The Role Of Endogenous Jasmonic Acid In Floret Opening Of Rice

Floret opening of rice which includes opening of palea and lemma, filament elongation and anther dehiscence is a highly organized developmental process, and is controlled by genetic factors and environmental factors (weather conditions and artificial treatments). The regulation of floret opening can contribute to avoid high temperature-induced floret sterility at anthesis, and obtain the synchronization of anthesis between some cytoplasmic male sterile lines and restorer lines in hybrid seed production. It was well documented that exogenously applied jasmonic acid (JA) and methyl jasmonate (MeJA) significantly induce floret opening of rice and other gramineous plants such as wheat, sorghum and ryegrass. However, whether the endogenous JA is involved in the regulation of rice floret opening has remained unclear. To elucidate the role of endogenous JA in rice floret opening, we measured JA levels with HPLC-electrospray ionization tandem mass spectrometry system (HPLC-MS/MS), and analyzed the expression of JA pathway related genes with real-time reverse transcription-PCR (Real-time RT-PCR) in florets of Wuyunjing7(japonica cultivar) and Minghui63(indica cultivar) during natural anthesis. Four transgenic rice lines with overexpression or RNAi-suppression of JA biosynthetic genes OsAOSl or OsOPR7were also generated and analyzed. The main results are as follows:(1) JA levels in mature florets kept relatively constant before floret opening in the morning, and rapidly reached a peak during floret opening, and then followed a sharp decrease after the floret closing. The peak level of JA in opening florets was4-5times higher than that in florets1h before anthesis.(2) The changes of JA levels in rice florets during anthesis were supported by our molecular findings. The expression of OsDADl-3, OsLOX-RCIl, OsAOSl, OsAOC and OsOPR7that encode key enzymes in JA biosynthetic pathway was up-regulated to some extent during floret opening, and was down-regulated after floret closing.(3) The levels of JA and expression of JA biosynthetic genes in unopened mature florets under the condition of overcast did not show obvious changes in the daytime and didn’t appear a peak.(4) The levels of JA and transcripts of JA biosynthetic genes were higher in opening florets than in closed or unopened florets at the same panicle.(5) The expression of JA related biosynthesis genes was also detected in floret organs before and after anthesis. The results showed that the significantly accumulated JA in opening florets was mainly originated from JA made in stamens and lodicules.(6) The expression levels of OsJARland OsCOI1involved in JA signal transduction pathway kept rather constant during floret opening, suggesting that JA signal transduction pathway may not be a controlling site for JA pathway during floret opening.(7) Using the Agrobacterium-mediated rice transformation, we generated four transgenic rice lines, including OsAOS1over-expressed line (AOS1OE), OsAOS1RNAi-suppressed line (AOS1Ri), OsOPR7over-expressed line (OPR7OE) and OsOPR7RNAi-suppressed line (OPR7Ri). mRNA expression level analysis confirmed that OsAOSl transcripts in leaves and florets of AOS1OE lines and OsOPR7transcripts in OPR7OE lines significantly increased, and that OsAOS1transcripts in leaves and florets of AOS1Ri lines and OsOPR7transcripts in OPR7Ri lines significantly reduced.(8) No obvious difference of spikelet development and floret opening time between the four transgenic lines (AOS1OE, AOS1Ri, OPR7OE and OPR7Ri) and wild-type line was observed in the field condition. The peak time of floret opening in these transgenic and wild-type lines was about at11:50in the morning.(9) The JA levels in florets of two over-expressed transgenic lines (AOS1OE and OPR7OE) had no significant difference with those of wild-type lines before floret opening and at the time of floret opening. The JA levels in florets of two RNAi-suppressed transgenic lines (AOSIRi and OPR7Ri) were in the same ranges as those of wild-type line before floret opening, and were below half of wild-type line at the time of floret opening. However, the JA levels in RNAi florets at the time of floret opening were significantly increased than that before floret opening. These results indicated that overexpression of OsAOS1and OsOPR7did not change the basal JA level of transgenic plants, and RNAi suppression of OsAOS1and OsOPR7just partially inhibited JA biosynthesis in transgenic plants. This provided a reasonable explanation for that no obvious alteration of floret opening time was observed in the four different transgenic lines in the field condition.Our study first reported that the changes of JA levels and expression of JA biosynthetic genes in mature florets were associated with process of floret opening, and this strongly indicated that endogenous JA is involved in the regulation of floret opening in rice.