Improving Crop Lodging Resistance by Adjusting Plant Height and Stem Strength

Crop height not only determines plant resistance to lodging and crowding, but also affects crop architecture, apical dominance, biomass, and mechanical harvesting. Plant height is determined by the internode elongation, regulated by genes involved in gibberellin (GA) and brassinosteroid (BR) biosynthesis or related signaling networks. Plants’ genetic inability to synthesize or respond to GAs and BRs induce dwarfness. However, the signaling mechanisms of GAs and BRs for controlling plant height individually or collectively are still unclear. Since stem mechanically supports plant during the whole life span, components that affect stem physical strength are also important to crop lodging resistance. One of the major components is lignin, which forms stem structure, thus contributing to crop lodging resistance. In this review, we looked into the reported genes involved in lignin, GAs, and BRs biosynthesis and summarized the signaling networks centered by these genes. Then, we filled the knowledge gap by modifying plant height through interrupting normal GA and BR metabolism utilizing core gene inhibitors. Therefore, we highly endorsed the current approaches of using plant growth regulators (PRGs) to maintain an ideal plant height under lodging stress, and proposed possibilities of modifying crop culm strength against lodging as well.

Genome-wide identification and expression profiling of the COBRA-like genes reveal likely roles in stem strength in rapeseed (Brassica napus L.)

The COBRA-like (COBL) genes play key roles in cell anisotropic expansion and the orientation of microfibrils. Mutations in these genes cause the brittle stem and induce pathogen responsive phenotypes in Arabidopsis and several crop plants. In this study, an in silico genome-wide analysis was performed to identify the COBL family members in Brassica. We identified 44, 20 and 23 COBL genes in B. napus and its diploid progenitor species B. rapa and B. oleracea, respectively. All the predicted COBL genes were phylogenetically clustered into two groups: the AtCOB group and the AtCOBL7 group. The conserved chromosome locations of COBLs in Arabidopsis and Brassica, together with clustering, indicated that the expansion of the COBL gene family in B. napus was primarily attributable to whole-genome triplication. Among the BnaCOBLs, 22 contained all the conserved motifs and derived from 9 of 12 subgroups. RNA-seq analysis was used to determine the tissue preferential expression patterns of various subgroups. BnaCOBL9, BnaCOBL35 and BnaCOBL41 were highly expressed in stem with high-breaking resistance, which implies these AtCOB subgroup members may be involved in stem development and stem breaking resistance of rapeseed. Our results of this study may help to elucidate the molecular properties of the COBRA gene family and provide informative clues for high stem-breaking resistance studies.

Pre- and/or Postharvest Silicon Application Prolongs the Vase Life and Enhances the Quality of Cut Peony (Paeonia lactiflora Pall.) Flowers

Peony is an important ornamental plant and has become increasingly popular for cut flower cultivation. However, a short vase life and frequent poor vase quality severely restrict its market value. The study described herein was conducted to investigate the effects of silicon application on the vase life and quality of two cut peony (Paeonia lactiflora Pall.) cultivars, ‘Taebaek’ and ‘Euiseong’. For pre- and/or postharvest silicon application, four experimental groups based on treatments were designed. With silicon treatment, the relevant growth attributes, including the shoot and leaf lengths, stem and bud diameters as well as the leaf width were all remarkably increased. In the postharvest storage, the addition of silicon to the holding solution in the vase was able to significantly extend vase life, delay fresh weight decrease, and improve vase quality, as characterized by the antioxidant enzyme activities and mechanical stem strength. Taken together, silicon application, regardless of the approach, was able to effectively prolong the vase life and enhance the quality of cut peony flowers.

Soil Moisture Levels Affect the Anatomy and Mechanical Properties of Basil Stems (Ocimum basilicum L.)

As plants would benefit from adjusting and optimizing their architecture to changing environmental stimuli, ensuring a strong and healthy plant, it was hypothesized that different soil moisture levels would affect xylem and collenchyma development in basil (Ocimum basilicum L. cv. Marian) stems. Four different irrigation set-points (20, 30, 40 and 50% VWC), corresponding respectively to pF values of 1.95, 1.65, 1.30 and 1.15, were applied. Basil plants grown near the theoretical wilting point (pF 2) had a higher xylem vessel frequency and lower mean vessel diameter, promoting water transport under drought conditions. Cultivation at low soil moisture also impacted the formation of collenchyma in the apical stem segments, providing mechanical and structural support to these fast-growing stems and vascular tissues. The proportion of collenchyma area was significantly lower for the pF1.15 treatment (9.25 ± 3.24 %) compared to the pF1.95 and pF1.30 treatments (16.04 ± 1.83% and 13.28 ± 1.38 %, respectively). Higher fractions of collenchyma resulted in a higher mechanical stem strength against bending. Additionally, tracheids acted as the major support tissues in the basal stem segments. These results confirm that the available soil moisture impacts mechanical stem strength and overall plant quality of basil plants by impacting xylem and collenchyma development during cultivation, ensuring sufficient mechanical support to the fast-growing stem and to the protection of the vascular tissues. To our knowledge, this study is the first to compare the mechanical and anatomical characteristics of plant stems cultivated at different soil moisture levels.

