Premature Fruit Drop Due to Huanglongbing Infection in Citrus Sinensis Is Predicted by Cell Wall Hydrolase Gene Expression

Citrus greening, also called Huanglongbing (HLB), is caused by a fastidious, phloem-restricted bacterium, Candidatus Liberibacter spp.  HLB has been discovered in many major citrus producting countries worldwide and affects all known cultivars.  Specific symptoms on HLB leaves are vein yellowing and blotchy mottle.  Infected fruit are lopsided, smaller in size, poorly  colored and pre-maturely drop at the calyx abscission zone before fully mature.  Trees typically die within 3 to 5 years post-infection.  Our study with Citrus sinensis ‘Valencia’ fruit at the green mature stage indicated 10-fold higher fruit drop in HLB-infected trees compared with healthy trees.  The fruit detachment force (FDF) was reduced 75% in symptomatic fruit when compared with healthy fruit.  The expression of abscission-induced genes such as 1-aminocyclopropane-1-carboxylate synthase (CsACS1), cellulose-a1 (CsCel-a1), polygalacturonase (CsPG), phospholipaseA₂α (CssPLA₂α) and PhospholipaseA₂β (CsPLA₂β) were 17-, 7.4-, 531-, 1.4-, and 1.6-fold induced in symptomatic fruit abscission zones (SFAZ) compared with healthy fruit abscission zones (HFAZ), respectively.  There was no difference of FDF between asymptomatic fruit and healthy fruit.  However, expression of CsACS1 and CsCel-a1 were 5-fold increased and CsPG was 150-fold increased in asymptomatic fruit abscission zones (ASFAZ) compared with HFAZ.  Bacterial titer was higher in SFAZ than ASFAZ.  The titer of bacteria was correlated with percentage of fruit drop.  These results confirm that abscission is induced prematurely in ASFAZ and SFAZ through induction of abscission cell wall hydrolase genes known to be associated with organ loosening.