Allelopathy in crop/weed interactions — an update

Abstract

Since varietal differences in allelopathy of crops against weeds were discovered in the 1970s, much research has documented the potential that allelopathic crops offer for integrated weed management with substantially reduced herbicide rates. Research groups worldwide have identified several crop species possessing potent allelopathic interference mediated by root exudation of allelochemicals. Rice, wheat, barley and sorghum have attracted most attention. Past research focused on germplasm screening for elite allelopathic cultivars and the identification of the allelochemicals involved. Based on this, traditional breeding efforts were initiated in rice and wheat to breed agronomically acceptable, weed-suppressive cultivars with improved allelopathic interference. Promising suppressive crosses are under investigation. Molecular approaches have elucidated the genetics of allelopathy by QTL mapping which associated the trait in rice and wheat with several chromosomes and suggested the involvement of several allelochemicals. Potentially important compounds that are constitutively secreted from roots have been identified in all crop species under investigation. Biosynthesis and exudation of these metabolites follow a distinct temporal pattern and can be induced by biotic and abiotic factors. The current state of knowledge suggests that allelopathy involves fluctuating mixtures of allelochemicals and their metabolites as regulated by genotype and developmental stage of the producing plant, environment, cultivation and signalling effects, as well as the chemical or microbial turnover of compounds in the rhizosphere. Functional genomics is being applied to identify genes involved in biosynthesis of several identified allelochemicals, providing the potential to improve allelopathy by molecular breeding. The dynamics of crop allelopathy, inducible processes and plant signalling is gaining growing attention; however, future research should also consider allelochemical release mechanisms, persistence, selectivity and modes of action, as well as consequences of improved crop allelopathy on plant physiology, the environment and management strategies. Creation of weed-suppressive cultivars with improved allelopathic interference is still a challenge, but traditional breeding or biotechnology should pave the way.

Effect of sorghum seedlings, and previous crop, on soil fluorescent Pseudomonas spp.

“…Of particular interest have been potentially allelopathic compounds that may inhibit growth of weeds (Belz 2007), but also have been implicated in allelopathic interactions with crop plants (Roth et al 1999;Wu et al 2007). Besides effects on plants, allelochemicals may be involved in modifications of physical properties of soil or activities of soil microorganisms (Bertin et al 2003).…”

“…Wheat root exudates that may have allelopathic potential are phenolic compounds (Wu et al 2001). The major group of root exudates from sorghum with allelopathic properties is the benzoquinonones called sorgoleone {2-hydroxy-5-methoxy-e- [(8Z,11Z-8,11,14-pentadecatriene]-p-benzoquinone} (Chang et al 1986;Belz 2007). Production of allelopathic phenolics by wheat seedling roots grown on agar medium have been shown to vary by types and levels, depending upon the cultivar (Wu et al 2001).…”

Temporal expression of the sor1 gene and inhibitory effects of Sorghum bicolor L. Moench on three weed species

“…However, the efficacy of control against some weeds might not occur immediately, as does that of commercial herbicides, but could build over time, as with most natural products (Belz 2007). Netzley et al (1988) reported that the concentration of sorgoleone exuded into the soil from S. bicolor roots can reach 10-100 μM.…”

“…Such chemicals therefore constitute an important mediator of population dynamics because they determine the pattern and density of vegetation, in natural and crop systems alike. Numerous plants have been reported to be allelopathic, including lichens, alfalfa, cucumber, rye, barley, wheat, rice, soybeans, and sorghum (Singh et al 2003;Belz 2007;Albuquerque et al 2010).…”

Allelochemicals of the phenoxazinone class act at physiologically relevant concentrations

“…2 This particular class of allelochemicals derives from the cyclic hydroxamic acids DIBOA (4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one) and DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one), which are contained in root exudates from many grasses, among them several major crops such as wheat, rice and maize. 3,4 We were able to show that APO and AMPO have the potential to bind to histone deacetylases (HDACs), 2 an evolutionarily highly conserved class of enzymes that govern the accessibility of the DNA by removal of acetyl groups from various amino acid residues of histone proteins. Histone acetylation is a dynamic epigenetic mark that correlates with relaxed chromatin conformation, accessible DNA and transcriptional activity (Fig.…”