Plants are subject to attack and infection by a remarkable variety of symbiotic species and have evolved a diverse array of mechanisms designed to frustrate the potential colonists. These can be divided into preformed or passive defense mechanisms and inducible or active systems. Passive plant defense comprises physical and chemical barriers that prevent entry of pathogens, such as bacteria, or render tissues unpalatable or toxic to the invader. The external surfaces of plants, in addition to being covered by an epidermis and a waxy cuticle, often carry spiky hairs known as trichomes, which either prevent feeding by insects or may even puncture and kill insect larvae. Other trichomes are sticky and glandular and effectively trap and immobilize insects.
If the physical barriers of the plant are breached, then preformed chemicals may inhibit or kill the intruder, and plant tissues contain a diverse array of toxic or potentially toxic substances, such as resins, tannins, glycosides, and alkaloids, many of which are highly effective deterrents to insects that feed on plants. The success of the Colorado beetle in infesting potatoes, for example, seems to be correlated with its high tolerance to alkaloids that normally repel potential pests. Other possible chemical defenses, while not directly toxic to the parasite, may inhibit some essential step in the establishment of a parasitic relationship. For example, glycoproteins in plant cell walls may inactivate enzymes that degrade cell walls. These enzymes are often produced by bacteria and fungi.
Active plant defense mechanisms are comparable to the immune system of vertebrate animals, although the cellular and molecular bases are fundamentally different. Both, however, are triggered in reaction to intrusion, implying that the host has some means of recognizing the presence of a foreign organism. The most dramatic example of an inducible plant defense reaction is the hypersensitive response. In the hypersensitive response, cells undergo rapid necrosis — that is, they become diseased and die — after being penetrated by a parasite; the parasite itself subsequently ceases to grow and is therefore restricted to one or a few cells around the entry site. Several theories have been put forward to explain the basis of hypersensitive resistance.
植物受到各种各样的共生物种的攻击和感染，并且已经发展出各种各样的机制，旨在挫败潜在的殖民者。这些可以分为预制或被动防御机制和诱导或主动系统。被动植物防御包括物理和化学屏障，其阻止病原体（例如细菌）进入，或使组织对入侵者不可接受或有毒。植物的外表面除了被表皮和蜡状角质层覆盖之外，还经常携带称为毛状体的尖状毛发，其可防止昆虫进食或甚至刺穿和杀死昆虫幼虫。其他毛状体具粘性和腺体，有效地捕获和固定昆虫。 如果植物的物理屏障被破坏，那么预先形成的化学物质可能会抑制或杀死入侵者，植物组织中含有多种有毒或潜在有毒物质，如树脂，单宁，甙和生物碱，其中许多都是高度的对以植物为食的昆虫的有效威慑。例如，科罗拉多甲虫在侵袭马铃薯方面的成功似乎与其对通常排斥潜在害虫的生物碱的高耐受性相关。其他可能的化学防御虽然对寄生虫没有直接毒性，但可能会抑制建立寄生关系的一些重要步骤。例如，植物细胞壁中的糖蛋白可以使降解细胞壁的酶失活。这些酶通常由细菌和真菌产生。 活性植物防御机制与脊椎动物的免疫系统相当，尽管细胞和分子基础是根本不同的。然而，两者都是针对入侵引发的，这意味着宿主有一些方法可以识别外来生物的存在。诱导型植物防御反应最引人注目的例子是过敏反应。在过敏反应中，细胞在被寄生虫穿透后经历快速坏死 – 即它们变得患病和死亡;寄生虫本身随后不再生长，因此仅限于入口处周围的一个或几个细胞。已经提出了几种理论来解释过敏抵抗的基础。