Rice stripe virus coat protein induces the accumulation of jasmonic acid, activating plant defence against the virus while also attracting its vector to feed

Abstract

The jasmonic acid (JA) pathway plays crucial roles in plant defence against pathogens and herbivores. Rice stripe virus (RSV) is the type member of the genus Tenuivirus. It is transmitted by the small brown planthopper (SBPH) and causes damaging epidemics in East Asia. The role(s) that JA may play in the tripartite interaction against RSV, its host, and vector are poorly understood. Here, we found that the JA pathway was induced by RSV infection and played a defence role against RSV. The coat protein (CP) was the major viral component responsible for inducing the JA pathway. Methyl jasmonate treatment attracted SBPHs to feed on rice plants while a JA-deficient mutant was less attractive than wild-type rice. SBPHs showed an obvious preference for feeding on transgenic rice lines expressing RSV CP. Our results demonstrate that CP is an inducer of the JA pathway that activates plant defence against RSV while also attracting SBPHs to feed and benefitting viral transmission. This is the first report of the function of JA in the tripartite interaction between RSV, its host, and its vector.

Keywords: Laodelphax striatellus; coat protein; jasmonic acid; rice stripe virus; small brown planthopper (SBPH).

FIGURE 1

The jasmonic acid (JA) pathway is induced by rice stripe virus (RSV) infection. Relative expression levels of genes in the JA pathway in RSV‐infected rice (a) and Nicotiana benthamiana (b). JA content in RSV‐infected rice (c) and N. benthamiana (d). The Actin (OsActin or NbActin) gene was used as an internal control. Bars represent the standard errors of the means from three biological repeats. A two‐sample unequal variance directional t test was used to test the significance of the difference (*p < .05, **p < .01). ng/g is JA amount (ng) in leaves (fresh weight, g)

FIGURE 2

The jasmonic acid (JA) pathway plays a defence role against rice stripe virus (RSV). (a) RSV symptoms on rice plants treated with methyl jasmonate (MeJA) and salicylhydroxamic acid (SHAM) (an inhibitor of the JA pathway) at 25 days postinfection (dpi). 0.1% ethanol was used as the control (CK). (b) The incidence of RSV on MeJA‐, SHAM‐, and 0.1% ethanol (CK)‐treated rice plants at 25 dpi. The percentage of plants infected with RSV (incidence) was determined by reverse transcription‐PCR at 25 dpi. Error bars show the mean ± SD of three replicates (at least 30 plants per replicate). A two‐sample unequal variance directional Student's t test was used to test the significance of the differences (*p < .05; n.s., not significant). (c) Western and northern blotting results showing the accumulation levels of RSV coat protein (CP) and viral RNA, respectively, in rice plants treated with MeJA or SHAM. (d) RSV symptoms on Nicotiana benthamiana plants treated with MeJA or SHAM. (e) Western and northern blotting results showing the accumulation levels of RSV CP and viral RNA, respectively, in the inoculated leaves (IL) and systemically infected leaves (SL) of N. benthamiana plants treated with MeJA or SHAM. Anti‐CP, RSV CP antibody was used in western blot to detect the protein level of RSV CP. Ponceau S‐stained RuBisCO was used as the loading control. CP probe, the partial sequence of RSV CP gene labelled with digoxigenin was used in northern blot to detect the accumulation of viral RNA. Ethidium bromide‐stained total RNAs was used as the loading control in northern blot. The relative intensity of the blot signal quantified by ImageJ is shown above the lanes

FIGURE 3

Rice stripe virus (RSV) coat protein (CP) is the major viral component responsible for induction of the jasmonic acid (JA) pathway. (a) Relative expression levels of JA pathway genes in leaves expressing RSV p2, p3, p4, pc4, or CP. (b) JA content was enhanced in leaves expressing RSV CP. (c) The phenotype of wild type (WT) (Nip) and CP transgenic rice (CP#2‐1, CP#5‐3, and CP#9‐1). Photographs were taken 15 days after germination. Scale bar = 5 cm. (d) Western blot confirmed the expression of CP in three transgenic lines. Total protein was extracted from rice seedlings 15 days after germination. Ponceau S‐stained RuBisCO was used as the loading control. (e) Relative expression levels of JA pathway genes in three transgenic rice lines expressing CP. (f) JA content was enhanced in transgenic lines. Bars represent the standard errors of the means from three biological repeats. A two‐sample unequal variance directional t test was used to test the significance of the difference (*p < .05, **p < .01)

FIGURE 4

RSV coat protein (CP) increases the attractiveness of plants to small brown leafhopper (SBPH) vectors and depends on the jasmonic acid (JA) pathway. SBPH performance on (a) methyl jasmonate (MeJA)‐ and (b) salicylhydroxamic acid (SHAM)‐treated rice plants, with 0.1% ethanol‐treated plants as control (CK). (c) SBPH performance on as‐lox line L145‐1 and wild‐type (WT) (Xiushui11) plants. SBPH performance on CP transgenic lines CP#2‐1 (d), CP#5‐3 (e), and CP#9‐1 (f) and WT (Nip) plants. Mean number of SBPHs per plant (±SEM) on pairs of plants, 1–48 hr after 8–10 replicated plant pairs were exposed to 10 insects. A two‐sample unequal variance directional t test was used to test the significance of the difference (*p < .05). (g) A proposed model for the dual effect of JA in RSV infection. The JA pathway is induced by CP and activates the plant defence against RSV. Meanwhile, the enhanced JA also attracts SBPHs to feed and thus to benefit viral transmission