Maize expressing the sunflower transcription factor HaHB11 has improved productivity in controlled and field conditions

Delaying or promoting? Manipulation of leaf senescence to improve crop yield and quality

MAIN CONCLUSION

Senescence influences leaf productivity through two aspects: photosynthesis and nutrient remobilization. Through distinctively manipulating progress of leaf senescence, it is promising to improve crop yield and quality simultaneously. Crop yield and quality are two chief goals pursued in agricultural and horticultural production. The basis of crop yield is leaf photosynthesis. Senescence is the last stage of leaf development, which usually causes decreasing of leaf photosynthetic activity. Delaying leaf senescence through physiological or molecular strategies may result in higher photosynthetic activity with a longer duration, thus producing more photoassimilates for biomass accumulation. On the other side, leaf senescence always induces degradation of macromolecular nutrients (including chlorophylls and proteins), and nutritional elements in leaves are then resorbed for development of other organs. For those crops with non-leaf organs as harvested biomass, translocating nutritional elements from leaves to harvested biomass is an indispensable physiological process to increase crop yield and quality. This review summarized successful studies about effects of delaying or promoting senescence on crop yield or quality improvement. Considering the distinctiveness of various crops, manipulation of leaf senescence should be specialized during agricultural and horticultural practices. Rational regulation of leaf senescence, such as inhibiting senescence to maintain leaf photosynthesis and then promoting senescence (with appropriate onset and efficiency) to remobilize more nutrients from leaves to target organs, may ultimately improve both crop yield and quality.

The Transcription Factor HaHB11 Boosts Grain Set and Yield in Rice Plants, Allowing Them to Approach Their Ideal Phenotype

The ideal rice phenotype is that of plants exhibiting fewer panicles with high biomass, large grain number, flag leaf area with small insertion angles, and an erected morphology improving light interception. The sunflower transcription factor HaHB11, homeodomain-leucine zipper I, confers increased seed yield and abiotic stress tolerance to Arabidopsis and maize. Here, we report the obtaining and characterization of rice plants expressing HaHB11 driven by its promoter or the 35S constitutive one. Transgenic p35S:HaHB11 plants closely resembled the ideal high-yield phenotype, whereas those carrying the pHaHB11:HaHB11 construct were hard to distinguish from the wild type. The former had an erected architecture, enhanced vegetative leaf biomass, rolled flag leaves with a larger surface, sharper insertion angles insensitive to brassinosteroids, and higher harvest index and seed biomass than the wild type. The combination of the distinct features exhibited by p35S:HaHB11 plants, including the increased number of set grains per panicle, supports the high-yield phenotype. We wondered where HaHB11 has to be expressed to achieve the high-yield phenotype and evaluated HaHB11 expression levels in all tissues. The results indicate that its expression is particularly necessary in the flag leaf and panicle to produce the ideal phenotype.

Expressing the sunflower transcription factor HaHB11 in maize improves waterlogging and defoliation tolerance

The sunflower (Helianthus annuus) transcription factor HaHB11 (H. annuus  Homeobox 11) belongs to the homeodomain-leucine zipper family and confers improved yield to maize (Zea mays) hybrids (HiII × B73) and lines. Here we report that transgenic maize lines expressing HaHB11 exhibited better performance under waterlogging, both in greenhouse and field trials carried out during three growth cycles. Transgenic plants had increased chlorophyll content, wider stems, more nodal roots, greater total aerial biomass, a higher harvest index, and increased plant grain yield. Under severe defoliation caused by a windstorm during flowering, transgenic genotypes were able to set more grains than controls. This response was confirmed in controlled defoliation assays. Hybrids generated by crossing B73 HaHB11 lines with the contrasting Mo17 lines were also tested in the field and exhibited the same beneficial traits as the parental lines, compared with their respective controls. Moreover, they were less penalized by stress than commercial hybrids. Waterlogging tolerance increased via improvement of the root system, including more xylem vessels, reduced tissue damage, less superoxide accumulation, and altered carbohydrate metabolism. Multivariate analyses corroborated the robustness of the differential traits observed. Furthermore, canopy spectral reflectance data, computing 29 vegetation indices associated with biomass, chlorophyll, and abiotic stress, helped to distinguish genotypes as well as their growing conditions. Altogether the results reported here indicate that this sunflower gene constitutes a suitable tool to improve maize plants for environments prone to waterlogging and/or wind defoliation.

