Auxin and Cytokinin Interplay during Leaf Morphogenesis and Phyllotaxy

Enhancing sweet potato production: a comprehensive analysis of the role of auxins and cytokinins in micropropagation

MAIN CONCLUSION

This review emphasizes the prevalent auxins and cytokinins used in sweet potato micropropagation, their optimal concentrations for effective in vitro regeneration, various propagation techniques, and Africa's potential to improve sweet potato production. Ipomoea batatas (L.) Lam., or sweet potato, is a robust, nutritious, and adaptable crop traditionally propagated through conventional methods. These techniques, however, have limitations, prompting the adoption of micropropagation as an efficient alternative for producing healthy, cost-effective plantlets in reduced time. This review critically evaluates the influence of auxins and cytokinins, the most frequently utilized plant growth regulators (PGRs), in enhancing sweet potato micropropagation protocols. The study examines the crop's origins, distribution, and cultivation practices, as well as the morphophysiological effects of PGRs on sweet potatoes. Our analysis reveals that 6-benzylaminopurine (BAP) and N6-benzyladenine (BA) are the predominant cytokinins, while naphthaleneacetic acid (NAA) and indole-3-butyric acid (IBA) are the primary auxins employed in sweet potato micropropagation. The review also proposes strategies for increasing production, particularly in Africa, and identifies areas requiring further investigation to better understand how these growth regulators impact the physiological development and response of sweet potatoes to environmental stress. This comprehensive assessment contributes to the expanding knowledge base on sweet potato micropropagation and offers valuable insights for researchers and practitioners in the field.

Hormonal profiles in dormant turions of 22 aquatic plant species: do they reflect functional or taxonomic traits?

BACKGROUND AND AIMS

Turions are vegetative, dormant overwintering organs formed in aquatic plants in response to unfavourable ecological conditions. Contents of cytokinin (CK), auxin metabolites and abscisic acid (ABA) as main growth and development regulators were compared in innately dormant autumnal turions of 22 aquatic plant species of different functional ecological or taxonomic groups with those in non-dormant winter apices in three aquatic species and with those in spring turions of four species after their overwintering.

METHODS

The hormones were analysed in miniature turion samples using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry.

KEY RESULTS

In innately dormant turions, the total contents of each of the four main CK types, biologically active forms and total CKs differed by two to three orders of magnitude across 22 species; the proportion of active CK forms was 0.18-67 %. Similarly, the content of four auxin forms was extremely variable and the IAA proportion as the active form was 0.014-99 %. The ABA content varied from almost zero to 54 µmol kg-1 dry weight and after overwintering it usually significantly decreased. Of all functional traits studied, hormone profiles depended most on the place of turion sprouting (surface vs bottom) and we suggest that this trait is crucial for turion ecophysiology.

CONCLUSIONS

The key role of ABA in regulating turion dormancy was confirmed. However, the highly variable pattern of the ABA content in innately dormant and in overwintered turions indicates that the hormonal mechanism regulating the innate dormancy and its breaking in turions is not uniform within aquatic plants.

DRMY1 promotes robust morphogenesis by sustaining the translation of cytokinin signaling inhibitor proteins

Robustness is the invariant development of phenotype despite environmental changes and genetic perturbations. In the Arabidopsis flower bud, four sepals robustly initiate and grow to constant size to enclose and protect the inner floral organs. We previously characterized the mutant development related myb-like1 ( drmy1 ), where 3-5 sepals initiate variably and grow to different sizes, compromising their protective function. The molecular mechanism underlying this loss of robustness was unclear. Here, we show that drmy1 has reduced TARGET OF RAPAMYCIN (TOR) activity, ribosomal content, and translation. Translation reduction decreases the protein level of ARABIDOPSIS RESPONSE REGULATOR7 (ARR7) and ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6 (AHP6), two cytokinin signaling inhibitors that are normally rapidly produced before sepal initiation. The resultant upregulation of cytokinin signaling disrupts robust auxin patterning and sepal initiation. Our work shows that the homeostasis of translation, a ubiquitous cellular process, is crucial for the robust spatiotemporal patterning of organogenesis.

