“ <i>Candidatus</i> Liberibacter asiaticus” Infection Induces Citric Acid Accumulation and Immune Responses Mediated by the Transcription Factor CitPH4

Citrus huanglongbing (HLB), caused by "Candidatus Liberibacter" spp., is one of the most disastrous citrus diseases worldwide. HLB-affected citrus fruits are significantly more acidic than healthy fruits. However, the molecular mechanism behind this phenomenon remains to be elucidated. Here, we report that HLB-affected fruits have higher levels of citric acid (CA) than healthy fruits. Moreover, Citrus PH4 (CitPH4), which encodes a MYB transcription factor that functions as a key regulator of CA accumulation, was upregulated in HLB-affected fruits relative to healthy fruits. Heterologous overexpression of CitPH4 in tobacco (Nicotiana tabacum) plants enhanced tolerance to HLB. Subsequently, overexpression and gene-editing experiments indicated that CitPH4 can affect the salicylic acid (SA) pathway, which directly binds to and activates the promoter of CsPBS3, a key gene of SA biosynthesis. HLB-affected fruits had higher SA levels than healthy fruits. Furthermore, application of SA activated CA biosynthesis and application of CA activated SA biosynthesis and signalling in citrus fruits and decreased "Candidatus Liberibacter asiaticus" (CLas) titres in infected leaves. This work suggests that CitPH4 is a key node between CA and SA, thus revealing crosstalk between defence responses and fruit quality in citrus.