potato leaf was observed after exogenous JA treatment, however the cell wall was found to be thickened and some small cavities in the chloroplast were discovered under electronic microscope.3. The proteins in potato leaf were separated by 2-D PAGE, protein composition of potato leaf were partly changed by exogenous JA. Three proteins of Mira were lost, while five new specific proteins,8 up-regulated proteins and 3 down-regulated proteins were found in Mira leaf. One new specific protein,13 up-regulated proteins and 8 down-regulated proteins were found in S88 leaf. No new specific protein was found in PB06 leaf, but 10 up-regulated proteins and 10 down-regulated proteins were found, and ten proteins were lost. The proteins induced by exogenous JA in Mira leaf were identified by mass spectrometry. The results show:(1)Three from five new specific proteins were found to be cysteine proteinase inhibitors, the rest two proteins were a threonine deaminase and unknown protein. (2)Two down-regulated proteins were found to be ribulose-1,5- bisphosphate carboxylase/oxygenase(Rubisco) large subunit. (3)Three up-regulated proteins were found to be hypothetical proteins, computer match score is respectively 47,70 and 56. One protein has a match score of 72 for calcium binding protein. Another one protein has a match score of 79 for Rubisco small subunit 3 in the photosynthetic and respiration. One protein matches H+-ATPase B with a score of 170, the amino acids sequence homology was 39%. The protein may have function in signal transduction process of Ca+ and play further roles in plant growth and development or the response to stresses. (4) Histidine kinase, alcohol dehydrogenase and thioredoxin peroxidase were found lost after exogenous JA treatment.In conclusion, exogenous JA has two main effects on potato resistance to P. infestans. Firstly, exogenous JA directly inhibits P.infestans. Secondly, enzyme synthesis of resistance related enzymes is stimulated by exogenous JA as a signal molecule. These enzymes include the cysteine proteinase inhibitor, threonine deaminase and calcium binding proteins for photosynthesis. Therefore, potato shows resistance under exogenous JA by inhibiting P. infestans and improving the life vigor of itself.