Systemic acquired resistance (SAR), a highly desirable form of plant defense, provides broad-spectrum immunity against diverse pathogens. The recent identification of seemingly unrelated chemical inducers of SAR warrants an investigation of their mutual interrelationships. We show that SAR induced by the dicarboxylic acid azelaic acid (AA) requires the phosphorylated sugar derivative glycerol-3-phosphate (G3P). Pathogen inoculation induced the release of free unsaturated fatty acids (FAs) and thereby triggered AA accumulation, because these FAs serve as precursors for AA. AA accumulation in turn increased the levels of G3P, which is required for AA-conferred SAR. The lipid transfer proteins DIR1 and AZI1, both of which are required for G3P- and AA-induced SAR, were essential for G3P accumulation. Conversely, reduced G3P resulted in decreased AZI1 and DIR1 transcription. Our results demonstrate that an intricate feedback regulatory loop among G3P, DIR1, and AZI1 regulates SAR and that AA functions upstream of G3P in this pathway.

This study aimed to evaluate the effect of silicon (Si) rates on some components of sorghum resistance to anthracnose. Two 2×5 factorial experiments, consisting of two sorghum lines (BR005 and BR009, resistant and susceptible, respectively) and five Si application rates (0, 0.06, 0.12, 0.24 and 0.30 g Si kg −1 of soil) were arranged in a completely randomised design with three replications. Plants from both lines were inoculated with a conidial suspension of Colletotrichum sublineolum (1×10 6 conidia ml −1 ) 30 days after emergence. The incubation period (IP), latent period (LP 60 ), area under relative infection efficiency progress curve (AURIEPC), area under anthracnose index progress curve (AUAIPC), final disease severity (FDS), percentage of pigmented leaf area (PLA), and percentage of necrotic leaf area (NLA) were evaluated. Silicon and calcium (Ca) content in leaf tissue of both lines was also determined. The content of Si in leaf tissue increased relative to the control by 55 and 58%, respectively, for the susceptible and resistant lines. There was no significant change in Ca content in leaf tissue for either of the lines; therefore the variations in Si accounted for differences in the level of disease response. The IP for the resistant line was not affected by Si application rates. The LP 60 was not evaluated in the resistant line due to the absence of acervuli. For the resistant line, Si application rates had no significant effect on AUAIPC, FDS, percentage of PLA, and percentage of NLA. On the susceptible line, a quadratic regression model best described the effect of Si application rates on IP, LP 60 , AURIEPC, AUAIPC, FDS, percentage of PLA, and percentage of NLA. The correlation between Si content in leaf tissue of the susceptible line and the AURIEPC, AUAIPC, FDS, PLA, and NLA was negatively significant (r = −0.57, −0.37, −0.40, −0.67, and −0.77, respectively). There was no correlation between Si content and IP or LP 60 . The correlation between the percentage of PLA with the percentage of NLA was negatively significant (r = −0.74). In conclusion, the results from this study underscore the importance of Si in sorghum resistance to anthracnose particularly for the susceptible line.

Shortening the juvenile stage in citrus and inducing early flowering has been the focus of several citrus genetic improvement programs. FLOWERING LOCUS T (FT) is a small phloem-translocated protein that regulates precocious flowering. In this study, two populations of transgenic Carrizo citrange rootstocks expressing either Citrus clementina FT1 or FT3 genes under the control of the Arabidopsis thaliana phloem specific SUCROSE SYNTHASE 2 (AtSUC2) promoter were developed. The transgenic plants were morphologically similar to the non-transgenic controls (non-transgenic Carrizo citrange), however, only AtSUC2-CcFT3 was capable of inducing precocious flowers. The transgenic lines produced flowers 16 months after transformation and flower buds appeared 30–40 days on juvenile immature scions grafted onto transgenic rootstock. Gene expression analysis revealed that the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and APETALA1 (AP1) were enhanced in the transgenics. Transcriptome profiling of a selected transgenic line showed the induction of genes in different groups including: genes from the flowering induction pathway, APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) family genes, and jasmonic acid (JA) pathway genes. Altogether, our results suggested that ectopic expression of CcFT3 in phloem tissues of Carrizo citrange triggered the expression of several genes to mediate early flowering.

Huanglongbing (HLB), a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas), is a major threat to the citrus industry. In a previous study conducted by our laboratory, several citrus transgenic trees expressing the Arabidopsis thaliana NPR1 (AtNPR1) gene remained HLB-free when grown in a field site under high HLB disease pressure. To determine the molecular mechanisms behind AtNPR1-mediated tolerance to HLB, a transcriptome analysis was performed using AtNPR1 overexpressing transgenic trees and non-transgenic trees as control, from which we identified 57 differentially expressed genes (DEGs). Data mining revealed the enhanced transcription of genes encoding pathogen-associated molecular patterns (PAMPs), transcription factors, leucine-rich repeat receptor kinases (LRR-RKs), and putative ankyrin repeat-containing proteins. These proteins were highly upregulated in the AtNPR1 transgenic line compared to the control plant. Furthermore, analysis of protein–protein interactions indicated that AtNPR1 interacts with CsNPR3 and CsTGA5 in the nucleus. Our results suggest that AtNPR1 positively regulates the innate defense mechanisms in citrus thereby boosting resistance and effectively protecting the plant against HLB.