Genome-Wide Association Study Identified Novel Candidate Loci/Genes Affecting Lodging Resistance in Rice

Lodging reduces rice yield, but increasing lodging resistance (LR) usually limits yield potential. Stem strength and leaf type are major traits related to LR and yield, respectively. Hence, understanding the genetic basis of stem strength and leaf type is of help to reduce lodging and increase yield in LR breeding. Here, we carried out an association analysis to identify quantitative trait locus (QTLs) affecting stem strength-related traits (internode length/IL, stem wall thickness/SWT, stem outer diameter/SOD, and stem inner diameter/SID) and leaf type-associated traits (Flag leaf length/FLL, Flag leaf angle/FLA, Flag leaf width/FLW, leaf-rolling/LFR and SPAD/Soil, and plant analyzer development) using a diverse panel of 550 accessions and evaluated over two years. Genome-wide association study (GWAS) using 4,076,837 high-quality single-nucleotide polymorphisms (SNPs) identified 89 QTLs for the nine traits. Next, through “gene-based association analysis, haplotype analysis, and functional annotation”, the scope was narrowed down step by step. Finally, we identified 21 candidate genes in 9 important QTLs that included four reported genes (TUT1, OsCCC1, CFL1, and ACL-D), and seventeen novel candidate genes. Introgression of alleles, which are beneficial for both stem strength and leaf type, or pyramiding stem strength alleles and leaf type alleles, can be employed for LR breeding. All in all, the experimental data and the identified candidate genes in this study provide a useful reference for the genetic improvement of rice LR.

Knockout of the lignin pathway gene BnF5H decreases the S/G lignin composition ratio and improves S. sclerotiorum resistance in B. napus

Ferulate-5-hydroxylase (F5H) is a key enzyme involved in the conversion of the guaiacyl monolignol (G-monolignol) to the syringyl monolignol (S-monolignol) in angiosperms. The monolignol ratio has been proposed to affect biomass recalcitrance and the resistance to plant disease. Sclerotinia sclerotiorum (S. sclerotiorum) stem rot of Brassica napus (B. napus) causes heavy damage in oilseed rape production. To date, there is no information about the effect of the lignin monomer ratio on the resistance to S. sclerotiorum in B. napus. Four dominantly expressed BnF5H genes were knocked out by CRISPR/Cas9 simultaneously in B. napus, and the f5h mutant KO-7 was generated. The S/G lignin composition ratio was decreased compared to that of the wild type (WT) based on the results of Mӓule staining and 2D-NMR profiling. The resistance to S. sclerotiorum in stems and leaves increased in KO-7. Furthermore, we found that the stem strength of KO-7 was significantly increased compared to that of the WT. Collectively, for the first time, we demonstrate that knockout of the lignin pathway gene F5H decreases the S/G ratio, improves S. sclerotiorum resistance in B. napus, and increases stem strength.

Epidemiological Characterization of Lettuce Drop and Biophysical Features of the Host Identify Soft Stem as a Susceptibility Factor to Sclerotinia minor

The soilborne fungus Sclerotinia minor was not known to produce sclerotia in the stems of infected and uncollapsed Lactuca standing intact until our observation in a greenhouse in 2017. We investigated lettuce–environment–S. minor interactions in two tolerant and four susceptible Lactuca genotypes to determine putative risk factors and targets for disease control. Symptomatological, pathophysiological, developmental, basal stem biophysical, and microclimate responses (27 variables) of the genotypes were determined under field and/or greenhouse conditions. Distinct patterns of infection responses were observed between modern cultivars and their primitive/wild relatives. The modern cultivars were susceptible to rapid basal stem and root degradations by S. minor. The oil-seed lettuce PI 251246 and the wild L. serriola 11-G99 were resilient to degradations and significantly deterred mycelium emergence and symptom development, but sclerotia formed to a significantly higher height in their stems. Photosynthetic efficiency declined rapidly within 1-day postinoculation (dpi) in susceptible plants but remained intact ~5–6 dpi in the tolerant 11-G99. Stomatal conductance spiked rapidly in 11-G99 plants within 1–3 dpi, coinciding with the emergence of fungal mycelia at the crown. A strong negative correlation detected between basal stem degradation severity/collapse and stem mechanical strength indicated that stem strength-mediated genetic factors determine the outcome of Sclerotinia infections of the host. Soft stem is a prominent lettuce drop susceptibility factor that could be targeted in resistance breeding. It also provides the prelude for the analysis of the biological basis of plant architecture-mediated resistance to Sclerotinia spp. in lettuce and other hosts.

THE CONTROLLING FACTORS OF SILICON SOLUBILITY IN SOIL SOLUTION

Silica is a beneficial element for rice plants which can protect from blast disease, increase stem strength, and alleviate abiotic stress. Silicon in soil solution is affected by several factors such as pH, temperature, organic matter, and redox potential (Eh). This study aims to investigate the controlling factor of Si solubility in soil solution. In the present study, Japanese silica gel (JSG) and Ultisols were collected from Japan. In laboratory experiment, the effects of Ca (calcium), Mg (magnesium) and others on solubility of Si (silica) were investigated. Under submerged condition, ten gram of soil with silica gel, Ca and Mg in plastic tube were incubated at 300C for 29 days. Calcium and Mg were applied into soil, at the concentration of 5, 10, 15 mg Ca L-1(T2, T3, T4 respectively) and 5, 10, 15 mg Mg L-1(T5, T6, T7 respectively). There was two controls as a follow T0 (soil) and T1 (soil + silica gel). During incubation, Si, Ca, Mg, Fe, and Mn concentrations in surface water were measured using ICP spectroscopy at day 8, 15, and 29. The results show the soil before treatment was slightly acidic (pH 5.7) and extractable Si concentration was 267.1 mg SiO2 kg-1. It was classified to be below critical level of available Si for rice (300 mg SiO2 kg-1). Total concentration of Ca and Mg in soil solution were highest for treatment T4 and T7, respectively compared with other treatments. On the first 8 days of incubation, Si released into soil solution was higher in T1 and T2 compared to other treatments. The solubility of Si was significantly positive correlated with Mn, Eh, and negatively correlated with pH, that indicated these were the controlling factors of the Si release in soil solution. There was no correlation between Si and Ca or Mg concentration in soil solution.