Auxin accumulation in cereals after infection by Fusarium graminearum: putative biosynthetic pathways and preferences

Indole-3-acetic acid (IAA) is a major naturally occurring auxin that shows extensive accumulation in cereal plants during the first few days of infection by the phytopathogen Fusarium graminearum. Apart from its positive effects on plant growth, empirical studies have suggested that it is a virulence factor that alters the host's nutritional level and fine-tunes the plant's immune responses, especially salicylic acid-mediated defenses. Plant and fungus genomic studies have predicted that their genomes carry the required genes for L-tryptophan-dependent IAA biosynthetic pathways. In recent decades, genetic and genomic studies have facilitated the description of L-tryptophan (L-TRP)-dependent IAA biosynthetic pathways in F. graminearum and its host plants. The present review illustrates and summarizes the putative and preference molecular networks related to extensive IAA accumulation in wheat heads triggered by infection with F. graminearum, based on the available knowledge about the endogenous IAA biosynthetic pathways in F. graminearum and wheat plants. Meanwhile, infection by F. graminearum could preferentially trigger L-TRP's conversion into serotonin and even phytomelatonin via tryptamine in wheat heads as well. Lower concentrations of them have been shown to stimulate IAA accumulation or mimic IAA to promote plant growth. However, upon that hardly provides sufficient information for regarding alternative methods of controlling scab epidemics. In combination with dissecting IAA biosynthetic pathways using genetic approaches exhibits many difficulties, we thus highlight that ongoing efforts should focus more on identifying the fungal effectors involved in extensive IAA accumulation in cereals in order to understand their potential roles in wheat-F. graminearum interactions. Advancements in molecular breeding programs will further accelerate the application of these molecular targets, allowing for the development of more scab-resistant wheat cultivars and resulting in the effective and environmentally friendly suppression of scab epidemics.