Rice is one of most important crops in the world and is also the most advanced crop for breeding among cereal crops. Generally, rice is grown in lowland and flood-prone areas due to its ability to ...survive in semiaquatic environments. To adapt to excess water conditions, rice has developed specialized anatomical and morphological traits such as aerenchyma, radial oxygen loss barriers, adventitious roots, and leaf gas film formation ability. However, these strategies are insufficient for survival under continuous complete submergence, which leads to death due to oxygen starvation. To overcome prolonged submergence, some Asian rice varieties have further developed additional traits such as aerobic germination, quiescence of leaf elongation in response to flash floods, and internode elongation under periodic flooding. In this review, we describe recent progress in the understanding of the various molecular mechanisms and genetic factors regulating flooding tolerance in rice.
The genus Cuscuta comprises stem holoparasitic plant species with wide geographic distribution. Cuscuta spp. obtain water, nutrients, proteins, and mRNA from their host plants via a parasitic organ ...called the haustorium. As the haustorium penetrates into the host tissue, search hyphae elongate within the host tissue and finally connect with the host's vascular system. Invasion by Cuscuta spp. evokes various reactions within the host plant's tissues. Here, we show that, when Arabidopsis (Arabidopsis thaliana) is invaded by Cuscuta campestris, ethylene biosynthesis by the host plant promotes elongation of the parasite's search hyphae. The expression of genes encoding 1-aminocylclopropane-1-carboxylic acid (ACC) synthases, ACC SYNTHASE2 (AtACS2) and ACC SYNTHASE6 (AtACS6), was activated in the stem of Arabidopsis plants upon invasion by C. campestris. When the ethylene-deficient Arabidopsis acs octuple mutant was invaded by C. campestris, cell elongation and endoreduplication of the search hyphae were significantly reduced, and the inhibition of search hyphae growth was complemented by exogenous application of ACC. In contrast, in the C. campestris-infected Arabidopsis ethylene-insensitive mutant etr1-3, no growth inhibition of search hyphae was observed, indicating that ETHYLENE RESPONSE1-mediated ethylene signaling in the host plant is not essential for parasitism by C. campestris. Overall, our results suggest that C. campestris recognizes host-produced ethylene as a stimulatory signal for successful invasion.
Most plants do poorly when flooded. Certain rice varieties, known as deepwater rice, survive periodic flooding and consequent oxygen deficiency by activating internode growth of stems to keep above ...the water. Here, we identify the gibberellin biosynthesis gene,
(
), whose loss-of-function allele catapulted the rice Green Revolution, as being responsible for submergence-induced internode elongation. When submerged, plants carrying the deepwater rice-specific
haplotype amplify a signaling relay in which the
gene is transcriptionally activated by an ethylene-responsive transcription factor, OsEIL1a. The SD1 protein directs increased synthesis of gibberellins, largely GA
, which promote internode elongation. Evolutionary analysis shows that the deepwater rice-specific haplotype was derived from standing variation in wild rice and selected for deepwater rice cultivation in Bangladesh.
Summary
Cytokinins are phytohormones that play key roles in the maintenance of stem cell activity in plants. Although alternative single‐step and two‐step activation pathways for cytokinin have been ...proposed, the significance of the single‐step pathway which is catalyzed by LONELY GUY (LOG), is not fully understood. We analyzed the metabolic flow of cytokinin activation in Arabidopsis log multiple mutants using stable isotope‐labeled tracers and characterized the mutants’ morphological and developmental phenotypes. In tracer experiments, cytokinin activation was inhibited most pronouncedly by log7, while the other log mutations had cumulative effects. Although sextuple or lower‐order mutants did not show drastic phenotypes in vegetative growth, the log1log2log3log4log5log7log8 septuple T‐DNA insertion mutant in which the LOG‐dependent pathway is impaired, displayed severe retardation of shoot and root growth with defects in the maintenance of the apical meristems. Detailed observation of the mutants showed that LOG7 was required for the maintenance of shoot apical meristem size. LOG7 was also suggested to play a role for normal primary root growth together with LOG3 and LOG4. These results suggest a dominant role of the single‐step activation pathway mediated by LOGs for cytokinin production, and overlapping but differentiated functions of the members of the LOG gene family in growth and development.
