**5. Can phytochelatin synthase play a part in the phytoremediation of combined pollutions?**

With global industrialization and the development of modern cropping systems, massive amounts of toxic substances such as pesticides, heavy metals and inorganic fertilizers have been released into the environment and caused massive pollutions [97, 102]. Inevitably, the combined contaminations have damaged the ecosystems and become global issues [103, 104].

In the case of soil pollution, the primary sources of heavy metals include pesticides, fertilizers, mining, industrial processing and wastewater [102, 105]. One example of combined pollutants is glyphosate-based herbicides, which are highly toxic to the environment yet are the most-used pesticides in the world [106]. Heavy metals such as As, Ni, and Pb, which activate the catalytic activity of PCS, can be found as contaminants in many commercial glyphosate-based herbicides [3, 6, 106]. Interestingly, the *in vivo* chronic regulatory experiments showed that the toxicity of these herbicides might come from the heavy metals included in formulants instead of the active ingredients [106]. These findings suggest that heavy metal toxicity may occur in the biological materials used in the phytoremediation of xenobiotic compounds. Thus, heavy metal detoxification mechanisms in phytoremediation plants also need to be considered to improve their performance in co-contaminated soils and groundwaters.

GSH and its derivates are widely involved in plant development and stress response, and GSH itself serves as a hub for the mechanisms of heavy metal detoxification, xenobiotics biodegradation and oxidative stress response [7, 9]. Because PCS is a key enzyme in GSH metabolism, it is not surprising that PCS should be involved in both heavy metal stress and the turnover of xenobiotics. Other critical enzymes in the GSH metabolic pathway such as GST have been used in combating

multiple stresses, including heavy metal and xenobiotics degradation, and biotic stress [107]. However, the role of PCS is primarily emphasized in heavy metal stress response despite its contribution to the degradation of GS-metabolites and innate immunity. Based on the knowledge about the diverse functions of PCS, it is worth exploring whether PCS can be a useful tool in enhancing the tolerance and performance of the plants challenged by combined stresses.
