Fungal Pathogens of Maize Gaining Free Passage Along the Silk Road

Abstract

Silks are the long threads at the tips of maize ears onto which pollen land and sperm nuclei travel long distances to fertilize egg cells, giving rise to embryos and seeds; however fungal pathogens also use this route to invade developing grain, causing damaging ear rots with dangerous mycotoxins. This review highlights the importance of silks as the direct highways by which globally important fungal pathogens enter maize kernels. First, the most important silk-entering fungal pathogens in maize are reviewed, including Fusarium graminearum, Fusarium verticillioides, and Aspergillus flavus, and their mycotoxins. Next, we compare the different modes used by each fungal pathogen to invade the silks, including susceptible time intervals and the effects of pollination. Innate silk defences and current strategies to protect silks from ear rot pathogens are reviewed, and future protective strategies and silk-based research are proposed. There is a particular gap in knowledge of how to improve silk health and defences around the time of pollination, and a need for protective silk sprays or other technologies. It is hoped that this review will stimulate innovations in breeding, inputs, and techniques to help growers protect silks, which are expected to become more vulnerable to pathogens due to climate change.

Keywords: Aspergillus flavus; Fusarium graminearum; Fusarium verticillioides; Ustilago maydis; ear rot; fungal pathogen; maize; mycotoxin; silk; style.

Figure 1

Maize silks and their normal function. (A) Maize silks emerging on a young cob. (B) Initial pollen entry onto a hair (trichome) of an individual silk: A pollen grain lands on the exposed silk tissue, from which a pollen tube penetrates the silk channel. Silk hairs, or trichomes, help to guide the pollen into the silks. (C) The exit of the sperm nuclei from the pollen tube-silk complex into the ovule leading to double fertilization: The pollen tube reaches the base of the silk, transporting the two sperm cells to their destination in the ovule. The pollen tube is received by the synergid cell within the ovule. One sperm cell will fertilize the egg cell, while the other will fertilize the central cell.