General Plant Health & Protection Biology 4399/5399 Lecture 2: Disease Development

Infectious diseases: are those that result from infection of a plant by a pathogen. In such diseases, the pathogen can grow and multiply rapidly on diseased plants, it can spread from diseased to healthy plants, and it can cause additional plants to become diseased, thereby leading to the development of a small or large epidemic. Parasite & Plant parasite: Pathogenicity: is the ability of the parasite to interfere with one or more of the essential functions of the plant, thereby causing disease. Biotroph: An organism that derives nutrients from the living tissues of another organism (its host). Necrotroph: 1. An organism that kills part or all of another organism before deriving nutrients from it (usually applied to plant pathogens). 2. An organism that derives nutrients from dead plant or animal tissues, whether or not it is responsible for the death of those tissues. Some terminology 2

Some terminology (cont’d) Obligate parasite. An organism that they can grow and reproduce in nature only in living hosts. It is incapable of living as a saprophyte and must live as a parasite. Saprobe. An organism that obtains its nutrients from non-living organic matter (commonly dead and decaying plant or animal matter) by absorbing soluble organic compounds. (Also saprotroph.) Nonobligate parasites: Semibiotroph / facultative saprophytes: Obligate and nonobligate parasites generally differ in the ways in which they attack their host plants and procure their nutrients from the host. Parasitism of cultivated crops is a common phenomenon. In North America, Fungi : more than 8,000 species cause nearly 100,000 diseases; Bacteria : at least 200; Mollicutes : ~75; Viruses : more than 1,000; 40 viroids ; Nematodes : and more than 500 species attack crops. 3

Host Range of Pathogens Pathogens differ with respect to the kinds of plants that they can attack, the organs and tissues that they can infect, the age of the organ or tissue of the plant on which they can grow. Obligate parasites: usually more specific but with exceptions e.g. viruses and nematodes Nonobligate parasites: 4

Development of diseases 5

Disease Cycle 6

Inoculation, penetration, establishment of infection, colonization (invasion), growth and reproduction of the pathogen, dissemination of the pathogen, and survival of the pathogen in the absence of the host, i.e., overwintering or oversummering (overseasoning) of the pathogen. Primary events of disease cycle 7

Inoculation o Types of inoculum: primary and secondary inoculum o Sources of inoculum: o Landing or Arrival of Innoculum Prepenetration o Attachment of pathogen to host o Spore germination and perception of host surface: cAMP pathway and MAPK cascade signaling o Appressorium formation and maturation: o Recognition between host and pathogen o Hatching of nematode eggs: second-stage juveniles in the eggs Different stages of disease development 8

Types of inoculum and ways in which some pathogens enter a host plant 9

Penetration o Direct penetration through intact plant surfaces o Through wounds o Through Natural openings: stomata, hydathode, nectarthode, lenticels Infection o Invasion o Colonization (growth and reproduction of pathogen) Dissemination of the pathogen o By air o By water o By Insects, Mites, Nematodes and other vectors o By pollen, seed, trans plants, budwood and nursery stock Different stages of disease development (cont’d) 10

11

Methods of germination and penetration by fungi 12

13

14

Pattern recognition receptor (PRR) PAMP (flagellin, flg22 peptide) PAMP-triggered immunity (PTI) Pseudomonas Plant cell AvrPto Type III effectors AvrPtoB ? ~30 type III effectors Type III effectors AvrPto and AvrPtoB suppress PAMP-triggered immunity Effector-triggered immunity (ETI) including HR/PCD Resistance protein complex Prf Pto = ser/thr protein kinase Prf = NBARC-LRR protein Pto is a member of a clustered gene family. and elicit Pto/Prf-mediated immunity Pto 15

16

17

Types of invasion of pathogens 18

19

Overwintering and/or Oversummering of pathogens 20

Disease cycles and epidemics Smuts Tree rusts Some tree diseases Downy mildew Powdery mildew Leaf sports and blights Grain rusts etc “Polyetic” 21

Effects Of Pathogens On Plant Physiological Functions Reducing the photosynthesis capability of plants by pathogens What is photosynthesis Diseases that destruct of leaf tissues: leaf spot, viral mosaics, yellowing Some other diseases: reducing/affecting chloroplasts (e.g. chlorophyll) Affecting translocation of water and nutrients in host plants Upward of water and inorganic nutrients Absorption of water by roots: damping-off fungi, root-rotting fungi, bacteria, most nematodes, some viruses Translocation of water through xylem: damping-off, stem rots, and cankers-causing fungi and bacteria. The most typical and complete dysfunction of xylem in translocating water, however, is observed in the vascular wilts (Figs. 3-3 and 3-5) caused by the fungi Ceratocystis, Ophiostoma, Fusarium, and Verticillium and bacteria such as Pseudomonas, Ralstonia, and Erwinia. 22

23

24

25

26

Effects Of Pathogens On Plant Physiological Functions (cont’d) Increased transpiration: affect a plant protection mechanism Reduce translocation of organic nutrients through phloem: plastodesmata, then phloem Affecting host plant respiration (oxidative phosphorylation): Respiration of diseased plants generally increased: use up their reserve carbohydrates faster Effect on cell membranes by pathogens Structure and functions of cell membrane Change of cell membrane permeability: electrolytes (small water-soluable ions and molecules from the cell) Affect host transcription and translation Transcription: viruses and obligate fungi Translation: Affect host plant growth and reproduction : indirect and direct effects 27

Cellular respiration process 28

29

30

31

32

33

34

35

36

37

38

39

Central dogma of genetic information flow , ,-4 Features of processes Accuracy - relative Signals – Starts and stops Stages –Initiation, elongation, termination Genome Transcriptome Proteome

41