Powdery mildew (Oidium heveae Steinm.) is one of the most important leaf diseases in rubber tree (Hevea brasiliensis). However, physiological and molecular processes associated with the Hevea-powdery mildew interaction are still unclear. In this study, effects of powdery mildew infection on the mitochondrial and chloroplast functions in rubber tree were comprehensively investigated. Powdery mildew damaged the structure and function of mitochondria prior to chloroplasts, causing inner and outer membranes disruption. The intact rate of mitochondria membrane was reduced from 70% in control leaves to 23.1% in the leaves at 5 days after inoculation (dai). Significant decreases in the activities of cytochrome c oxidase, NADH oxidation and malate dehydrogenase (MDH) were observed in the powdery mildew-infected leaves. Tricarboxylic acid cycle (TCA) and electron transfer capacity were seriously impaired after powdery mildew invasion. Chlorophyll (Chl) contents, maximal photochemical efficiency (Fv/Fm), actual photochemical efficiency of photosystem II (ΦPSII), and electron transport rate (ETR) were dramatically decreased in the infected leaves from 10 dai. Our results provided new insights into understanding the mechanism of Hevea-powdery mildew interaction in rubber tree.

Mildew resistance locus O (Mlo) gene was first found in barley as a powdery mildew susceptibility gene, and recessive mlo alleles confer durable resistance to barley powdery mildew. To identify candidate Mlo susceptibility genes in rubber tree, HbMlo12 was cloned from rubber tree clone CATAS7-33-97, which is susceptible to powdery mildew. Protein architecture analysis showed that HbMlo12 was a typical Mlo protein with seven transmembrane domains. Protein blast search in the Arabidopsis thaliana proteome database showed that HbMlo12 shared the highest similarity with AtMlo12, with 63% sequence identity. Furthermore, HbMlo12 together with the dicot powdery mildew susceptible Mlo proteins (including AtMlo2, AtMlo6, AtMlo12, tomato SlMlo1, pepper CaMlo2, pea PsMlo1, etc.) were grouped into clade V. Subcellular localization analysis in tobacco epidermal cells revealed that HbMlo12 was localized to the endoplasmic reticulum membrane. HbMlo12 was preferentially expressed in the flower and leaf of rubber tree. Moreover, its expression was significantly upregulated in response to powdery mildew inoculation. Application of exogenous ethephon caused a distinct increase in HbMlo12 expression. Additionally, HbMlo12 transcript was quickly induced by spraying salicylic acid and gibberellic acid and reached the maximum at 0.5 h after treatments. By contrast, HbMlo12 expression was downregulated by methyl jasmonate, abscisic acid, and drought stress treatments. There was no significant change in HbMlo12 expression after indole-3-acetic acid, H₂O₂, and wounding stimuli. Taken together, these results suggested that HbMlo12 might be a candidate Mlo gene conferring susceptibility to powdery mildew in rubber tree. The results of this study are vital in understanding Mlo gene evolution and developing new rubber tree varieties with powdery mildew resistance using reverse genetics.

5-Phospho-d-ribosyl-1-diphosphate (PRPP) synthase (PRS) catalyzes the biosynthesis of PRPP, which is an important compound of metabolism in most organisms. However, no PRS genes have been cloned, let alone studied for their biological function in rubber tree. In this study, we identify a novel protein (PRS4) that interacts in vivo with rubber tree anaphase promoting complex/cyclosome (APC/C) subunit 10 (HbAPC10) by yeast two-hybrid assays. PRS4 has been cloned from rubber tree and named as HbPRS4. Blastp search in the genome of Arabidopsis thaliana showed that HbPRS4 shared the highest similarity with AtPRS4, with 80.71% identity. qRT-PCR was used to determine the expression of HbPRS4 in different tissues and under various treatments. HbPRS4 was preferentially expressed in the bark. Moreover, the expression level of HbPRS4 was significantly induced by the proteasome inhibitor MG132 treatment, suggesting it might be regulated by the ubiquitin/26S proteasome pathway. The amount of HbPRS4 transcript was obviously decreased after mechanical wounding and abscisic acid (ABA) treatments, while a slight increase was observed at 24 h after ABA treatment. HbPRS4 transcript in the latex was significantly upregulated by ethephon (ET) and methyl jasmonate (MeJA) treatments. These results suggested that HbPRS4 may be a specific substrate of HbAPC10 indirectly regulating natural rubber biosynthesis in rubber tree.

Fipronil, an insecticide for crop protection with good selectivity between insects and mammals, are toxic to plants and animals in wild. In this paper, the decay patterns of Fipronil in 3 kinds of arable layer soil in Hainan province were studied by means of simulated rain water leaching use tap water on the earthworms and duckweeds, respectively. The results showed that decays of Fipronil in different soils were high related with organic matter content in soil. The death rates of earthworms were 10% in up layer of clay loam (rice land) which organic matter content was 14.84 g/kg. After leaching for 14 days, Fipronil flowed into lower layer, death rates of earthworms was increased to 24%. The decreasing of death rate of earthworms were found in lower layer of three soils suggested that little Fipronil flowed into lower soils. Leaching water collected at 1 h from three soils was poisoned to cultured duckweeds which reduce chlorophyll contents and the maximal efficiency of PSII photochemistry (Fv/Fm). These studies confirmed that Fipronil was combined with organic matter at surface soil. It could not decay easily in field and will influence the crop which planted in Hainan province for long periods. Key words: Fipronil, leaching, decay, organic matter, earthworms, duckweeds.