The effect of cold stress on resistance in Cavendish bananas to Fusarium wilt

Abstract

Banana is an important staple food crop however, production of the world’s most widely planted variety, the Cavendish banana, is threatened by a devastating fungal disease, called Fusarium wilt. Fusarium wilt of Cavendish bananas in the tropics is caused by Fusarium oxysporum f. sp. cubense (Foc) ‘tropical’ race 4 (TR4), while a variant of the fungus causing the disease in the subtropics is called Foc ‘subtropical’ race 4 (STR4). The incidence of Fusarium wilt in the subtropics is usually aggravated after winter, which suggests that the plant is predisposed to Foc STR4 during cold temperatures. The objective of this study was to investigate the molecular processes, in both Foc and banana, which contribute to Fusarium wilt development under cold stress. cDNA-AFLP expression profiling was used to elucidate the transcriptome of Foc STR4, Foc TR4 and non-pathogenic F. oxysporum isolates on minimal medium (without carbon source). This resulted in the identification of 229 unique gene fragments which included transcript derived fragments (TDFs) encoding for chitinase class V (chsV), GTPase activating protein, Major Facilitator Superfamily (MFS) multidrug transporter and serine/threonine protein kinase (ste12) genes. We speculate that those genes play a role in escaping host defence responses, and result in cell wall degradation. Pathogenicity-related genes identified in other formae speciales of F. oxysporum, such as the sucrose non-fermenting, F-box protein required for pathogenicity genes (frp1) and cyp55, were significantly up-regulated in Foc STR4 and Foc TR4, but not in F. oxysporum isolates non-pathogenic to banana. We suggest that these genes are important for the Fusarium wilt pathogen to enter the host xylem tissue, as they regulate the abundance of cell wall degrading enzymes. The increase in expression of cyp55 in pathogenic F. oxysporum may give the pathogen the ability to regulate the nitrogen response pathway, which is essential for pathogenicity. This study provided the first identification of genes in Foc that potentially contribute to pathogenicity in banana. Cavendish banana plants subjected to cold temperatures and inoculated with Foc resulted in a significant increase in disease severity. Visual symptoms, however, only appeared in inoculated plants after they were transferred to 28oC. Transcriptome analysis showed that several general defence mechanisms are activated in Cavendish bananas infected with Foc. An important finding was that expression of defencerelated genes was delayed in cold-treated plants, which enhance disease severity. More specifically, the induction of PR genes (PR-1, PR-4, PR-6 and PR-10), C4H, involved in phenylpropanoid pathway and thus important for phytoalexin and lignin production, and Hin1, involved in the hypersensitive response, was significantly suppressed at an early stage during cold stress. This potentially provides an opportunity to Foc STR4 to invade the xylem and progress within the vascular bundles before plant defences are activated. Disease development mainly occurs at 28oC, as the pathogen prefers higher temperatures for optimal growth and sporulation. At this temperature, movement of water through the vascular vessels of the roots and pseudostem is increased, resulting in the more rapid wilting of affected plants. Thus, cold stress may enhance infection of Foc STR4.