Advancements in Rice Leaf Development Research

Rice leaf morphology is a pivotal component of the ideal plant architecture, significantly impacting rice yield. The process of leaf development unfolds through three distinct stages: the initiation of leaf primordia, the establishment and maintenance of polarity, and leaf expansion. Genes regulating leaf morphology encompass transcription factors, hormones, and miRNAs. An in-depth synthesis and categorization of genes associated with leaf development, particularly those successfully cloned, hold paramount importance in unraveling the complexity of rice leaf development. Furthermore, it provides valuable insights into the potential for molecular-level manipulation of rice leaf types. This comprehensive review consolidates the stages of rice leaf development, the genes involved, molecular regulatory pathways, and the influence of plant hormones. Its objective is to establish a foundational understanding of the creation of ideal rice leaf forms and their practical application in molecular breeding.

Enhancing drought, heat shock, and combined stress tolerance in Myrobalan 29C rootstocks with foliar application of potassium nitrate

BACKGROUND

Drought and heat stress are significant concerns to food security in arid and semi-arid regions, where global warming is predicted to increase both frequency and severity. To cope with these challenges, the use of drought-tolerant plants or technological interventions are essential. In this study, the effects of foliar potassium nitrate (KNO3) application on the stress tolerance and recovery of Myrobalan 29C rootstocks (Prunus cerasifera Ehrh.) were evaluated. These rootstocks are widely recognized for their adaptability and are extensively used in fruit production. To assess their response, the rootstocks were subjected to drought, heat shock, or a combination of both stressors. Additionally, they were treated with 1.0% KNO3 via foliar application. Throughout the stress and recovery periods, various morphological, physiological, and bio-chemical parameters were measured.

RESULTS

Based on our results, KNO3 treatment improved LRWC, Chl stability, SC, and key stress markers like proline, MDA, H2O2, along with antioxidant enzymes CAT, SOD, POD during both stress and recovery phases. Moreover, our results emphasized KNO3's critical role in hormone regulation under stress. KNO3 application significantly altered hormone levels, notably increasing ABA during drought and heat shock stress, essential for stress response and adaptation. In contrast, IAA, GA, and cytokinin's significantly increased during the recovery phase in KNO3-treated plants, indicating improved growth regulation and stress recovery. In addition, KNO3 application improved the recovery process of the rootstocks by restoring their physiological and biochemical functions.

CONCLUSION

This study suggests that the application of foliar KNO3 is an effective technique for enhancing the drought and heat tolerance as well as the recovery of Myrobalan 29C rootstocks. These results hold significant value for farmers, policymakers, and researchers, as they offer crucial insights into the development of drought-tolerant crops and the management of climate change's adverse effects on agriculture.

Characteristics of turion development in two aquatic carnivorous plants: Hormonal profiles, gas exchange and mineral nutrient content

Turions are vegetative, dormant, and storage overwintering organs formed in perennial aquatic plants in response to unfavorable ecological conditions and originate by extreme condensation of apical shoot segments. The contents of cytokinins, auxins, and abscisic acid were estimated in shoot apices of summer growing, rootless aquatic carnivorous plants, Aldrovanda vesiculosa and Utricularia australis, and in developing turions at three stages and full maturity to reveal hormonal patterns responsible for turion development. The hormones were analyzed in miniature turion samples using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry. Photosynthetic measurements in young leaves also confirmed relatively high photosynthetic rates at later turion stages. The content of active cytokinin forms was almost stable in A. vesiculosa during turion development but markedly decreased in U. australis. In both species, auxin content culminated in the middle of turion development and then decreased again. The content of abscisic acid as the main inhibitory hormone was very low in growing plants in both species but rose greatly at first developmental stages and stayed very high in mature turions. The hormonal data indicate a great strength of developing turions within sink-source relationships and confirm the central role of abscisic acid in regulating the turion development.

Ginkgo biloba GbbZIP08 transcription factor is involved in the regulation of flavonoid biosynthesis

Ginkgo biloba is the oldest relict plant on Earth and an economic plant resource derived from China. Flavonoids extracted from G. biloba are beneficial to the prevention and treatment of cardiovascular and cerebrovascular diseases. Basic leucine zipper (bZIP) transcription factors (TFs) have been recognized to play important roles in plant secondary metabolism. In this study, GbbZIP08 was isolated and characterized. It encodes a protein containing 154 amino acids, which belongs to hypocotyl 5 in group H of the bZIP family. Tobacco transient expression assay indicated that GbbZIP08 was localized in the plant nucleus. GbbZIP08 overexpression showed that the contents of total flavonoids, kaempferol, and anthocyanin in transgenic tobacco were significantly higher than those in the wild type. Transcriptome sequencing analysis revealed significant upregulation of structural genes in the flavonoid biosynthesis pathway. In addition, phytohormone signal transduction pathways, such as the abscisic acid, salicylic acid, auxin, and jasmonic acid pathways, were enriched with a large number of differentially expressed genes. TFs such as MYB, AP2, WRKY, NAC, bZIP, and bHLH, were also differentially expressed. The above results indicated that GbbZIP08 overexpression promoted flavonoid accumulation and increased the transcription levels of flavonoid-synthesis-related genes in plants.

