Sustainable agriculture of tropical spices: arbuscular mycorrhizal fungi as an ecotechnological tool − A critical review

A thorough critical analysis of the existing literature on arbuscular mycorrhizal fungi (AMF) in spices provides significant findings. Although the International Standard Organisation (ISO) has listed about 109 plant products as spices and fifty or more spice crops are commonly used worldwide, AMF studies in spices currently are limited. However, a high diversity of forty-one species of AMF is reported from the rhizosphere of ten spices included in this study. Among the AMF species, Glomus spp. (some of which are now known by new names), followed by Acaulospora spp., dominate spice fields. In spices, the AMF provides nutritional benefits such as increased accumulation of P and other nutrients, especially K, N, Fe, Zn, and Mn. Thereby, AMF boosts the photosynthetic activity of spices by enabling them to have more chlorophyll and carotenoid pigments. Thus, AMF directly contributes to diverse plant growth characteristics, including fruiting quality and seed yield in spices. AMF also stimulates secondary metabolite production in spice crops by directly enhancing biomass production or indirectly activating the secondary metabolite synthesis pathway. The synthesis of terpenoids in spices is connected with increased absorption of nutrients such as phosphorus. Thus, AMF contributes indirectly to the quality of spices. Experimental studies state that AMF is crucial in certain spice crops' biofortification, especially selenium (Se). The extra nutritional benefits of AMF in the cultivation of spice crops include overcoming diverse environmental stresses, pests, and diseases. Therefore, AMF is a well-known inevitable biological component in spice fields, essential to sustainable cultivation of spices and improved spice quality. However, among the AMF known in spices, identification up to the species level is not available in many cases. Knowledge of AMF concerning soil types, seasons, and other environmental variables is limited. Moreover, the ecology of AMF in spice fields, especially optimization of factors concerning the effectiveness of specific AMF in specific spices, is significant to its application as a natural tool for sustaining soil fertility and crop productivity. Therefore, intensive research on AMF diversity of more spice crops over diverse soil and agroclimatic regions in different seasons has become essential worldwide. In addition, AMF activity concerning the root morphology and architecture in specific spices is also significant. Such studies need to be complemented with intensive experimental studies for optimization in variables (plant, fungal, and other environmental factors) in the judicious application of AMF in cultivating spices. A comprehensive review of research on all such aspects of AMF applications in spice crops has yet to appear in the literature. Since the global demand for spices is increasing from the diversity of their uses, such as food-flavouring agents, food additives, nutraceuticals, and medicinal sources or ingredients, intensifying an organic means of spice cultivation is a worldwide need today. In the above contexts, the current literature review on AMF in spice fields and AMF applications in the sustainability of spice cultivation is timely and highly relevant. The study concludes by providing the exact required focus of research on AMF concerning spices for the future.