Of course plants can get sick Plant pathogens are a huge focus of agricultural research. Common illnesses would might be familiar with: Dutch Elm Disease, The Irish Potato Famine, canker... the list goes on
as for immune system in plants have it Classes of plant immune responses Basal response: transcription of genes in response to PAMP recognition. Hypersensitive response (HR); apoptosis of cells at the site of infection Systemic acquired immunity: The entire plant becomes resistant to infection Jasmonic acid/ethylene pathway: The entire plant and neighboring plants develop resistance to herbivores. Non-host immunity The basal response This is the response induced by PAMP elicited signaling. The effectors of this response are currently being characterized. Hypersensitive response This is a rapid apoptosis response that kills cells in the area of infection. It can be induced by the interaction of an R gene carrying plant with an Avr carrying microbe. In the lab one can infiltrate bacteria into the whole leaf, causing a massive cell death response but in the field the HR response is likely tiny and limits the growth of microbes to a small area on the leaf. This should stop the growth of biotrophic pathogens that require living tissue in order to survive. Nitric oxide (NO) and hydrogen peroxide (H2O2) regulate the response. The HR can trigger systemic acquired resistance described below. Stanford Microbiology and Immunology 104/204: Innate Immunology, Lecture 4 The immune system in plants Systemic Acquired resistance (SAR) Challenge a leaf with an infectious agent and distal tissues become resistant. The distal tissues have broad resistance – not just to the original pathogen. Can be induced by cell death; either HR or otherwise. The effects are broad range, acting on bacteria, fungi and viruses. A number of genes are induces by SAR but the mechanism behind the resistance in unknown. Salicylic acid must accumulate to induce the SAR but salicylic acid is not the systemic signal; an unidentified lipid likely serves as the signal. An SAR signal (but not salicylate) travels through the plant and increases the resistance of the plant to further infection. This response has found a use in agriculture in the form of inducers of SAR like Actigard, Messenger and Vacciplant. Jasmonic acid response (JA) This chemical is a volatile plant hormone involved in regulating immunity. JA synthesis is induced upon herbivory (crushing wounds). JA induces the transcription of a number of genes that are anticipated to reduce the digestion of the herbivore. For example, the induction of arginase in tomato plants. This reduces the availability of arginine to the insect gut and reduces growth of the caterpillar. Plant cells can recognize PAMPs The Arabidopsis receptor Fls2 is a flagellin receptor The structure is reminiscent of Toll in that the extracellular domain of the protein contains Leucine Rich Repeats. The intracellular domain contains a serine threonine kinase Signaling is transduced through a MAP kinase cascade and activates transcription factors in the WRKY family. As in animals, there are many PRRs in plants that presumably can recognize microbes by more than one PAMP. Fls2 can be shown to have an immune function because loss of function mutations sensitized the plants to infection. Forced expression of the MAP kinases or WRKY29 will force the activation of the pathway and protect the plant from fungal and bacterial infections This suggests that this PAMP activated pathway is required for fighting fungal as well as bacterial infections. The pathway may have originally been identified as responding to bacteria but its output can affect fungal growth as well