Friends in Arms: Flavonoids and the Auxin/Cytokinin Balance in Terrestrialization

Land plants survive the challenges of new environments by evolving mechanisms that protect them from excess irradiation, nutrient deficiency, and temperature and water availability fluctuations. One such evolved mechanism is the regulation of the shoot/root growth ratio in response to water and nutrient availability by balancing the actions of the hormones auxin and cytokinin. Plant terrestrialization co-occurred with a dramatic expansion in secondary metabolism, particularly with the evolution and establishment of the flavonoid biosynthetic pathway. Flavonoid biosynthesis is responsive to a wide range of stresses, and the numerous synthesized flavonoid species offer two main evolutionary advantages to land plants. First, flavonoids are antioxidants and thus defend plants against those adverse conditions that lead to the overproduction of reactive oxygen species. Second, flavonoids aid in protecting plants against water and nutrient deficiency by modulating root development and establishing symbiotic relations with beneficial soil fungi and bacteria. Here, we review different aspects of the relationships between the auxin/cytokinin module and flavonoids. The current body of knowledge suggests that whereas both auxin and cytokinin regulate flavonoid biosynthesis, flavonoids act to fine-tune only auxin, which in turn regulates cytokinin action. This conclusion agrees with the established master regulatory function of auxin in controlling the shoot/root growth ratio.

Is auxin enough? Cytokinins and margin patterning in simple leaves

The interplay between auxin and cytokinins affects facets of plant development as different as ovule formation and lateral root initiation. Moreover, cytokinins favor complexity in the development of Solanum lycopersicum and Cardamine hirsuta compound leaves. Nevertheless, no role has been proposed for cytokinins in patterning the margins of the simple leaves of Arabidopsis thaliana, a process that is assumed to be sufficiently explained by auxin localization. Here, we discuss evidence supporting the hypothesis that cytokinins play a role in simple leaf margin morphogenesis via crosstalk with auxin, as occurs in other plant developmental events. Indeed, mutant or transgenic arabidopsis plants defective in cytokinin biosynthesis or signaling, or with increased cytokinin degradation have leaf margins less serrated than the wild type.

Cytokinins and auxins in organs of aquatic carnivorous plants: what do they reflect?

BACKGROUND AND AIMS

Aquatic carnivorous plants have typical rootless linear shoots bearing traps and exhibit steep physiological polarity with rapid apical growth. The aim was to analyse auxin and cytokinin metabolites in traps, leaves/shoots and shoot apices in several species of genera Aldrovanda and Utricularia to elucidate how the hormonal profiles reflect the specific organ functions and polarity.

METHODS

The main auxin and cytokinin metabolites were analysed in miniature samples (>2 mg dry weight) of different organs of Aldrovanda vesiculosa and six Utricularia species using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry.

KEY RESULTS

Total contents of biologically active forms (free bases, ribosides) of all four main endogenously occurring cytokinin types were consistently higher in traps than in leaves in four Utricularia species with monomorphic shoots and/or higher than in shoots in two Utricularia species with dimorphic shoots. In Aldrovanda traps, the total content of different cytokinin forms was similar to or lower than that in shoots. In U. australis leaves, feeding on prey increased all cytokinin forms, while no consistent differences occurred in Aldrovanda. In four aquatic Utricularia species with monomorphic shoots, the content of four auxin forms was usually higher in traps than in leaves. Zero IAA content was determined in U. australis leaves from a meso-eutrophic site or when prey-fed.

CONCLUSIONS

Different cytokinin and auxin profiles estimated in traps and leaves/shoots of aquatic carnivorous plants indicate an association with different dominant functions of these organs: nutrient uptake by traps versus photosynthetic function of traps. Interplay of cytokinins and auxins regulates apical dominance in these plants possessing strong polarity.

