The rice bright green leaf (bgl) locus encodes OsRopGEF10, which activates the development of small cuticular papillae on leaf surfaces

Genome-Wide Identification and Characterization of RopGEF Gene Family in C4 Crops

In plants, RopGEF-mediated ROP signaling is pivotal in cellular signaling pathways, including apical growth, pollen germination and perception, intercellular recognition, as well as in responses to biotic and abiotic stresses. In this study, we retrieved a total of 37 RopGEF members from three C4 Crops, of which 11 are from millet, 11 from sorghum, and 15 from maize. Based on their phylogenetic relationships and structural characteristics, all RopGEF members are classified into four subfamilies. The qRT-PCR technique was utilized to evaluate the expression profiles of 11 SiRopGEFs across different tissues in foxtail millet. The findings indicated that the majority of the SiRopGEFs exhibited higher expression levels in leaves as opposed to roots and stems. The levels of expression of SiRopGEF genes were examined in response to abiotic stress and plant hormones. SiRopGEF1, SiRopGEF5, SiRopGEF6, and SiRopGEF8 showed significant induction under abiotic stresses such as salt, cold, and heat. On the other hand, SiRopGEF1, SiRopGEF2, and SiRopGEF7 were consistently upregulated, while SiRopGEF3, SiRopGEF4, SiRopGEF6, SiRopGEF9, and SiRopGEF10 were downregulated upon exposure to abscisic acid (ABA), ethylene (ET), salicylic acid (SA), and gibberellic acid (GA3) hormones. The alterations in the expression patterns of RopGEF members imply their potential functions in plant growth and development, abiotic stress response, and hormone signal transduction. These discoveries suggest that the RopGEF genes may function as a potential genetic marker to facilitate future studies in elucidating the functional characteristics of RopGEFs.

Development of activation-tagged gain-of-functional mutants in indica rice line (BPT 5204) for sheath blight resistance

BACKGROUND

The development of sheath blight (ShB) resistance varieties has been a challenge for scientists for long time in rice. Activation tagging is an efficient gain-of-function mutation approach to create novel phenotypes and to identify their underlying genes. In this study, a mutant population was developed employing activation tagging in the recalcitrant indica rice (Oryza sativa L.) cv. BPT 5204 (Samba Mahsuri) through activation tagging.

METHODS AND RESULTS

In this study, we have generated more than 1000 activation tagged lines in indica rice, from these mutant population 38 (GFP- RFP+) stable Ds plants were generated through germinal transposition at T2 generation based on molecular analysis and seeds selected on hygromycin (50 mg/L) containing medium segregation analyses confirmed that the transgene inherited as mendelian segregation ratio of 3:1 (3 resistant: 1 susceptible). Of them, five stable activation tagged Ds lines (M-Ds-1, M-Ds-2, M-Ds-3, M-Ds-4 and M-Ds-5) were selected based on phenotypic observation through screening for sheath blight (ShB) resistance caused by fungal pathogen Rhizoctonia solani (R. solani),. Among them, M-Ds-3 and M-Ds-5 lines showed significant resistance for ShB over other tagged lines and wild type (WT) plants. Furthermore, analysed for launch pad insertion through TAIL-PCR results and mapped on corresponding rice chromosomes. Flanking sequence and gene expression analysis revealed that the upregulation of glycoside hydrolase-OsGH or similar to Class III chitinase homologue (LOC_Os08g40680) in M-Ds-3 and a hypothetical protein gene (LOC_Os01g55000) in M-Ds-5 are potential candidate genes for sheath blight resistance in rice.

CONCLUSION

In the present study, we developed Ac-Ds based ShB resistance gain-of-functional mutants through activation tagging in rice. These activation tagged mutant lines can be excellent sources for the development of ShB resistant cultivars in rice.

Rice Stomatal Mega-Papillae Restrict Water Loss and Pathogen Entry

Rice (Oryza sativa) is a water-intensive crop, and like other plants uses stomata to balance CO₂ uptake with water-loss. To identify agronomic traits related to rice stomatal complexes, an anatomical screen of 64 Thai and 100 global rice cultivars was undertaken. Epidermal outgrowths called papillae were identified on the stomatal subsidiary cells of all cultivars. These were also detected on eight other species of the Oryza genus but not on the stomata of any other plant species we surveyed. Our rice screen identified two cultivars that had "mega-papillae" that were so large or abundant that their stomatal pores were partially occluded; Kalubala Vee had extra-large papillae, and Dharia had approximately twice the normal number of papillae. These were most accentuated on the flag leaves, but mega-papillae were also detectable on earlier forming leaves. Energy dispersive X-Ray spectrometry revealed that silicon is the major component of stomatal papillae. We studied the potential function(s) of mega-papillae by assessing gas exchange and pathogen infection rates. Under saturating light conditions, mega-papillae bearing cultivars had reduced stomatal conductance and their stomata were slower to close and re-open, but photosynthetic assimilation was not significantly affected. Assessment of an F₃ hybrid population treated with Xanthomonas oryzae pv. oryzicola indicated that subsidiary cell mega-papillae may aid in preventing bacterial leaf streak infection. Our results highlight stomatal mega-papillae as a novel rice trait that influences gas exchange, stomatal dynamics, and defense against stomatal pathogens which we propose could benefit the performance of future rice crops.

Genome-wide analysis of RopGEF gene family to identify genes contributing to pollen tube growth in rice (Oryza sativa)

BACKGROUND

In plants, the key roles played by RopGEF-mediated ROP signaling in diverse processes, including polar tip growth, have been identified. Despite their important roles in reproduction, a comprehensive analysis of RopGEF members has not yet been performed in rice (Oryza sativa). To determine whether RopGEF regulators are involved in rice pollen tube growth, we performed genome-wide analysis of this family in rice.

