Plant Science

AbstractZinnia peruviana (Asteraceae) is an annual flowering plant cultivated in various gardens of Mysore, the south of Karnataka state, India. During 2021, phytoplasma‐like symptoms such as virescence and phyllody were observed on Zinnia in Mysore local gardens. A total of 10 symptomatic and five asymptomatic samples were collected, and the DNA was amplified by PCR using primers specific to 16S RNA and secA genes. The nucleotide sequence analysis showed that the 16S RNA and SecA gene sequences were most similar to those of ‘Ca. P. aurantifolia’, with identities greater than 99%. The phytoplasma sequence from this study shared a common clade with ‘Ca. P. aurantifolia’ when phylogenetic analysis was done. Similarly, the reference sequences for the 16S rRNA and SecA region confirmed the identity of the phytoplasma associated with the leaves of Zinnia. This is the new host record for ‘Ca. P. aurantifolia’ (16SrII‐D) phytoplasma associated with Z. peruviana from India.

AbstractInfection with the bacterium ‘Candidatus Liberibacter solanacearum’ (Lso) is suspected to cause severe damage in carrot leading to high carrot weight loss. This study investigates three main aspects: (i) whether there is a reduction of carrot root weight under field conditions due to Lso infection; (ii) the correlation between Lso infection rate in carrot plants and occurrence of the psyllid Trioza apicalis as the known vector for Lso in carrot, and (iii) the comparison between symptoms described in literature and observed symptoms associated with Lso infection. Therefore, field surveys were conducted from 2018 to 2021 on organically managed carrot fields in Lower Saxony, Germany. Two Rebell orange sticky traps were placed per field replaced on a weekly basis. Captured T. apicalis were morphologically identified and counted. Carrot plant samples were collected from T. apicalis infested fields. Discolouration of foliage and further symptoms on carrots were investigated. For detection of Lso in carrot plants and psyllids, the samples were analysed using PCR. The infection rate of carrot plants varied between 2.5% and 80% per field with low abundance of T. apicalis. Of the asymptomatic plants, 39.4% were infected with Lso, while 80.9% of the symptomatic plants were infected with Lso. The weight of Lso-positive carrot samples was not significantly reduced compared to Lso-negative samples. No economically relevant losses of carrot yield were reported by farmers participating in this study. This indicates that there was no effect of Lso infection on carrot root weight during this study. Of the symptoms associated with a Lso infection in carrots, only leaf discolouration could be confirmed. Overall, the bacterium is present in organically grown carrot plants in Lower Saxony but no major carrot root weight losses were observed.

Citrus Huanglongbing, one of the most devastating citrus diseases, is caused by Candidatus Liberibacter asiaticus (CLas). Polyamines are aliphatic nitrogen-containing compounds that play important roles in disease resistance. However, the role of polyamine metabolism in the tolerance of citrus to infection with CLas have not been extensively studied. We used HPLC and UPLC-Q/TOF-MS to detect the contents of nine polyamine metabolism-related compounds (PMRCs) in six citrus cultivars with varying levels of tolerance to CLas. Moreover, we systematically detected the changes of PMRC and H2O2 contents, and compared the gene expression levels and activities of enzymes involved in the polyamine metabolic pathway among healthy, asymptomatic, and symptomatic leaves of Newhall navel oranges infected with CLas. The tolerant and moderately tolerant varieties showed higher PMRC levels than those of susceptible varieties. Compared with the healthy group, the symptomatic group showed significantly increased contents of arginine, ornithine, γ-aminobutyric acid, and putrescine by approximately 180, 19, 1.5, and 0.2 times, respectively, and up-regulated expression of biosynthetic genes. Arginase and ornithine decarboxylase enzyme activities were the highest in the symptomatic group, while arginine decarboxylase and agmatine deiminase enzyme activities were the highest in the asymptomatic group. The two polyamine biosynthetic pathways showed different trends with the increase of the titer of CLas, indicating that polyamines were mainly synthesized through the arginine decarboxylase pathway in the asymptomatic leaves, and were synthesized via the ornithine decarboxylase pathway in symptomatic leaves. These findings provide new insight into the changes of polyamine metabolism in citrus infected with CLas.

