Aspergillus flavus infection induces transcriptional and physical changes in developing maize kernels

Abstract

Maize kernels are susceptible to infection by the opportunistic pathogen Aspergillus flavus. Infection results in reduction of grain quality and contamination of kernels with the highly carcinogenic mycotoxin, aflatoxin. To understanding host response to infection by the fungus, transcription of approximately 9000 maize genes were monitored during the host-pathogen interaction with a custom designed Affymetrix GeneChip® DNA array. More than 4000 maize genes were found differentially expressed at a FDR of 0.05. This included the up regulation of defense related genes and signaling pathways. Transcriptional changes also were observed in primary metabolism genes. Starch biosynthetic genes were down regulated during infection, while genes encoding maize hydrolytic enzymes, presumably involved in the degradation of host reserves, were up regulated. These data indicate that infection of the maize kernel by A. flavus induced metabolic changes in the kernel, including the production of a defense response, as well as a disruption in kernel development.

Keywords: Aspergillus flavus; aflatoxins; genetic; maize; pathogenesis; transcription.

Figure 1

Volcano plot of test results showing statistical significance vs. fold change. Each point represents the results of one gene, where the x-axis is the difference in expression between A. flavus-infected samples and mock-infected samples (log₂ [A. flavus]—log₂ [mock]). The y-axis is the –log₁₀ transformed p-value. The dashed line indicates the significance threshold based on an FDR of 0.05 (Benjamini and Hochberg, 1995); all points above the line are considered statistically significant.

Figure 2

Enzymes in maize seed carbohydrate metabolism whose biosynthetic genes are differentially expressed in the A. flavus-maize interaction. Black arrows denote described carbohydrate metabolic pathways. Up or down directed block arrows indicate increased or decreased expression of genes, respectively. Numbers beside the enzymes corresponds to their description in Table 2.

Figure 3

Mature B73 kernels naturally infected with Aspergillus flavus. A sagittal (1) and frontal (2) and transversal (3) section of healthy B73 kernels (left) were compared to disease kernels (right) to discern any physical changes that occurred as a result of A. flavus. Bolded letters denote kernel parts and tissues: a—crown; b—pericarp; c—aleurone; d—starchy endosperm; e—hard endosperm; f—scutellar tissue; g—leaf primordia (plumule); h—primary root; i—transfer cells; j—pedicel; k—embryo; and l—germ.