RESULTS

Phylogenomic and meta-expression analysis of eleven RopGEFs in rice showed that four genes were preferentially expressed in mature pollen. These four genes contain the plant-specific Rop nucleotide exchanger (PRONE) domain and possible phosphorylated residues, suggesting a conserved role in polar tip growth with Arabidopsis thaliana. In subcellular localization analysis of the four RopGEFs through tobacco (Nicotiana benthamiana) infiltration, four proteins were predominantly identified in plasma membrane. Moreover, double mutants of RopGEF2/8 exhibited reduced pollen germination, causing partial male sterility. These genes possess unique cis-acting elements in their promoters compared with the other RopGEF genes.

CONCLUSIONS

In this study, four RopGEF genes were identified as pollen-specific gene in eleven members of rice, and the expression pattern, promoter analysis, and evolutionary relationship of the RopGEF family were studied compared with Arabidopsis. Our study indicated that four RopGEF genes might function during pollen germination in distinct subcellular localization. Our study could provide valuable information on the functional study of RopGEF in rice.

Guanine Nucleotide Exchange Factor 7B (RopGEF7B) is involved in floral organ development in Oryza sativa

BACKGROUND

RAC/ROP GTPase are versatile signaling molecules controlling diverse biological processes including cell polarity establishment, cell growth, morphogenesis, hormone responses and many other cellular processes in plants. The activities of ROPs are positively regulated by guanine nucleotide exchange factors (GEFs). Evidence suggests that RopGEFs regulate polar auxin transport and polar growth in pollen tube in Arabidopsis thaliana. However, the biological functions of rice RopGEFs during plant development remain largely unknown.

RESULTS

We investigated a member of the OsRopGEF family, namely OsRopGEF7B. OsRopGEF7B_pro:GUS analysis indicates that OsRopGEF7B is expressed in various tissues, especially in the floral meristem and floral organ primordia. Knock-out and -down of OsRopGEF7B by T-DNA insertion and RNA interference, respectively, predominantly caused an increase in the number of floral organs in the inner whorls (stamen and ovary), as well as abnormal paleae/lemmas and ectopic growth of lodicules, resulting in decline of rice seed setting. Bimolecular fluorescence complement (BiFC) assays as well as yeast two-hybrid assays indicate that OsRopGEF7B interacts with OsRACs.

CONCLUSIONS

OsRopGEF7B plays roles in floral organ development in rice, affecting rice seed setting rate. Manipulation of OsRopGEF7B has potential for application in genetically modified crops.

Rice leaf hydrophobicity and gas films are conferred by a wax synthesis gene (LGF1) and contribute to flood tolerance

Floods impede gas (O₂ and CO₂ ) exchange between plants and the environment. A mechanism to enhance plant gas exchange under water comprises gas films on hydrophobic leaves, but the genetic regulation of this mechanism is unknown. We used a rice mutant (dripping wet leaf 7, drp7) which does not retain gas films on leaves, and its wild-type (Kinmaze), in gene discovery for this trait. Gene complementation was tested in transgenic lines. Functional properties of leaves as related to gas film retention and underwater photosynthesis were evaluated. Leaf Gas Film 1 (LGF1) was identified as the gene determining leaf gas films. LGF1 regulates C30 primary alcohol synthesis, which is necessary for abundant epicuticular wax platelets, leaf hydrophobicity and gas films on submerged leaves. This trait enhanced underwater photosynthesis 8.2-fold and contributes to submergence tolerance. Gene function was verified by a complementation test of LGF1 expressed in the drp7 mutant background, which restored C30 primary alcohol synthesis, wax platelet abundance, leaf hydrophobicity, gas film retention, and underwater photosynthesis. The discovery of LGF1 provides an opportunity to better understand variation amongst rice genotypes for gas film retention ability and to target various alleles in breeding for improved submergence tolerance for yield stability in flood-prone areas.

AtPRK2 promotes ROP1 activation via RopGEFs in the control of polarized pollen tube growth

The ROP1 GTPase-based signaling network controls tip growth in Arabidopsis pollen tubes. Our previous studies imply that ROP1 might be directly activated by RopGEF1, which belongs to a plant-specific family of Rho guanine nucleotide exchange factors (RopGEFs) and in turn may be activated by an unknown factor through releasing RopGEF1's auto-inhibition. In this study, we found that RopGEF1 forms a complex with ROP1 and AtPRK2, a receptor-like protein kinase previously shown to interact with RopGEFs. AtPRK2 phosphorylated RopGEF1 in vitro and the atprk1,2,5 triple mutant showed defective pollen tube growth, similar to the phenotype of the ropgef1,9,12,14 quadruple mutant. Overexpression of a dominant negative form of AtPRK2 (DN-PRK2) inhibited pollen germination in Arabidopsis and reduced pollen elongation in tobacco. The DN-PRK2-induced pollen germination defect was rescued by overexpressing a constitutively active form of RopGEF1, RopGEF1(90-457), implying that RopGEF1 acts downstream of AtPRK2. Moreover, AtPRK2 increased ROP1 activity at the apical plasma membrane whereas DN-PRK2 reduced ROP1 activity. Finally, two mutations at the C-terminal putative phosphorylation sites of RopGEF1 (RopGEF1S460A and RopGEF1S480A) eliminated the function of RopGEF1 in vivo. Taken together, our results support the hypothesis that AtPRK2 acts as a positive regulator of the ROP1 signaling pathway most likely by activating RopGEF1 through phosphorylation.