Abstract Huanglongbing (HLB) primarily caused by Candidatus Liberibacter asiaticus (CLas) has been threatening citrus production globally. Under HLB conditions, an excessive accumulation of the polysaccharide callose in citrus phloem occurs, leading to phloem blockage and starch accumulation in leaves. The callose production is controlled by callose synthases (CalS), which have multiple members within plants. However, the knowledge of callose production in the citrus upon infection with CLas is limited. In this study, we firstly identified 11 CalSs in the Citrus sinensis genome through bioinformatics and found the expression pattern of CsCalS11 exhibited a positive correlation with callose deposition in CLas-infected leaves (correlation coefficient of 0.77, P ≤ 0.05). Knockdown of CsCalS11 resulted in a reduction of callose deposition and starch accumulation in CLas-infected citrus. Interestingly, we observed significantly higher concentrations of abscisic acid (ABA) in HLB-infected citrus leaves compared to uninfected ones. Furthermore, the expressions of CsABI5, CsPYR, and CsSnRK2 in the ABA pathway substantially increased in citrus leaves upon CLas infection. Additionally, the expression of CsCalS11 was significantly upregulated in citrus leaves following the application of exogenous ABA. We confirmed that CsABI5, a pivotal component of the ABA signaling pathway, regulates CsCalS11 expression by binding to its promoter using yeast one-hybrid assay, dual luciferase assay, and transient expression in citrus leaves. In conclusion, our findings strongly suggest that the CsABI5-CsCalS11 module plays a crucial role in regulating callose deposition through the ABA signaling pathway during CLas infection. The results also revealed new function of the ABA signaling pathway in plants under biotic stress.

Abstract Phytoplasmas manipulate host plant development to benefit insect vector colonization and their own invasion. However, the virulence factors and mechanisms underlying small-leaf formation caused by jujube witches’ broom (JWB) phytoplasmas remain largely unknown. Here, effectors SJP1 and SJP2 from JWB phytoplasmas were identified to induce small-leaf formation in jujube (Ziziphus jujuba). In vivo interaction and expression assays showed that SJP1 and SJP2 interacted with and stabilized the transcription factor ZjTCP2. Overexpression of SJP1 and SJP2 in jujube induced ZjTCP2 accumulation. In addition, the abundance of miRNA319f_1 was significantly reduced in leaves of SJP1 and SJP2 transgenic jujube plants and showed the opposite pattern to the expression of its target, ZjTCP2, which was consistent with the pattern in diseased leaves. Overexpression of ZjTCP2 in Arabidopsis promoted ectopic leaves arising from the adaxial side of cotyledons and reduced leaf size. Constitutive expression of the miRNA319f_1 precursor in the 35S::ZjTCP2 background reduced the abundance of ZjTCP2 mRNA and reversed the cotyledon and leaf defects in Arabidopsis. Therefore, these observations suggest that effectors SJP1 and SJP2 induced small-leaf formation, at least partly, by interacting with and activating ZjTCP2 expression both at the transcriptional and the protein level, providing new insights into small-leaf formation caused by phytoplasmas in woody plants.

IntroductionAsian citrus psyllid (ACP, Diaphorina citri) is an important transmission vector of “Candidatus Liberibacter asiaticus” (CLas), the causal agent of Huanglongbing (HLB), the most destructive citrus disease in the world. As there are currently no HLB-resistant rootstocks or varieties, the control of ACP is an important way to prevent HLB. Some viruses of insect vectors can be used as genetically engineered materials to control insect vectors.MethodsTo gain knowledge on viruses in ACP in China, the prevalence of five RNA and DNA viruses was successfully determined by optimizing reverse transcription polymerase chain reaction (RT-PCR) in individual adult ACPs. The five ACP-associated viruses were identified as follows: diaphorina citri bunyavirus 2, which was newly identified by high-throughput sequencing in our lab, diaphorina citri reovirus (DcRV), diaphorina citri picorna-like virus (DcPLV), diaphorina citri bunyavirus (DcBV), and diaphorina citri densovirus-like virus (DcDV).ResultsDcPLV was the most prevalent and widespread ACP-associated virus, followed by DcBV, and it was detected in more than 50% of all samples tested. DcPLV was also demonstrated to propagate vertically and found more in salivary glands among different tissues. Approximately 60% of all adult insect samples were co-infected with more than one insect pathogen, including the five ACP-associated viruses and CLas.DiscussionThis is the first time these viruses, including the newly identified ACP-associated virus, have been detected in individual adult ACPs from natural populations in China’s five major citrus-producing provinces. These results provide valuable information about the prevalence of ACP-associated viruses in China, some of which have the potential to be used as biocontrol agents. In addition, analysis of the change in prevalence of pathogens in a single insect vector is the basis for understanding the interactions between CLas, ACP, and insect viruses.