WOX family transcriptional regulators modulate cytokinin homeostasis during leaf blade development in Medicago truncatula and Nicotiana sylvestris

The plant-specific family of WUSCHEL (WUS)-related homeobox (WOX) transcription factors is key regulators of embryogenesis, meristem maintenance, and lateral organ development in flowering plants. The modern/WUS clade transcriptional repressor STENOFOLIA/LAMINA1(LAM1), and the intermediate/WOX9 clade transcriptional activator MtWOX9/NsWOX9 antagonistically regulate leaf blade expansion, but the molecular mechanism is unknown. Using transcriptome profiling and biochemical methods, we determined that NsCKX3 is the common target of LAM1 and NsWOX9 in Nicotiana sylvestris. LAM1 and NsWOX9 directly recognize and bind to the same cis-elements in the NsCKX3 promoter to repress and activate its expression, respectively, thus controlling the levels of active cytokinins in vivo. Disruption of NsCKX3 in the lam1 background yielded a phenotype similar to the knockdown of NsWOX9 in lam1, while overexpressing NsCKX3 resulted in narrower and shorter lam1 leaf blades reminiscent of NsWOX9 overexpression in the lam1 mutant. Moreover, we established that LAM1 physically interacts with NsWOX9, and this interaction is required to regulate NsCKX3 transcription. Taken together, our results indicate that repressor and activator WOX members oppositely regulate a common downstream target to function in leaf blade outgrowth, offering a novel insight into the role of local cytokinins in balancing cell proliferation and differentiation during lateral organ development.

Involvement of Auxin-Mediated CqEXPA50 Contributes to Salt Tolerance in Quinoa (Chenopodium quinoa) by Interaction with Auxin Pathway Genes

Soil salinization is a global problem that limits crop yields and threatens agricultural development. Auxin-induced expansins contribute to plant salt tolerance through cell wall loosening. However, how auxins and expansins contribute to the adaptation of the halophyte quinoa (Chenopodium quinoa) to salt stress has not yet been reported. Here, auxin was found to contribute to the salt tolerance of quinoa by promoting the accumulation of photosynthetic pigments under salt stress, maintaining enzymatic and nonenzymatic antioxidant systems and scavenging excess reactive oxygen species (ROS). The Chenopodium quinoa expansin (Cqexpansin) family and the auxin pathway gene family (Chenopodium quinoa auxin response factor (CqARF), Chenopodium quinoa auxin/indoleacetic acid (CqAux/IAA), Chenopodium quinoa Gretchen Hagen 3 (CqGH3) and Chenopodium quinoa small auxin upregulated RNA (CqSAUR)) were identified from the quinoa genome. Combined expression profiling identified Chenopodium quinoa α-expansin 50 (CqEXPA50) as being involved in auxin-mediated salt tolerance. CqEXPA50 enhanced salt tolerance in quinoa seedlings was revealed by transient overexpression and physiological and biochemical analyses. Furthermore, the auxin pathway and salt stress-related genes regulated by CqEXPA50 were identified. The interaction of CqEXPA50 with these proteins was demonstrated by bimolecular fluorescence complementation (BIFC). The proteins that interact with CqEXPA50 were also found to improve salt tolerance. In conclusion, this study identified some genes potentially involved in the salt tolerance regulatory network of quinoa, providing new insights into salt tolerance.

Integrated application of plant bioregulator and micronutrients improves crop physiology, productivity and grain biofortification of delayed sown wheat

Delay sowing of wheat is a common problem in Punjab that exacerbates serious yield loss. To reduce yield loss and improve yield, different combinations of foliar-applied bioregulator and micronutrients, control (CK), zinc (Zn), boron (B), thiourea (TU), Zn + B (ZnB), Zn + TU (ZnTU), B + TU (BTU), Zn + B + TU (ZnBTU) were applied at booting and grain filling stages in delayed sown wheat in 2017-18 and 2018-19. The results showed that ZnBTU treatment significantly increased leaf area index by 25.06% and 23.21%, spike length by 15.11% and 19.65% in 2017 and 2018, respectively, compared to CK. The ZnBTU treatment also increased 1000-grain weight by 21.96% and 22.01% in 2017 and 2018, respectively, compared to CK. Similarly, higher Zn, B and N contents in straw and grain were recoded for ZnBTU treatment which was statistically similar to ZnB and ZnTU treatments. Overall, ZnBTU treatment also increased the photosynthetic rate, transpiration rate, stomatal conductance by 46.67%, 26.03%, 76.25% and decreased internal CO2 by 28.18%, compared to CK, respectively. Moreover, ZnBTU also recorded the highest grain yield in 2017-18 (25.05%) and 2018-19 (28.49%) than CK. In conclusion, foliar application of ZnBTU at the booting and grain filling stages of delayed sown wheat could be a promising strategy to increase grain yield.