Allelopathy is prevalent in agricultural ecosystems and mediated by plant-derived secondary metabolites (allelochemicals).  Allelochemicals are released by donor plants and affect the root growth and development of receptor plants.  Allelopathy is responsible for the continuous cropping obstacles in cucumber (Cucumis sativus L.).  p-Hydroxybenzoic acid (pHBA), an autotoxin from root exudates of cucumber, has been proposed to be an important allelopathic chemical.  However, the molecular mechanism by which pHBA affect root growth and development in cucumber is unknown.  Here, we found that pHBA treatment suppressed root growth of cucumber by reducing the meristem activity and cell length.  This root growth defect is caused by reduced reactive oxygen species (ROS) accumulation in root tips.  After pHBA treatment, the expression levels of several ROS-scavenging-related genes were increased, including peroxidase (POD), catalase (CAT) and metallothionein (MT).  Moreover, exogenously application of salicylhydroxamate (SHAM), a peroxidase inhibitor, can partially restore the pHBA treatment induced root growth inhibition.  Furthermore, we found that there is natural variation for the inhibitory effect of pHBA on root growth.  We also showed that pHBA treatment could maintain higher level of ROS accumulated in the pHBA less sensitive cucumber than that in the pHBA-sensitive cucumber.  These results suggest that pHBA inhibits root growth by reducing root tip ROS level in cucumber.

 

Phosphorus (P) is a limiting nutrient element for crop.  To obtain maximum crop yield, P fertilizer is often over-applied, which leads to accelerating exhaustion of phosphate resources and serious environmental problems.  Reducing the application of P fertilizer and enhancing the P utilization efficiency of crops are significant for the sustainable development of agriculture.  Cadmium (Cd) contamination in rice is another serious agricultural issue.  However, whether reducing the application of P fertilizer and enhancing the P utilization efficiency of crops will increase the risk of Cd accumulation in crops remains obscure.  In this study, we are aiming to elucidate the relationship between Cd and P in rice from physiological and genetic perspectives.  For this purpose, the wild type (WT) rice plants and phosphate (Pi)-starvation signaling repressed mutant phr2 were used to analyze the relationship between Cd and P.  Here, we found that Cd stress could promote P accumulation and induce Pi-starvation signaling in WT and phr2 shoots under Pi-sufficient condition in a PHOSPHATE STARVATION RESPONSE 2 (PHR2) independent manner.  Besides, the expression level of Cd transporter of OsNramp5 and the uptake speed of Cd²⁺ were not obviously changed under Pi-sufficient and Pi-deficient conditions.  Furthermore, our Cd determination results showed that the Cd concentrations in WT and phr2 were not obviously changed under Pi-sufficient and Pi-deficient conditions.  These results indicate that the external P availability and internal Pi-starvation signaling cannot obviously affect the accumulation of Cd in rice seedling.