Background

The importance of methylglyoxal in plant response and signaling to various stresses is only just beginning to be understood. Various abiotic stresses have been shown to lead to an accumulation of methylglyoxal in plants (Yadav et al., 2005; Borysiuk et al., 2018). This accumulation suggests that methylglyoxal has a signaling role in plants (Hossain et al., 2009; Hoque et al., 2016; Mostofa et al., 2018). Exogenous application of methylglyoxal has been found to upregulate antioxidant and defense genes in plants corroborating the role methylglyoxal has as a signaling molecule (Kaur et al., 2015; Li et al., 2017). The accumulation of methylglyoxal in plants has also been described in response to biotic stresses including bacterial, viral and fungal infections (Melvin et al., 2017). Recently, it has been shown that cowpea aphid infestation also leads to an increase in methylglyoxal level expanding its role in defense against herbivore pests (MacWilliams et al., 2020). Three methods have been established for quantification of methylglyoxal. Of these three methods, the N-acetyl-L-cysteine method by Wild et al. (2012) has been found to measure methylglyoxal in the most economical and safest way. The other two methods involve expensive enzyme purification or derivatization with an explosive chemical (Racker, 1951; Gilbert and Brandt, 1975). N-acetyl-L-cysteine method involves mixing methylglyoxal with N-acetyl-L-cysteine to generate N-α-acetyl-S-(1-hydroxy-2-oxo-prop-1-yl)cysteine which is detected and measured at absorbance 288 nm by a spectrophotometer (Figure 1). In this protocol, we combine the method by Wild et al. (2012), that simply measures methylglyoxal levels, with extraction of methylglyoxal from plant tissues, and measuring its levels in cowpea (Vigna unguiculata) leaves after cowpea aphid infestation (Aphis craccivora).


Figure 1. Reaction of methylglyoxal with N-acetyl-L-cysteine to form N-α-acetyl-S-(1-hydroxy-2-oxo-prop-1-yl)cysteine. Structures were drawn using the online resource at http://molview.org.