Strigolactones restore vegetative and reproductive developments in Huanglongbing (HLB) affected, greenhouse-grown citrus trees by modulating carbohydrate distribution

Two-year-old greenhouse-grown Huanglongbing (HLB)-affected and healthy 'Hamlin' sweet orange (Citrus sinensis L. Osbeck) trees were spray treated with 10 μM strigolactone (SL) during the floral induction period (November 2016 and January 2017), and the flowering and fruit-setting period (April and June of 2017). Trees were grouped in four treatments: SL-treated HLB-affected trees; untreated HLB-affected trees; SL-treated healthy trees, and healthy control trees. Strigolactone application on HLB-affected trees during floral induction accelerated and augmented reproductive growth by inducing a 2.3-fold increase in flower bearing branches and an average of 2.6-fold increase in flowers per tree within two months from application. On the contrary, the same trees produced 31% less vegetative branches, 55% less flower bearing branches and 85% fewer flowers than HLB trees during the spring flush (April 2017). For the entire season, the number of fruit was higher in HLB + SL trees compared to untreated HLB-affected trees. Strigolactone application on healthy trees increased sucrose and starch content in mature leaves by 26% and 166%, respectively, with a 47% increase in the number of flowers compared to untreated trees. Strigolactone application during the flowering and fruit-setting period induced additional summer flush. The results of iodine staining and analytical observations demonstrated a decline in starch content in leaves and an increase in roots of SL-treated HLB-affected trees. We conclude that strigolactone ameliorates the effect of HLB by balancing vegetative and reproductive developmental response in HLB-affected citrus trees through the partitioning of stored carbohydrate.