| One of the benefits of the agricultural use of rye (Secale cereale L.) as a cover crop is the suppression of weed growth, by both physical competition and chemical interference through the production and release of allelopathic chemicals. The main allelochemical in rye is the benzoxazinone (BX) DIBOA (2,4-dihydroxy-2H-1,4-benzoxazin-3(4H )-one). The environmental, physiological, and molecular factors that regulate BX synthesis are not understood, nor are the dates or growth stages for maximum BX content. This research was conducted to elucidate (1) the changes in BX allelochemical levels in fall-planted, field-grown rye shoot tissue during the growing season in North Carolina; (2) the fundamental patterns of BX synthesis in rye shoots and (3) the conditions in which BX synthesis genes are expressed. In understanding more fully rye's natural allelopathic potential, it may be possible to optimize the allelochemical content in foliar tissues to coincide with the needs of the farmer.;Four different rye varieties, two winter and two facultative types were planted in the fall, and 0.5 m2 plots were harvested at intervals the following spring. Two different measures of allelochemical content were taken. Seed germination bioassays were used as an estimate of rye allelopathic potential. Dilutions of aqueous extracts from rye tissue were tested using perennial ryegrass (Lolium perenne) and pigweed (Amaranthus retroflexus) as indicator species to compare the relative toxicity of the tissue harvested at different times during the season. Levels of DIBOA, DIBOA-glycoside and BOA (benzoxazolin-2(3 H)-one, the degradation product of DIBOA) in rye shoot tissue were also directly determined using a GC method. Rye tissue harvested at Julian Date (JD) 37 was the most toxic to both indicator species, and the toxicity decreased over time. The total BX concentration of the tissue increased from JD 37 to JD 65 and then decreased over time. BX levels and phytotoxicity as measured by the bioassay have a similar, but not identical temporal profile. Total BX field rate peaked twice, the first time due to maximum BX concentration, the second time due to biomass accumulation.;To understand the fundamental conditions under which rye plants synthesize BX, the sequence and expression profile of key pathway genes were determined, as well as the concentration of BX in individual leaves of 21 day-old vegetatively growing rye plants. The sequences of the genes coding for anthranilate synthase alpha 1 (ASA1), tryptophan synthase alpha (TSA), the indole-3-glycerol phosphate lyase Bx1 (Bx1) and the cytochrome P450 monooxygenase BX2 (Bx2) were obtained. The rye genes proved to be closely related to homologous genes in other Triticeae. Maximum benzoxazinone levels were measured in the youngest tissue, as were the expression levels of genes Bx1 and Bx2. Bx1 and Bx2 exhibited a coordinated, developmentally-responsive expression patterns, which best followed the expected profile for biosynthesis of the BX found in the tissue. In contrast, ASA1 and TSA were expressed at a constitutive basal level. In order to test the inducibility of the BX synthesis pathway in rye, the concentration of BX and expression levels of genes ASA1, TSA, Bx1 and Bx2 were determined in 21 day-old plants that were treated by wounding or an application of methyl jasmonate. BX levels increased significantly in both mechanically and chemically induced tissue, and the highest response was observed in young tissue. Gene expression, however, did not appear to change following treatment, or the pattern did not reflect expectations for the observed changes in BX biosynthesis.;These results suggest possibilities to increase the levels of allelochemicals in rye cover crops, or that by different management strategies, growers could better maintain the existing levels for longer times. Allelochemicals are important tools in weed suppression and cover crops could be better managed for optimum use of their allelopathic potential. |
