Ekamber Kariali

Hormonal regulation of tiller dynamics in differentially-tillering rice cultivars

Tiller number can contribute significantly to yield potential of rice, but little knowledge is available on hormonal regulation of tillering and tiller dynamics. In the present study, Indole-3-acetic acid (IAA), kinetin (6-furfuryl amino purine) and Gibberellic acid (GA3) treatments have been applied at the early tillering stage to two rice cultivars that contrast for tiller number. The responses of the hormones were studied on growth, development, grain yield, senescence patterns, assimilate concentration of the panicle and ethylene production in different classes of tillers. The leaf area, panicle grain number, fertility percentage and grain yield of tillers were higher in the low-tillering cultivar than that of high-tillering cultivar; the treatment of kinetin was more effective in the latter than in the former. High ethylene production was responsible for reduction of growth duration and grain yield of the tillers. Kinetin application reduced ethylene production of the late-tillers significantly for the benefit of grain yield.

High ethylene production slackens seed filling in compact panicled rice cultivar.

Change of plant type in rice resulting in increased compactness of the panicle, allows space for accommodation of a larger number of spikelets, but grain yield does not increase proportionately because of limitations in grain filling. The objective of this study was to evaluate potential causes of poor filling of spikelets by comparing the physiological processes that influence source and sink activities between a compact- (OR-1920-7) and a loose-panicled (Lalat) rice cultivars growing in the open field conditions in the farm of Regional Research and Technology Transfer Station, Orissa University of Agriculture and Technology, Chiplima, India during dry season of 2007. Although grain number per unit length of the branches was higher in the compact-panicled cultivar than the loose-panicled cultivar, average grain weight was lower on the primary and secondary branches at top, middle and basal positions of the panicle in the former compared to the corresponding positions of the panicle in the latter. Compared to Lalat, ethylene production rate was considerably higher in the boot of the flag leaf sheath of OR-1920-7 during the pre-anthesis period. Ethylene evolution rate correlated negatively with growth and cell division rates and starch concentration of the juvenile endosperm. Because spikelet growth was slower in OR-1920-7 than in Lalat, unused carbohydrates accumulated in the endosperm. The stomatal conductance of the flag leaf during this period was also lower in the former than that of the latter and it correlated negatively with ethylene evolution rate of the boot. It is concluded that high ethylene production slackened grain filling of compact-panicled rice cultivar OR-1920-7 because of its adverse influence on both source and sink activities.

Apoplasmic assimilates and grain growth of contrasting rice cultivars differing in grain dry mass and size

Apical dominance in assimilate filling impacts grain growth in basal spikelets of rice panicle. In this study, organic materials of the pericarp, apoplasmic space and endosperm of the apical and basal caryopses, and photosynthesis of the flag leaf were measured during early part of grain development in three types of rice cultivars with similar phenology, but difference in grain weight and size in the dry and wet seasons of 2006 and 2007, respectively. Photosynthetic activity of the flag leaf was consistently low in small-seeded cultivars. Rates of grain filling and cell division of endosperm and concentration of assimilates, starch, proteins and chlorophylls of the caryopsis were lower, but spikelet ethylene production and peroxidase activity were higher in a small-seeded cultivar compared to a big-seeded cultivar. Similar disparities in grain filling and other attributes were noticed for the inferior basal spikelets of the panicle compared to the superior apical spikelets, except the assimilate concentration of the pericarp and endosperm. Temporal fluctuation in assimilate concentration of the organs were similar between the cultivars. Concentration of apoplasmic assimilates mostly exhibited negative correlation with that of pericarp and endosperm. Compared to the apical spikelets, correlation was more negative for the basal spikelets. Conversely, correlation was positive between the concentration of apoplasmic assimilates and endosperm cell number and grain weight of the cultivars. Ethylene released from the spikelets at anthesis affected growth and cell division rates of endosperm and enhanced protein and chlorophyll degradation and peroxidase activity of the caryopsis. It was concluded that variation in spikelet ethylene production may be responsible for differences in size or weight of grains among rice cultivars and spikelets at different locations of the panicle. The concentration of apoplasmic assimilates could be an indicator for grain filling capacity, and ethylene regulated the concentration by affecting pericarp activity for assimilate unloading.

