A few sequence polymorphisms among isolates of Maize bushy stunt phytoplasma associate with organ proliferation symptoms of infected maize plants

Abstract

Background and Aims Maize bushy stunt phytoplasma (MBSP) is a bacterial pathogen of maize (Zea mays L.) across Latin America. MBSP belongs to the 16SrI-B sub-group within the genus ‘Candidatus Phytoplasma’. MBSP and its insect vector Dalbulus maidis (Hemiptera: Cicadellidae) are restricted to maize; both are thought to have coevolved with maize during its domestication from a teosinte-like ancestor. MBSP-infected maize plants show a diversity of symptoms. and it is likely that MBSP is under strong selection for increased virulence and insect transmission on maize hybrids that are widely grown in Brazil. In this study it was investigated whether the differences in genome sequences of MBSP isolates from two maize-growing regions in South-east Brazil explain variations in symptom severity of the MBSP isolates on various maize genotypes. Methods MBSP isolates were collected from maize production fields in Guaíra and Piracicaba in South-east Brazil for infection assays. One representative isolate was chosen for de novo whole-genome assembly and for the alignment of sequence reads from the genomes of other phytoplasma isolates to detect polymorphisms. Statistical methods were applied to investigate the correlation between variations in disease symptoms of infected maize plants and MBSP sequence polymorphisms. Key Results MBSP isolates contributed consistently to organ proliferation symptoms and maize genotype to leaf necrosis, reddening and yellowing of infected maize plants. The symptom differences are associated with polymorphisms in a phase-variable lipoprotein, which is a candidate effector, and an ATP-dependent lipoprotein ABC export protein, whereas no polymorphisms were observed in other candidate effector genes. Lipoproteins and ABC export proteins activate host defence responses, regulate pathogen attachment to host cells and activate effector secretion systems in other pathogens. Conclusions Polymorphisms in two putative virulence genes among MBSP isolates from maize-growing regions in South-east Brazil are associated with variations in organ proliferation symptoms of MBSP-infected maize plants.

Incidence of corn stunt disease in off-season corn hybrids in different sowing seasons

“…Therefore, the incidence of corn stunt disease increased with the delay in sowing (Figure 2), as also observed by Oliveira et al (2003). This may be explained by the fact that corn leafhoppers, which acquire Mollicutes by ingesting the infected sap of diseased plants, only become permanently infectious after three to four weeks (Orlovskis et al, 2017). In addition, since corn leafhoppers migrate from older to younger plants , later plantings also resulted in the exposure to an increased number of infectious insect vectors.…”

“…Mollicutes interfere with corn plant growth and development, reduce nutrient uptake, and affect photoassimilate translocation for grain filling (Magalhães et al, 2005), stimulating the proliferation of unproductive ears (Oliveira et al, 2008;Teixeira et al, 2013;Orlovskis et al, 2017) and decreasing plant resistance to water stress (Magalhães et al, 2001). This explains the greater difference in grain yield between resistant and susceptible hybrids in the later planting, when stunt disease incidence was the highest.…”

Phytoplasmas–The “Crouching Tiger” Threat of Australian Plant Pathology

“…To date, complete genomes of six phytoplasma strains (Oshima et al, 2004; Bai et al, 2006; Kube et al, 2008; Tran-Nguyen et al, 2008; Andersen et al, 2013; Orlovskis et al, 2017) and an additional 16 draft or incomplete genomes have been reported (refer Genomes OnLine database [GOLD]; https://gold.jgi.doe.gov). …”

Phytoplasma SAP11 effector destabilization of TCP transcription factors differentially impact development and defence of Arabidopsis versus maize

“…This is in agreement with BRC1 not directly affecting A. thaliana flower architecture [52,53], and maize TB1 being a direct positive regulator of MADS-box transcription factors that control maize female inflorescence architecture [40]. Interestingly, many phytoplasmas have SAP54 effectors, which degrade MADS-box transcription factors leading to the formation of leaf-like sterile flowers [9,10,54,55] whereas no effector gene with sequence similarity to SAP54 was identified in MBSP [56]. It is possible that the maize-specialist phytoplasma strain does not require an additional effector (such as SAP54) to modulate floral development of its host, as SAP11 MBSP indirectly targets flowering via TB1.…”

“…MBSP and the insect vectors D. maids and D. elimatus are thought to have co-evolved with maize since its domestication from teosinte [23]. We previously sequenced the genomes of MBSP isolates from geographically distant locations and found single nucleotide polymorphisms (SNPs) throughout the genomes of these isolates but that SAP11 MBSP remained conserved [56]. The effector may be subject to purifying selection because the destabilization of maize TB1 TCPs and subsequent induction of axillary branching and inhibition of female flower production promote MBSP fitness in maize in a manner that is so far unknown.…”