Rice ORMDL Controls Sphingolipid Homeostasis Affecting Fertility Resulting from Abnormal Pollen Development

The orosomucoids (ORM) are ER-resisdent polypeptides encoded by ORM and ORMDL (ORM-like) genes. In humans, ORMDL3 was reported as genetic risk factor associated to asthma. In yeast, ORM proteins act as negative regulators of sphingolipid synthesis. Sphingolipids are important molecules regulating several processes including stress responses and apoptosis. However, the function of ORM/ORMDL genes in plants has not yet been reported. Previously, we found that temperature sensitive genetic male sterility (TGMS) rice lines controlled by tms2 contain a deletion of about 70 kb in chromosome 7. We identified four genes expressed in panicles, including an ORMDL ortholog, as candidates for tms2. In this report, we quantified expression of the only two candidate genes normally expressed in anthers of wild type plants grown in controlled growth rooms for fertile and sterile conditions. We found that only the ORMDL gene (LOC_Os07g26940) showed differential expression under these conditions. To better understand the function of rice ORMDL genes, we generated RNAi transgenic rice plants suppressing either LOC_Os07g26940, or all three ORMDL genes present in rice. We found that the RNAi transgenic plants with low expression of either LOC_Os07g26940 alone or all three ORMDL genes were sterile, having abnormal pollen morphology and staining. In addition, we found that both sphingolipid metabolism and expression of genes involved in sphingolipid synthesis were perturbed in the tms2 mutant, analogous to the role of ORMs in yeast. Our results indicated that plant ORMDL proteins influence sphingolipid homeostasis, and deletion of this gene affected fertility resulting from abnormal pollen development.     

Differential Drought Responses in F-box Gene Expression and Grain Yield Between Two Rice Groups with Contrasting Drought Tolerance

Understanding the molecular mechanisms regulating rice’s response to drought stress is important for cereal crop development. We investigated the physiological and gene expression responses of three drought-tolerant and two drought-sensitive unrelated rice cultivars at the reproductive stage. Leaf water loss and leaf rolling tests distinguished these two groups from each other. Both 7 and 14 days of drought stress affected most of the tested agronomic traits including grain yield, and the effects were stronger in the drought-sensitive cultivars. Fourteen days of drought stress severely reduced grain yield in the sensitive cultivars. Expression levels of 50 genes previously published were examined in panicles. Of these, 25 genes were expressed in panicles and could be classified into 6 groups. LOC_Os02g51350, a Kelch domain-containing F-box gene, was selected for further studies including expression analysis in panicles during the 14-day drought stress, domain analysis, and analysis of cis-acting elements in the promoters. All three drought-resistant cultivars possessed the F-box domain, which was absent in the two drought-sensitive cultivars. In addition, drought-related ABRE and DRE/CRT cis-acting elements were more abundant in Os02g51350 promoters of cultivars with good grain yield under drought stress than in promoters of cultivars with severe yield reduction. Our results suggest that the F-box version of Os02g51350 is important for maintenance of grain yield under drought.     

Phylogeography of the Asian rice gall midge <Emphasis Type="Italic">Orseolia oryzae</Emphasis> (Wood Mason) (Diptera: Cecidomyiidae) in Thailand

The Asian rice gall midge (RGM) Orseolia oryzae (Wood Mason) (Diptera: Cecidomyiidae) is a major pest of rice, leading to yield losses in Thailand and many Asian countries. Despite an increasing number of reported midge outbreaks and the presence of many susceptible rice varieties, only a few studies have focused on the genetic variation of the midges. Therefore, we analyzed the phylogeography among Thai RGM populations covering north, northeast and central Thailand. Two mitochondrial DNA genes, cytochrome C oxidase I (COI) and 12S, and a non-coding repeat region (RR) situated just before COI were amplified. Overall, the haplotype diversity for COI and 12S genes of the Thai population was high, but the nucleotide diversity was quite low. Altogether, the phylogenetic tree and pairwise F _st values indicated that Thai RGM populations recently expanded and were homogeneously distributed throughout the country, except for some populations in the north, which most likely became recently isolated from the main population. Two non-coding repeat motifs, that were recently observed in the mitogenome of RGM in India, were absent in Thai populations and replaced by an 89 bp non-coding sequence. Tandem nucleotide repeats of the sequence TA were also observed. The repeat copy number varied from 2 to 11 and was not correlated with geographical repartition of the midge. Finally, COI barcoding divergence between Indian and Thai populations was high (6.3% in average), giving insights into the potential existence of an RGM species complex in Asia.