Transcriptional and microscopic analyses of citrus stem and root responses to Candidatus Liberibacter asiaticus infection.

Huanglongbing (HLB) is the most destructive disease that affects citrus worldwide. The disease has been associated with Candidatus Liberibacter. HLB diseased citrus plants develop a multitude of symptoms including zinc and copper deficiencies, blotchy mottle, corky veins, stunting, and twig dieback. Ca. L. asiaticus infection also seriously affects the roots. Previous study focused on gene expression of leaves and fruit to Ca. L. asiaticus infection. In this study, we compared the gene expression levels of stems and roots of healthy plants with those in Ca. L. asiaticus infected plants using microarrays. Affymetrix microarray analysis showed a total of 988 genes were significantly altered in expression, of which 885 were in the stems, and 111 in the roots. Of these, 551 and 56 were up-regulated, while 334 and 55 were down-regulated in the stem and root samples of HLB diseased trees compared to healthy plants, respectively. Dramatic differences in the transcriptional responses were observed between citrus stems and roots to Ca. L. asiaticus infection, with only 8 genes affected in both the roots and stems. The affected genes are involved in diverse cellular functions, including carbohydrate metabolism, cell wall biogenesis, biotic and abiotic stress responses, signaling and transcriptional factors, transportation, cell organization, protein modification and degradation, development, hormone signaling, metal handling, and redox. Microscopy analysis showed the depletion of starch in the roots of the infected plants but not in healthy plants. Collapse and thickening of cell walls were observed in HLB affected roots, but not as severe as in the stems. This study provides insight into the host response of the stems and roots to Ca. L. asiaticus infection.

Huanglongbing alters the structure and functional diversity of microbial communities associated with citrus rhizosphere.

The diversity and stability of bacterial communities present in the rhizosphere heavily influence soil and plant quality and ecosystem sustainability. The goal of this study is to understand how 'Candidatus Liberibacter asiaticus' (known to cause Huanglongbing, HLB) influences the structure and functional potential of microbial communities associated with the citrus rhizosphere. Clone library sequencing and taxon/group-specific quantitative real-time PCR results showed that 'Ca. L. asiaticus' infection restructured the native microbial community associated with citrus rhizosphere. Within the bacterial community, phylum Proteobacteria with various genera typically known as successful rhizosphere colonizers were significantly greater in clone libraries from healthy samples, whereas phylum Acidobacteria, Actinobacteria and Firmicutes, typically more dominant in the bulk soil were higher in 'Ca. L. asiaticus'-infected samples. A comprehensive functional microarray GeoChip 3.0 was used to determine the effects of 'Ca. L. asiaticus' infection on the functional diversity of rhizosphere microbial communities. GeoChip analysis showed that HLB disease has significant effects on various functional guilds of bacteria. Many genes involved in key ecological processes such as nitrogen cycling, carbon fixation, phosphorus utilization, metal homeostasis and resistance were significantly greater in healthy than in the 'Ca. L. asiaticus'-infected citrus rhizosphere. Our results showed that the microbial community of the 'Ca. L. asiaticus'-infected citrus rhizosphere has shifted away from using more easily degraded sources of carbon to the more recalcitrant forms. Overall, our study provides evidence that the change in plant physiology mediated by 'Ca. L. asiaticus' infection could elicit shifts in the composition and functional potential of rhizosphere microbial communities. In the long term, these fluctuations might have important implications for the productivity and sustainability of citrus-producing agro-ecosystems.