Spikelet-specific variation in ethylene production and constitutive expression of ethylene receptors and signal transducers during grain filling of compact- and lax-panicle rice (Oryza sativa) cultivars

Grain yields in modern super rice cultivars do not always meet the expectations because many spikelets are located on secondary branches in closely packed homogeneous distribution in these plants, and they do not fill properly. The factors limiting grain filling of such spikelets, especially in the lower panicle branches, are elusive. Two long-duration rice cultivars differing in panicle density, Mahalaxmi (compact) and Upahar (lax), were cultivated in an open field plot. Grain filling, ethylene production and constitutive expression of ethylene receptors and ethylene signal transducers in apical and basal spikelets of the panicle were compared during the early post-anthesis stage, which is the most critical period for grain development. In another experiment, a similar assessment was made for the medium-duration cultivars compact-panicle OR-1918 and lax-panicle Lalat. Grain weight of the apical spikelets was always higher than that of the basal spikelets. This gradient of grain weight was wide in the compact-panicle cultivars and narrow in the lax-panicle cultivars. Compared to apical spikelets, the basal spikelets produced more ethylene at anthesis and retained the capacity for post-anthesis expression of ethylene receptors and ethylene signal transducers longer. High ethylene production enhanced the expression of the RSR1 gene, but reduced expression of the GBSS1 gene. Ethylene inhibited the partitioning of assimilates of developing grains resulting in low starch biosynthesis and high accumulation of soluble carbohydrates. It is concluded that an increase in grain/spikelet density in rice panicles reduces apical dominance to the detriment of grain filling by production of ethylene and/or enhanced perception of the ethylene signal. Ethylene could be a second messenger for apical dominance in grain filling. The manipulation of the ethylene signal would possibly improve rice grain yield.

Compact panicle architecture is detrimental for growth as well as sucrose synthase activity of developing rice kernels

The increase of spikelet number in the panicles of modern super rice has made the architecture compact, as the extra spikelets are accommodated mostly on secondary branches than on primary branches. However, the grain yield did not improve because of poor grain filling, which was more visible in the basal spikelets than apical spikelets. The objective of this study was to examine the effect of the compactness and positional difference of spikelets in the panicle on grain filling by comparing the activity and genetic expression of starch synthesising enzymes in the developing kernels of lax-(Upahar and CR3856-45-11-2-7-2-5 (CR-45)) and compact-(Mahalaxmi and CR3856-29-14-2-1-1-1 (CR-29)) panicle cultivars. Upahar and Mahalaxmi are genetically related, whereas CR-45 and CR-29 are recombinant inbred lines. The grain carbohydrate concentration and activity of sucrose synthase (SUS) enzyme were estimated during the active period of grain filling. Further, expression of isoforms of SUS, ADP glucose pyrophosphorylase (APL and APS for large and small units respectively) and starch synthase (SS and GBSS for soluble and granule bound starch synthases respectively) were also assayed through PCR studies. The genotype approach used revealed grain SUS activity and starch concentration high and sugar concentration low in the lax- compared with compact-panicle cultivars and in the apical spikelets compared with basal ones. The margin of variation between apical and basal spikelets was higher in the compact- than the lax-panicle cultivars. Genetic expression of most of the isoforms of the enzymes was higher in the lax- than the compact-panicle cultivars as seen in RT-PCR studies. A quantitative appraisal of transcript levels of isoforms in the qRT-PCR identified greater expression of SUS3 in the basal spikelets of Upahar than that in Mahalaxmi and in CR-45 over CR-29, most prominently during the active period of grain filling. We conclude that proximal location as well as increased density of spikelets on panicles affected SUS3 expression in the basal spikelets. The metabolic dominance of a spikelet in rice panicle is dependent on the expression of the genes for different isoforms of starch synthesising enzymes, but the expression of SUS3 could be more specific than the others. SUS3 expression is most active during grain filling of the lax-panicle cultivars, but its dominance is reduced significantly in the kernels of the compact-panicle cultivars.

Plasticity of Tiller Dynamics in Wild Rice Oryza rufipogon Griff.: A Strategy for Resilience in Suboptimal Environments

Rice cultivation in tropical Asia is susceptible to drought and flood and the need is high for stress resistant genes. Wild rice Oryza rufipogon Griff., grows in close sympatric association with cultivated rice in various habitats across the globe and possesses traits for survival under challenging environments. The species adapts according to the level of soil moisture available and modifies phenology, biomass production and grain yield. Variation in tiller dynamics of the species between contrasting environments gives an estimate of the adaptation. The species possesses AA genome, which permits genetic compatibility for cross breeding with cultivated rice. Utility of the species as possible repository of stress resistant genes is evaluated in this review by examining variation in assimilate partitioning between different classes of tillers of ecotypes growing across a gradation of habitats against background knowledge available for cultivated rice. Models have been constructed to explain mechanisms of tillering and tiller dynamics, and reveal the genotypic permissibility for resilience in sub-optimal environments. It is concluded that environmentally cued alteration in assimilate production and partitioning mask genetic potential for tiller production and survival. Tiller number in excess of resource capacity is corrected by senescence of late-tillers possibly through an ethylene-mediated signal.

