Differential response of wild and cultivated wheats to water deficits during grain development: changes in soluble carbohydrates and invertases

A proteomic analysis of grain yield-related traits in wheat

Grain yield, which is mainly contributed by tillering capacity as well as kernel number and weight, is the most important trait to plant breeders and agronomists. Label-free quantitative proteomics was used to analyse yield-contributing organs in wheat. These were leaf sample, tiller initiation, spike initiation, ovary and three successive kernel development stages at 5, 10 and 15 days after anthesis (DAA). We identified 3182 proteins across all samples. The largest number was obtained for spike initiation (1673), while the smallest was kernel sample at 15 DAA (709). Of the 3182 proteins, 296 of them were common to all seven organs. Organ-specific proteins ranged from 148 in ovary to 561 in spike initiation. When relative protein abundances were compared to that of leaf sample, 347 and 519 proteins were identified as differentially abundant in tiller initiation and spike initiation, respectively. When compared with ovary, 81, 35 and 96 proteins were identified as differentially abundant in kernels sampled at 5, 10 and 15 DAA, respectively. Our study indicated that two Argonaute proteins were solely expressed in spike initiation. Of the four expansin proteins detected, three of them were mainly expressed during the first 10 days of kernel development after anthesis. We also detected cell wall invertases and sucrose and starch synthases mainly during the kernel development period. The manipulation of these proteins could lead to increases in tillers, kernels per spike or final grain weight, and is worth exploring in future studies.

Molecular evolution and structural variations in nuclear encoded chloroplast localized heat shock protein 26 (sHSP26) from genetically diverse wheat species

Heat shock proteins are an important class of molecular chaperones known to impart tolerance under high temperature stress. sHSP26, a member of small heat shock protein subfamily is specifically involved in protecting plant's photosynthetic machinery. The present study aimed at identifying and characterizing sequence and structural variations in sHSP26 from genetically diverse progenitor and non-progenitor species of wheat. In silico analysis identified three paralogous copies of TaHSP26 to reside on short arm of chromosome 4A while one homeologue each was localized on long arm of chromosome 4B and 4D of cultivated bread wheat. Wild DD-genome donor Aegilops tauschii carried an additional sHSP26 gene (AET4Gv20569400) which was absent in the cultivated DD genome of bread wheat. In vitro amplification of this novel gene in wild accessions of Ae. tauschii and synthetic hexaploid wheat but not in cultivated bread wheat validated this finding. Further, significant length polymorphism could be identified in exon1 from diverse sHSP26 sequences. Multiple sequence alignment of procured sequences revealed numerous sSNPs and nsSNPs. D3A, P125 L, Q242 K were designated as homeolog specific- while A49 G as non-progenitor specific amino acid replacements. A 9-bp indel in TmHSP26-1(GA) translated into a deletion of SPM amino acid segment in chloroplast specific conserved consensus region III. High degree of divergence in nucleotide sequence between cultivated and wild species appeared in the form of higher ω values (Ka/Ks >1) indicating positive selection during the course of evolution. Phylogenetic analysis elucidated ancestral relationships between wheat sHSP26 proteins and orthologous proteins across plant kingdom. Overall, data mining approach may be employed as an effective pre-breeding strategy to identify and mobilize novel stress responsive genes and distinct allelic variants from wider germplasm collections of wheat to enhance climate resilience of present day elite wheat cultivars.

Stress Adaptive Plasticity: Aegilops tauschii and Triticum dicoccoides as Potential Donors of Drought Associated Morpho-Physiological Traits in Wheat