Valves on the plant epidermis called stomata develop according to positional cues, which likely involve putative ligands (EPIDERMAL PATTERNING FACTORS EPFs) and putative receptors (ERECTA family ...receptor kinases and TOO MANY MOUTHS TMM) in Arabidopsis. Here we report the direct, robust, and saturable binding of bioactive EPF peptides to the ERECTA family. In contrast, TMM exhibits negligible binding to EPF1 but binding to EPF2. The ERECTA family forms receptor homomers in vivo. On the other hand, TMM associates with the ERECTA family but not with itself. While ERECTA family receptor kinases exhibit complex redundancy, blocking ERECTA and ERECTA-LIKE1 (ERL1) signaling confers specific insensitivity to EPF2 and EPF1, respectively. Our results place the ERECTA family as the primary receptors for EPFs with TMM as a signal modulator and establish EPF2-ERECTA and EPF1-ERL1 as ligand-receptor pairs specifying two steps of stomatal development: initiation and spacing divisions.
Grain weight is an important crop yield component; however, its underlying regulatory mechanisms are largely unknown. Here, we identify a grain-weight quantitative trait locus (QTL) encoding a ...new-type GNAT-like protein that harbors intrinsic histone acetyltransferase activity (OsglHAT1). Our genetic and molecular evidences pinpointed the QTL- OsglHAT1 ’s allelic variations to a 1.2-kb region upstream of the gene body, which is consistent with its function as a positive regulator of the traits. Elevated OsglHAT1 expression enhances grain weight and yield by enlarging spikelet hulls via increasing cell number and accelerating grain filling, and increases global acetylation levels of histone H4. OsglHAT1 localizes to the nucleus, where it likely functions through the regulation of transcription. Despite its positive agronomical effects on grain weight, yield, and plant biomass, the rare allele elevating OsglHAT1 expression has so far escaped human selection. Our findings reveal the first example, to our knowledge, of a QTL for a yield component trait being due to a chromatin modifier that has the potential to improve crop high-yield breeding.
Significance Grain weight is an important crop yield component; however, its underlying regulatory mechanisms are largely unknown. Here, we identify a grain-weight quantitative trait locus (QTL) in rice encoding a new-type GNAT-like protein that harbors intrinsic histone acetyltransferase activity (OsglHAT1). Elevated OsglHAT1 expression enhances grain weight and yield by enlarging spikelet hulls via increasing cell number and accelerating grain filling, and increases global acetylation levels of histone H4. Our findings reveal the first example, to our knowledge, of a QTL for a yield component trait being due to a chromatin modifier that has the potential to improve crop high-yield breeding.
Reef-building corals thrive in oligotrophic environments due to their possession of endosymbiotic algae. Confined to the low pH interior of the symbiosome within the cell, the algal symbiont provides ...the coral host with photosynthetically fixed carbon. However, it remains unknown how carbon is released from the algal symbiont for uptake by the host. Here we show, using cultured symbiotic dinoflagellate,
Breviolum
sp., that decreases in pH directly accelerates the release of monosaccharides, that is, glucose and galactose, into the ambient environment. Under low pH conditions, the cell surface structures were deformed and genes related to cellulase were significantly upregulated in
Breviolum
. Importantly, the release of monosaccharides was suppressed by the cellulase inhibitor, glucopyranoside, linking the release of carbon to degradation of the agal cell wall. Our results suggest that the low pH signals the cellulase-mediated release of monosaccharides from the algal cell wall as an environmental response in coral reef ecosystems.
Coral reefs are known as ‘treasure troves of biodiversity’ because of the enormous variety of different fish, crustaceans and other marine life they support. Colonies of marine animals, known as corals, which are anchored to rocks on the sea bed, form the main structures of a coral reef. Many corals rely on partnerships with microscopic algae known as dinoflagellates for most of their energy needs. The dinoflagellates use sunlight to make sugars and other carbohydrates and they give some of these to the coral. In exchange, the coral provides a home for the dinoflagellates inside its body.
The algae live inside special compartments within coral cells known as symbiosomes. These compartments have a lower pH (that is, they are more acidic) than the rest of the coral cell. Previous studies have shown that the algae release sugars into the symbiosome but it remains unclear what triggers this release and whether it only occurs when the algae are in a partnership.