Individual Tiller Dynamics of Two Wild Oryza species in Contrasting Habitats

In the eastern part of India, the cultivated rice Oryza sativa has two wild relatives namely, Oryza nivara and Oryza rufipogon. O. nivara is annual and grows in dryland aerobic habitats compared to the perennial O. rufi pogon that lives in less aerobic swampy habitats. Tillering is generally more restricted in the O. nivara than O. rufipogon. Both environmental parameters and genotypic variation determine tiller number and dynamics of rice in the fi eld (De Datta, 1981;Yoshida, 1981). However, the stability of tiller hierarchy among genotypes across a range of growth habitats and the relationship of tiller dynamics and hierarchy with the assimilate concentration of their panicle are not known. In this study, the natural habitat of these two species has been altered to simulate cultivated conditions in order to assess the impact of environmental conditions on the mechanism of assimilate partitioning between different types of tillers and determination of tiller number.

Biochemical and molecular characterisation of salt-induced poor grain filling in a rice cultivar

Despite the prevalence of poor grain filling in rice (Oryza sativa L.) under abiotic stress, the reason for this is largely unexplored. Application of 0.75% NaCl to a salt-sensitive rice cultivar at late booting resulted in a >20% yield loss. Spikelets per panicle and the percentage of filled grain decreased significantly in response to NaCl application. The inhibitory effect of NaCl on grain filling was greater in basal than in apical spikelets. Sucrose synthase (SUS) activity was positively correlated with grain weight. The transcript levels of the SUS isoforms differed greatly: the levels of SUS2 increased significantly in response to salt; those of SUS4 decreased drastically. Gene expression studies of starch synthase and ADP-glucose pyrophosphorylase showed that the decreased transcript levels of one isoform was compensated by an increase in those of the other. Salt application also significantly increased the gene expression of the ethylene receptors and the ethylene signalling proteins. The increase in their transcript levels was comparatively greater in basal than in apical spikelets. Significant enhancement in the transcript levels of the ethylene receptors and the increase in the production of ethylene indicated that the salt-induced inhibition of grain filling might be mediated by ethylene. Additionally, the inhibition of chromosomal endoreduplication mediated by decreased transcript levels of B-type cyclin could explain poor grain filling under salt stress. A significant increase in the transcript levels of the ethylene-responsive factors in the spikelets during grain filling in response to salt indicated their possible protective role in grain filling under stress.

Comparative proteomics of the superior and inferior spikelets at the early grain filling stage in rice cultivars contrast for panicle compactness and ethylene evolution

The breeding programmes in rice aimed at increasing the number of spikelets per panicle have been accompanied by poor grain filling in the inferior spikelets of large panicle rice, leading to yield disadvantage. The present study attempted to understand the reason for differential grain filling in the inferior and superior spikelets by comparative proteomics considering a compact-panicle rice cultivar Mahalaxmi and a lax-panicle rice cultivar Upahar, which show poor and good grain filling, respectively. An initial study of two rice cultivars for panicle compactness and grain filling revealed an inverse correlation between the two parameters. It was further observed that the panicle compactness in Mahalaxmi was associated with a higher evolution of ethylene by the spikelets, both superior and inferior, compared with the lax-panicle Upahar. The proteomic studies revealed that the superior and inferior spikelets of Mahalaxmi differentially expressed 21 proteins that were also expressed in Upahar. However, in Upahar, only two of these proteins were differentially expressed between the superior and inferior spikelets, indicating that the metabolic activities of the spikelets occupying the superior and inferior positions on the panicle were very different in Mahalaxmi compared with those in Upahar. Among the proteins that were downregulated in the inferior spikelets compared with the superior ones in Mahalaxmi were importin-α, elongation factor 1-β and cell division control protein 48, which are essential for cell cycle progression and cell division. Low expression of these proteins might inhibit endosperm cell division in the inferior spikelets, limiting their sink capacity and leading to poor grain filling compared to that in the superior spikelets. The poor grain filling in Mahalaxmi may also be a result of the high evolution of ethylene in the inferior spikelets, which is reflected from the observation that these spikelets showed significantly higher expression of S-adenosylmethionine synthase and the gene encoding the enzyme than the superior spikelets in this cultivar, but not in Upahar; S-adenosynlmethionine synthase catalyses the synthesis of S-adenosylmethionine, the precursor of ethylene biosynthesis.

MicroRNAs as biomarkers and prospective therapeutic targets in colon and pancreatic cancers

Colon and pancreatic cancers have high mortality rates due to early metastasis prior to the onset of symptoms. Screening tests for colorectal cancer are invasive and expensive. No effective screening is available for pancreatic cancer. Identification of biomarkers for early detection in both of these cancers is being extensively researched. MicroRNAs (miRNA) are small non-coding molecule biomarkers that regulate cancers. Measurement of miRNAs in pancreatic fluid or blood could be a preferred non-invasive screening method. The regulation of colon and pancreatic cancers by miRNA is complex. miRNA play a central role in inflammation, invasiveness, and tumor progression in these two cancers, as well as regulation of the NF-κB pathway. miRNA’s evolving role in screening is also reviewed.