Differential Exudation of Two BenzoxazinoidsOne of the Determining Factors for Seedling Allelopathy of Triticeae Species

Abstract

Benzoxazinoids (Bx) are natural phytotoxins that function as chemical defense compounds in several species. The release of Bx by intact plant roots associated these compounds with root allelopathy in Triticeae species; however, the significance of exudate concentrations of Bx for plant-plant interactions is still a controversial question. A biological screening of 146 cultivars of four Triticeae species (Triticum aestivum L., Triticum durum Desf., Triticum spelta L., and Secale cereale L.) demonstrated a high cultivar dependence to suppress the root growth of Sinapis alba L. by root allelopathy in a dose-response bioassay. Only a few cultivars possessed a marked high or low allelopathic activity, whereby high-performance liquid chromatography-diode array detection analysis of root exudates revealed that these cultivars differed considerably in their ability to exude the two Bx aglucones, DIBOA [2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one] and DIMBOA [2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one]. The total amount of DIBOA and DIMBOA exuded showed a significant correlation to the growth inhibition in bioassay with a statistically estimated contribution to the overall allelopathic effect of 48-72%. In a bioassay with pure phytotoxins, Bx concentrations consistent with the amounts quantified in the screening bioassay caused detrimental effects on S. alba and almost reproduced the statistically estimated contribution. The observed causal association between the allelopathic activity under laboratory conditions and the exudate concentrations of Bx suggests that this association might have implications for the interference of Triticeae species in natural plant communities.

Plants Release Precursors of Histone Deacetylase Inhibitors to Suppress Growth of Competitors

“…The presence of these allelochemicals in the rhizosphere depends on factors such as species, cultivar, or plant density (Huang et al, 2003;Belz and Hurle, 2005;Macías et al, 2014). Maybe the best estimate for naturally occurring APO concentrations can be gained from studies of exudation of the APO precursor DIBOA from roots: In hydroponic cultures, DIBOA released from rye (Secale cereale) and durum (Triticum durum) roots accumulated to concentrations of 300 and 30 µM, respectively (Belz and Hurle, 2005;Macías et al, 2014). Given the equimolar conversion of DIBOA to BOA, and a conversion rate of 10:1 of BOA into APO (Gents et al, 2005;Understrup et al, 2005), this would result in APO concentrations of 30 and 3 µM, respectively.…”

“…The conservation of the target raises the question how the donor can avoid HDAC inhibition by its own allelochemicals. APO and AMPO are the main stable intermediates that usually develop after precursor substances have been released by plant roots into the rhizosphere; thus, these compounds are not directly produced within the donor plant itself (Belz and Hurle, 2005;Macías et al, 2006Macías et al, , 2009Belz, 2007). Accordingly, neither the APO and AMPO precursors DIBOA and DIMBOA nor the BOA and MBOA intermediates had strong effects on HDAC activity (Supplemental Figure 5).…”

“…Grasses in particular have the ability to interfere with and to outcompete neighboring plants via the biosynthesis of phytotoxic root exudates. Cyclic hydroxamic acids (e.g., benzoxazinoids or benzoxazinones), with their main representatives DIBOA [4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one] and its methoxylated analog DIMBOA [2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one], were among the first active allelochemicals identified and are present in root exudates of several major cereal crops (Belz and Hurle, 2005;Macías et al, 2006;Belz, 2007). In nature, these substances typically undergo rapid degradation via the intermediates benzoxazolin-2(3H)-one (BOA) and 6-methoxybenzoxazolin-2(3H)-one (MBOA).…”

Spatial and Temporal Dynamics of Root Exudation: How Important is Heterogeneity in Allelopathic Interactions?

“…Transport of allelochemicals through the CMN may also protect allelochemicals from rapid degradation by moving them away from microbial "hot zones" in the rhizosphere. Others (Belz and Hurle 2005;Tharayil et al 2009Tharayil et al , 2010) have reported diurnal fluctuations in the exudation of allelochemicals, and Thayaril and Triebwasser (2010) suggest that the pulsing of allelochemical release may be important in overcoming biotic and abiotic degradation mechanisms in soil. While we have measured greater release of thiophenes over daylight hr, the magnitude of diurnal variation seems generally modest in comparison to other variation observed in this study.…”

Allelochemicals of the phenoxazinone class act at physiologically relevant concentrations

“…Experiments on durum wheat and rye showed that DIBOA accumulated in the leachate at concentrations of 30 mM and 300 mM, respectively. 7,8 Taking into account a 10:1 conversion rate of DIBOA to APO, 9,10 the resulting APO concentration reaches 3 mM to 30 mM. In our previous experiments we had determined the half-maximal-effect concentration (EC 50 ) of APO to be 75 mM, 2 and thus slightly above the concentrations found in the root leachates.…”