Wound-induced pectin methylesterases enhance banana (Musa spp. AAA) susceptibility to Fusarium oxysporum f. sp. cubense

LiMa 2 ShuangJiang 2 GuimeiLin 1 JianghuaCai 2 XiaoxiYe 2 HoubinChen 2 MinhuiLi 0 HuapingLi 0 TomTak 3 Jozefamaj 3 ChunxiangXu 2 0 College of Natural Resources & Environment, South China Agricultural University , Guangzhou 510642, China 1 Institute of Biotechnology, Guangxi Academy of Agricultural Sciences , Nanning 530007, China 2 College of Horticulture, South China Agricultural University , Guangzhou 510642, China 3 Centre of the Region Han for Biotechnological and Agricultural Research, Department of Cell Biology, Faculty of Science, Palack University , 783 01 Olomouc, Czech Republic Recent studies suggest that plant pectin methylesterases (PMEs) are directly involved in plant defence besides their roles in plant development. However, the molecular mechanisms of PME action on pectins are not well understood. In order to understand how PMEs modify pectins during banana (Musa spp.)-Fusarium interaction, the expression and enzyme activities of PMEs in two banana cultivars, highly resistant or susceptible to Fusarium, were compared with each other. Furthermore, the spatial distribution of PMEs and their effect on pectin methylesterification of 10 individual homogalacturonan (HG) epitopes with different degrees of methylesterification (DMs) were also examined. The results showed that, before pathogen treatment, the resistant cultivar displayed higher PME activity than the susceptible cultivar, corresponding well to the lower level of pectin DM. Asignificant increase in PME expression and activity and a decrease in pectin DM were observed in the susceptible cultivar but not in the resistant cultivar when plants were wounded, which was necessary for successful infection. With the increase of PME in the wounded susceptible cultivar, the JIM5 antigen (low methyestrified HGs) increased. Forty-eight hours after pathogen infection, the PME activity and expression in the susceptible cultivar were higher than those in the resistant cultivar, while the DM was lower. In conclusion, the resistant and the susceptible cultivars differ significantly in their response to wounding. Increased PMEs and thereafter decreased DMs acompanied by increased low methylesterified HGs in the root vascular cylinder appear to play a key role in determination of banana susceptibility to Fusarium. - Banana Fusarium wilt caused by Fusarium oxysporum f. sp. cubense (Foc) is one of the major global constraints limiting banana (Musa spp.) production. Cavendish banana varieties (Musa spp. AAA) are highly susceptible to Foc race 4 (van den Berg etal., 2007). The vascular pathogen penetrates banana roots through the wound of the vascular tissues (Sequeira et al., 1958), and colonizes and occludes the xylem vessels. Fungal proliferation in the xylem causes a reddish-brown discoloration of the rhizome and pseudo-stem, leaf wilting, and ultimately death of the whole plant (Saravanan etal., 2003). The cell wall forms the first barrier for pathogen penetration into the intracellular space in plants. Homogalacturonan (HG) is the major group of cell wall pectins, playing a key role in providing a barrier against environmental stresses including pathogen attacks (Vorwerk etal., 2004). As a strategy to overcome the cell wall barrier, Fusarium oxysporum, a hemibiotrophic and/or necrotrophic pathogen, secretes a substantial array of cell wall-degrading enzymes (CWDEs) such as pectin methylesterases (PMEs), polygalacturonases (PGs), and polymethylgalacturonases (PMGs) to digest the host plant cell wall (Gothoskar et al., 1953; Vorwerk et al., 2004; Cantu etal., 2008). On the other hand, in response to cell wall damage by pathogens, plants have evolved a battery of defence responses including hydroxylation of plant cell wall components and the CWDEs (Lionetti et al., 2007; Cantu et al., 2008). Plant PMEs have major roles in pectin remodelling and they control the methylesterification status of most abundant pectin polysaccharide HG (Pelloux et al., 2007; Wolf et al., 2009). In plantpathogen interactions, some cell wall HG-derived compounds act as elicitors whereas others act as defence suppressors (Vorwerk et al., 2004; Osorio et al., 2008). In addition, PME activities (An et al., 2008; Raiola et al., 2011; Volpi et al., 2011), the degree of HG methylesterification (DM) (Wiethlter etal., 2003; Boudjeko etal., 2006; Lionetti etal., 2007, 2012; Pelloux etal., 2007; Curvers et al., 2010; Raiola et al., 2011), and the molecular properties of HG (Wiethlter etal., 2003; Cantu etal., 2008) have been correlated with changes in the susceptibility of plants to pathogens. More recently, some reports have shown that overexpression of PME inhibitor (PMEI) protein reduced the activity or expression level of PME, resulting in higher resistance of plants to pathogens (Lionetti etal., 2007; An etal., 2008; Volpi etal., 2011). On the other hand, silencing of the PMEI gene enhanced susceptibility of plants to pathogens (An et al., 2008). It was reported that PMEs were involved in plant defence by influencing the pectin structure and DM (Lionetti etal., 2007; Osorio etal., 2008; Raiola etal., 2011; Volpi etal., 2011). Molecular mechanisms of PME action on pectins are not well understood, and only very few studies addressed the role of PMEs in plantpathogen interactions and in the response to wounding. In addition, contradictory data were published on the PME expression level (Raiola etal., 2011; Wojtasik etal., 2011) and cell wall composition changes in response to pathogens in diverse plant species (Simon etal., 2005; Boudjeko etal., 2006). The specific aim of the present study was to understand how PMEs modify specific groups of HGs and to monitor the changes of individual groups of HGs during bananaFusarium interaction. Expression of the MaPME1 gene (PME1 of Musa accuminata) as well as enzyme activities and immunolocalization of PMEs were compared in two banana cultivars, one highly resistant and one susceptible to Fusarium. Furthermore, the spatial distributions of PMEs and their correlations with pectin methylesterification of 10 individual HGs with different DMs were also examined. Materials and methods Musa spp. AAA cv. Yueyoukang 1 and Brazil were selected as plant material. Brazil is susceptible (S) to Foc race 4 whereas Yueyoukang 1, a somaclonal variant, is resistant (R) to this pathogen (Chen etal., 2006). Inoculation of two banana cultivars with pathogen Tissue cultured banana plants were cultured in liquid rooting medium, incubated at 28 2C under light on a reciprocal shaker. Two weeks after induction of roots, one root of each plant was cut off to facilitate the penetration of the pathogen. Afterwards, such treated plants were transferred to medium containing Foc race 4 at a final concentration of 5 102 ml1. Plants transferred to a medium without fungus served as the cut controls, The samples were collected 6 h and 48 h after the transfer, respectively. Intact plants collected 6 h after transfer to new liquid roo (...truncated)