Rice physical defenses and their role against insect herbivores

Abstract

Understanding surface defenses, a relatively unexplored area in rice can provide valuable insight into constitutive and induced defenses against herbivores. Plants have evolved a multi-layered defense system against the wide range of pests that constantly attack them. Physical defenses comprised of trichomes, wax, silica, callose, and lignin, and are considered as the first line of defense against herbivory that can directly affect herbivores by restricting or deterring them. Most studies on physical defenses against insect herbivores have been focused on dicots compared to monocots, although monocots include one of the most important crops, rice, which half of the global population is dependent on as their staple food. In rice, Silica is an important element stimulating plant growth, although Silica has also been found to impart resistance against herbivores. However, other physical defenses in rice including wax, trichomes, callose, and lignin are less explored. A detailed exploration of the morphological structures and functional consequences of physical defense structures in rice can assist in incorporating these resistance traits in plant breeding and genetic improvement programs, and thereby potentially reduce the use of chemicals in the field. This mini review addresses these points with a closer look at current literature and prospects on rice physical defenses.

Keywords: Oryza sativa; Callose; Epicuticular wax; Lignin; Silicon; Trichomes.

Fig. 1

Schematic representation of the physical defense structures in rice and the effects against the major feeding guilds: chewing and sucking herbivores. Wax, trichomes, silica, and callose can be considered as the physical defense mechanisms evolved by plants in response to insect herbivory. Among these, epicuticular wax and trichomes can be considered as surface defense structures, and wax is known as the first line of defense acting as a vigorous barrier against rice insect pests regardless of feeding guilds. Wax in rice restricts the free movement of insect pests resulting in the reduction of mass gain and increased mortality. Trichomes on the other hand penetrate the midgut impeding their feeding. Different types of trichomes are seen in rice such as macro trichomes, micro trichomes, prickle, papillae [non-glandular trichomes], and glandular trichomes. Silica is important in the normal growth and development of rice and has a vital role in defense causing mandibular wear and reducing the longevity and fertility in insect pests. Deposition of callose is another defense mechanism by rice on sucking herbivores such as Brown Plant Hopper, by obstructing the sieve tubes, interfering the stylet penetration