The inconsistent prevalence of abiotic stress in most of the agroecosystems can be addressed through deployment of plant material with stress adaptive plasticity. The present study explores water stress induced plasticity for early root-shoot development, proline induction and cell membrane injury in 57 accessions of Aegilops tauschii (DD-genome) and 26 accessions of Triticum dicoccoides (AABB-genome) along with durum and bread wheat cultivars. Thirty three Ae. tauschii accessions and 18 T. dicoccoides accessions showed an increase in root dry weight (ranging from 1.8 to 294.75%) under water stress. Shoot parameters- length and biomass, by and large were suppressed by water stress, but genotypes with stress adaptive plasticity leading to improvement of shoot traits (e.g., Ae tauschii accession 14191 and T. dicoccoides accession 7130) could be identified. Water stress induced active responses, rather than passive repartitioning of biomass was indicated by better shoot growth in seedlings of genotypes with enhanced root growth under stress. Membrane injury seemed to work as a trigger to activate water stress adaptive cellular machinery and was found positively correlated with several root-shoot based adaptive responses in seedlings. Stress induced proline accumulation in leaf tissue showed marked inter- and intra-specific genetic variation but hardly any association with stress adaptive plasticity. Genotypic variation for early stage plasticity traits viz., change in root dry weight, shoot length, shoot fresh weight, shoot dry weight and membrane injury positively correlated with grain weight based stress tolerance index (r = 0.267, r = 0.404, r = 0.299, r = 0.526, and r = 0.359, respectively). In another such trend, adaptive seedling plasticity correlated positively with resistance to early flowering under stress (r = 0.372 with membrane injury, r = 0.286 with change in root length, r = 0.352 with change in shoot length, r = 0.268 with change in shoot dry weight). Overall, Ae. tauschii accessions 9816, 14109, 14128, and T. dicoccoides accessions 5259 and 7130 were identified as potential donors of stress adaptive plasticity. The prospect of the study for molecular marker tagging, cloning of plasticity genes and creation of elite synthetic hexaploid donors is discussed.

Bread wheat progenitors: Aegilops tauschii (DD genome) and Triticum dicoccoides (AABB genome) reveal differential antioxidative response under water stress

Antioxidant enzymes are known to play a significant role in scavenging reactive oxygen species and maintaining cellular homeostasis. Activity of four antioxidant enzymes viz., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) was examined in the flag leaves of nine Aegilops tauschii and three Triticum dicoccoides accessions along with two bread wheat cultivars under irrigated and rain-fed conditions. These accessions were shortlisted from a larger set on the basis of field performance for a set of morpho-physiological traits. At anthesis, significant differences were observed in enzyme activities in two environments. A 45% elevation in average GR activity was observed under rain-fed conditions. Genotypic variation was evident within each environment as well as in terms of response to stress environment. Aegilops tauschii accession 3769 (86% increase in SOD, 41% in CAT, 72% in APX, 48% in GR activity) and acc. 14096 (37% increase in SOD, 32% CAT, 25% APX, 42% GR) showed up-regulation in the activity of all the four studied antioxidant enzymes. Aegilops tauschii accessions-9809, 14189 and 14113 also seemed to have strong induction mechanism as elevated activity of at least three enzymes was observed in them under rain-fed conditions. T. dicoccoides, on the other hand, maintained active antioxidative machinery under irrigated condition with relatively lower induction under stress. A significant positive correlation (r = 0.760) was identified between change in the activity of CAT and GR under stress. Changes in plant height, spike length and grain weight were recorded under stress and non-stress conditions on the basis of which a cumulative tolerance index was deduced and accessions were ranked for drought tolerance. Overall, Ae. tauschii accession 3769, 14096, 14113 (DD-genome) and T. dicoccoides accession 7054 (AABB-genome) may be used as donors to combine beneficial stress adaptive traits of all the three sub-genomes into a synthetic hexaploid for improving wheat for water stress conditions.

Staying Alive or Going to Die During Terminal Senescence-An Enigma Surrounding Yield Stability

Breeding programs with the aim to enhance yield productivity under abiotic stress conditions during the reproductive stage of crops is a top priority in the era of climate change. However, the choice of exploring stay-green or senescence phenotypes, which represent an opposing physiological bearing, are explored in cereal breeding programs for enhanced yield stability to a different extent. Thus, the consideration of stay-green or senescence phenotypes is still an ongoing debate and has not been comprehensively addressed. In this review, we provide arguments for designing a target phenotype to mitigate abiotic stresses during pre- and post-anthesis in cereals with a focus on hormonal balances regulating stay-green phenotype versus remobilization. The two major hypothesis for grain yield improvement are (i) the importance of the stay-green trait to elevate grain number under pre-anthesis and anthesis stress and (ii) fine tuning the regulatory and molecular physiological mechanisms to accelerate nutrient remobilization to optimize grain quality and seed weight under post-anthesis stress. We highlight why a cautious balance in the phenotype design is essential. While stay-green phenotypes promise to be ideal for developing stress-tolerant lines during pre-anthesis and fertilization to enhance grain number and yield per se, fine-tuning efficient remobilizing behavior during seed filling might optimize grain weight, grain quality and nutrient efficiency. The proposed model provides novel and focused directions for cereal stress breeding programs to ensure better seed-set and efficient grain-filling in cereals under terminal drought and heat stress exposure.