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

[摘要] Banana is an important staple food crop however, production of the world's mostwidely planted variety, the Cavendish banana, is threatened by a devastating fungaldisease, called Fusarium wilt. Fusarium wilt of Cavendish bananas in the tropics iscaused by Fusarium oxysporum f. sp. cubense (Foc) 'tropical' race 4 (TR4), while avariant 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 aggravatedafter winter, which suggests that the plant is predisposed to Foc STR4 during coldtemperatures. The objective of this study was to investigate the molecular processes,in both Foc and banana, which contribute to Fusarium wilt development under coldstress.cDNA-AFLP expression profiling was used to elucidate the transcriptome of FocSTR4, Foc TR4 and non-pathogenic F. oxysporum isolates on minimal medium(without carbon source). This resulted in the identification of 229 unique genefragments which included transcript derived fragments (TDFs) encoding for chitinaseclass V (chsV), GTPase activating protein, Major Facilitator Superfamily (MFS)multidrug transporter and serine/threonine protein kinase (ste12) genes. We speculatethat those genes play a role in escaping host defence responses, and result in cell walldegradation. Pathogenicity-related genes identified in other formae speciales of F.oxysporum, such as the sucrose non-fermenting, F-box protein required forpathogenicity genes (frp1) and cyp55, were significantly up-regulated in Foc STR4and Foc TR4, but not in F. oxysporum isolates non-pathogenic to banana. We suggestthat these genes are important for the Fusarium wilt pathogen to enter the host xylemtissue, as they regulate the abundance of cell wall degrading enzymes. The increase inexpression of cyp55 in pathogenic F. oxysporum may give the pathogen the ability toregulate the nitrogen response pathway, which is essential for pathogenicity. Thisstudy provided the first identification of genes in Foc that potentially contribute topathogenicity in banana.Cavendish banana plants subjected to cold temperatures and inoculated with Focresulted in a significant increase in disease severity. Visual symptoms, however, onlyappeared in inoculated plants after they were transferred to 28oC. Transcriptomeanalysis showed that several general defence mechanisms are activated in Cavendish bananas infected with Foc. An important finding was that expression of defencerelatedgenes 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 ligninproduction, and Hin1, involved in the hypersensitive response, was significantlysuppressed at an early stage during cold stress. This potentially provides anopportunity to Foc STR4 to invade the xylem and progress within the vascularbundles before plant defences are activated. Disease development mainly occurs at28oC, as the pathogen prefers higher temperatures for optimal growth and sporulation.At this temperature, movement of water through the vascular vessels of the roots andpseudostem is increased, resulting in the more rapid wilting of affected plants. Thus,cold stress may enhance infection of Foc STR4.

[发布机构] University of Pretoria