Ishii et al. studied a type of dinoflagellate known as
Breviolum sp.
that had been grown in sea water-like liquid in a laboratory. The experiments found that the alga released two sugar molecules known as glucose and galactose into its surroundings even in the absence of a host coral.
Increasing the acidity of the liquid caused the alga to release more sugars and resulted in changes to some of the structures on the surface of its cells. The alga also produced an enzyme, called cellulase, to degrade the wall that normally surrounds the cell of an alga. Treating the alga with a drug that inhibits the activity of cellulase also suppressed the release of sugars from the cells.
These findings suggest that when dinoflagellates enter acidic environments, like the guts of marine animals or symbiosomes inside coral cells, the decrease in pH can activate the algal cellulase enzyme, which in turn triggers the release of sugars for the coral. This research will provide a new viewpoint to those interested in how partnerships between animals and algae are sustained in marine environments. It also highlights the importance of the alga cell wall in establishing partnerships with corals. Further work will seek to clarify the precise biological mechanisms involved.
Rhamnogalacturonan I (RG-I) comprises approximately one quarter of the pectin molecules in land plants, and the backbone of RG-I consists of a repeating sequence of 2)-α-L-Rha(1-4)-α-D-GalUA(1- ...disaccharide. Four
Arabidopsis thaliana
genes encoding RG-I rhamnosyltransferases (AtRRT1 to AtRRT4), which synthesize the disaccharide repeats, have been identified in the glycosyltransferase family (GT106). However, the functional role of RG-I in plant cell walls and the evolutional history of RRTs remains to be clarified. Here, we characterized the sole ortholog of AtRRT1–AtRRT4 in liverwort,
Marchantia polymorpha
, namely, MpRRT1. MpRRT1 had RRT activity and genetically complemented the At
RRT1
-deficient mutant phenotype in
A. thaliana
. However, the Mp
RRT1
-deficient
M. polymorpha
mutants showed no prominent morphological changes and only an approximate 20% reduction in rhamnose content in the cell wall fraction compared to that in wild-type plants, suggesting the existence of other
RRT
gene(s) in the
M. polymorpha
genome. As expected, we detected RRT activities in other GT106 family proteins such as those encoded by Mp
RRT3
in
M. polymorpha
and
FRB1/
At
RRT8
in
A. thaliana
, the deficient mutant of which affects cell adhesion. Our results show that
RRT
genes are more redundant and diverse in GT106 than previously thought.
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.
Under flooded conditions, the leaves and internodes of deepwater rice can elongate above the water surface to capture oxygen and prevent drowning. Our previous studies showed that three major ...quantitative trait loci (QTL) regulate deepwater‐dependent internode elongation in deepwater rice. In this study, we investigated the age‐dependent internode elongation in deepwater rice. We also investigated the relationship between deepwater‐dependent internode elongation and the phytohormone gibberellin (GA) by physiological and genetic approach using a QTL pyramiding line (NIL‐1 + 3 + 12). Deepwater rice did not show internode elongation before the sixth leaf stage under deepwater condition. Additionally, deepwater‐dependent internode elongation occurred on the sixth and seventh internodes during the sixth leaf stage. These results indicate that deepwater rice could not start internode elongation until the sixth leaf stage. Ultra‐performance liquid chromatography tandem mass‐spectrometry (UPLC‐MS/MS) method for the phytohormone contents showed a deepwater‐dependent GA1 and GA4 accumulation in deepwater rice. Additionally, a GA inhibitor abolished deepwater‐dependent internode elongation in deepwater rice. On the contrary, GA feeding mimicked internode elongation under ordinary growth conditions. However, mutations in GA biosynthesis and signal transduction genes blocked deepwater‐dependent internode elongation. These data suggested that GA biosynthesis and signal transduction are essential for deepwater‐dependent internode elongation in deepwater rice.
Deepwater rice obtained the ability for rapid internode elongation to avoid drowning and adapt to flooded condition. How does it regulate internode elongation? Using both physiological and genetic approach, this paper shows that the plant hormone, gibberellin (GA) regulates internode elongation.
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