Systemic Acquired Resistance and Induced Systemic Resistance in Plants

Jasmonic acid and salicylic acid are plant hormones primarily involved in plant defense against biotic stress. Jasmonic acid regulates flower development, pollen viability, and seed germination while activating defense responses through the systemin-mediated pathway, where wounded leaves release systemin that triggers jasmonic acid biosynthesis in companion cells, leading to long-distance signaling via phloem and activation of defense genes including protease inhibitors. Salicylic acid, discovered in 1937 as 'calorigen' and identified as salicylic acid in 1987, regulates thermogenesis by shifting mitochondrial respiration to the alternative oxidase pathway, delays flower senescence by inhibiting ethylene biosynthesis, and triggers hypersensitive response and systemic acquired resistance (SAR) through the synthesis of pathogenesis-related proteins. Both hormones have distinct biosynthetic pathways: jasmonic acid is synthesized from alpha-linolenic acid through multiple enzymatic steps in plastids and peroxisomes, while salicylic acid is produced via the benzoic acid pathway (in Nicotiana tabacum) or the isochorismate pathway (in Arabidopsis).

Jasmonic acid is a plant hormone synthesized from alpha-linolenic acid through a series of enzymatic reactions including lipoxygenase-mediated oxygenation and hydroperoxide rearrangement to form 12-oxo-phytodienoic acid (OPDA), which is then transported via ABC transporters and converted to jasmonic acid; it functions as a key signaling molecule in plant defense responses against herbivores and pathogens by inducing the expression of defense-related genes that produce proteins making plant tissues unpalatable or indigestible, and it works in conjunction with other stress hormones like salicylic acid and ethylene to coordinate plant responses to environmental stresses.

Plants use specific hormones for defense: salicylic acid against pathogens and jasmonic acid against herbivores. Salicylic acid, extracted from willow (Salix), is the basis for aspirin. It is produced through the shikimic acid pathway from phenylalanine. When infected, salicylic acid signals locally and systemically through phloem transport, and can be released as volatile compounds to warn neighboring plants. This signaling triggers defense responses throughout the plant.

Salicylic acid is a key hormone in plant defense, particularly effective against biotrophic pathogens. It activates defense genes including PR proteins and phytoalexins. Salicylic acid can move systemically through the plant, providing protection to uninfected tissues. Jasmonate is a key hormone in plant defense, particularly effective against necrotrophic pathogens. It works synergistically with ethylene to activate defense genes. Jasmonate is particularly important for defense against insect pests, as it triggers the production of defensive compounds and physical barriers like trichomes.

Salicylic acid (SA) and jasmonic acid (JA) are two recently discovered plant hormones that work in conjunction to regulate plant responses to biotic and abiotic stresses. SA, derived from willow bark and precursor to aspirin, primarily activates defense responses through the NPR1 protein-mediated signaling pathway, while JA regulates development and defense through JAZ repressor degradation. These hormones exhibit complex cross-talk, with SA generally antagonizing JA signaling by sequestering transcription factors, and their biosynthesis occurs in chloroplasts and cytosol through distinct enzymatic pathways. SA promotes thermotolerance through calcium homeostasis and antioxidant system activation, while JA accelerates fruit ripening in non-climacteric fruits.

Plants possess immunity formed through induced resistance (similar to vaccination) and acquired resistance (salicylic acid-dependent). Induced systemic resistance is formed through rhizobacteria synthesizing signaling molecules that induce protection in plant tissues, independent of salicylic acid but under jasmonic acid and ethylene influence. Acquired systemic resistance involves a cascade of defense reactions where a mobile signal is generated at the invasion site and transported throughout the plant, generalizing the induced state. This requires salicylic acid as a signaling molecule and involves phytoalexin synthesis, cell wall changes, and defense enzyme activation including phenylalanine ammonia-lyase (PAL), a key enzyme connecting primary and secondary metabolism.

Systemic acquired resistance (SAR) is a plant defense mechanism where pathogenic attack on one part induces resistance throughout the plant. Infected leaves release volatile salicylic acid, which transmits airborne to adjacent plant parts, providing community-level protection. In contrast, induced systemic resistance (ISR) is activated by non-pathogenic microbes colonizing root zones (like nitrogen-fixing bacteria). SAR involves salicylic acid signaling, while ISR uses jasmonic acid and ethylene pathways. Both mechanisms prime plants for enhanced defense responses, representing sophisticated immune strategies evolved over millions of years of plant-pathogen interactions.

After hypersensitive response occurs locally, systemic acquired resistance (SAR) develops throughout the entire plant body. SAR provides long-lasting, broad-spectrum protection against future pathogen attacks. Additionally, induced systemic resistance (ISR) is activated by non-pathogenic organisms like beneficial rhizobacteria, representing a proactive defense strategy where plants prepare for potential future threats based on past encounters with harmless microorganisms.

Induced Systemic Resistance (ISR) is a plant defense mechanism activated by beneficial rhizobacteria or fungi (PGPR/PGPF) that interact with plant roots, triggering systemic protection against pathogens through signaling pathways involving jasmonic acid, ethylene, and abscisic acid, with key transcription factors like MYC2 and MYB72 playing crucial roles; unlike Systemic Acquired Resistance (SAR), ISR is independent of salicylic acid and provides broad-spectrum protection against both soil-borne and foliar pathogens, offering a sustainable alternative to chemical pesticides in agriculture.

Plants develop resistance through two mechanisms: Systemic Acquired Resistance (SAR) mediated by salicylic acid, triggered by pathogen infection, producing pathogenesis-related proteins (PR1, PR2, kinases, peroxidases) causing localized cell death and membrane reinforcement; and Induced Systemic Resistance (ISR) mediated by jasmonic acid and ethylene, triggered by beneficial organisms like plant growth-promoting fungi or bacteria, providing broad-spectrum resistance against subsequent pathogen attacks.

This lecture introduces fundamental physics concepts including the MKS unit system (meter, kilogram, second), significant figures for precision in measurements, and the distinction between mass and weight. It covers motion along a straight line, defining average velocity as total distance divided by time interval, and instantaneous velocity as the limit of average velocity as the interval approaches zero—a concept fundamental to differential calculus. The lecture traces the historical development of calculus, noting it was independently invented by Isaac Newton and Gottfried Wilhelm Leibniz in the late 17th century, with Leibniz's notation (dx/dt) becoming the standard used today.

本讲座系列系统介绍杨振宁先生对现代物理学的代表性贡献，本期聚焦于李-黄-杨硬核玻色气理论（1957年）。该工作研究了硬球玻色气体的量子行为，引入伪势方法以处理粒子间短程强相互作用，避免直接求解复杂多体波函数。该方法通过有效势替代真实硬核排斥，使理论计算在保持物理本质的同时变得可行。讲座引用原始论文（Lee, Huang, Yang, PR 1957）及费曼相关研究（PR 1954），并参考黄克孙《五十年硬球玻色气》综述，说明该理论对冷原子系统与量子多体物理的深远影响。该工作是理解玻色-爱因斯坦凝聚中相互作用效应的基础，推动了后续量子气体与规范场理论的发展。讲座面向物理研究生、有准备的本科生及研究人员，每两周一次，持续进行，旨在教学性地解析杨振宁理论的物理内涵与数学结构。