Leaf senescence: progression, regulation, and application

Leaf senescence, the last stage of leaf development, is a type of postmitotic senescence and is characterized by the functional transition from nutrient assimilation to nutrient remobilization which is essential for plants' fitness. The initiation and progression of leaf senescence are regulated by a variety of internal and external factors such as age, phytohormones, and environmental stresses. Significant breakthroughs in dissecting the molecular mechanisms underpinning leaf senescence have benefited from the identification of senescence-altered mutants through forward genetic screening and functional assessment of hundreds of senescence-associated genes (SAGs) via reverse genetic research in model plant Arabidopsis thaliana as well as in crop plants. Leaf senescence involves highly complex genetic programs that are tightly tuned by multiple layers of regulation, including chromatin and transcription regulation, post-transcriptional, translational and post-translational regulation. Due to the significant impact of leaf senescence on photosynthesis, nutrient remobilization, stress responses, and productivity, much effort has been made in devising strategies based on known senescence regulatory mechanisms to manipulate the initiation and progression of leaf senescence, aiming for higher yield, better quality, or improved horticultural performance in crop plants. This review aims to provide an overview of leaf senescence and discuss recent advances in multi-dimensional regulation of leaf senescence from genetic and molecular network perspectives. We also put forward the key issues that need to be addressed, including the nature of leaf age, functional stay-green trait, coordination between different regulatory pathways, source-sink relationship and nutrient remobilization, as well as translational researches on leaf senescence.

The sunflower TLDc-containing protein HaOXR2 confers tolerance to oxidative stress and waterlogging when expressed in maize plants

The sunflower (Helianthus annuus L.) genome encodes six proteins containing a TLDc domain, typical of the eukaryotic OXidation Resistance (OXR) protein family. Expression of sunflower HaOXR2 in Arabidopsis generated plants with increased rosette diameter, higher number of leaves and increased seed production. Maize inbred lines expressing HaOXR2 also showed increased total leaf area per plant. In addition, heterologous expression of HaOXR2 induced an increase in the oxidative stress tolerance in Arabidopsis and maize. Maize transgenic plants expressing HaOXR2 experienced less oxidative damage and exhibited increased photosynthetic performance and efficiency than non-transgenic segregant plants after treatment of leaves with the reactive oxygen species generating compound Paraquat. Expression of HaOXR2 in maize also improved tolerance to waterlogging. The number of expanded leaves, aerial biomass, and stem height and cross-section area were less affected by waterlogging in HaOXR2 expressing plants, which also displayed less aerial tissue damage under these conditions. Transgenic plants also showed an increased production of roots, a typical adaptive stress response. The results show the existence of functional conservation of OXR proteins in dicot and monocot plants and indicate that HaOXR2 could be useful to improve plant performance under conditions that increase oxidative stress.

Key role of the motor protein Kinesin 13B in the activity of homeodomain-leucine zipper I transcription factors

The sunflower (Helianthus annuus) homeodomain-leucine zipper I transcription factor HaHB11 conferred differential phenotypic features when it was expressed in Arabidopsis, alfalfa, and maize plants. Such differences were increased biomass, seed yield, and tolerance to flooding. To elucidate the molecular mechanisms leading to such traits and identify HaHB11-interacting proteins, a yeast two-hybrid screening of an Arabidopsis cDNA library was carried out using HaHB11 as bait. The sole protein identified with high confidence as interacting with HaHB11 was Kinesin 13B. The interaction was confirmed by bimolecular fluorescence complementation and by yeast two-hybrid assay. Kinesin 13B also interacted with AtHB7, the Arabidopsis closest ortholog of HaHB11. Histochemical analyses revealed an overlap between the expression patterns of the three genes in hypocotyls, apical meristems, young leaves, vascular tissue, axillary buds, cauline leaves, and cauline leaf nodes at different developmental stages. AtKinesin 13B mutants did not exhibit a differential phenotype when compared with controls; however, both HaHB11 and AtHB7 overexpressor plants lost, partially or totally, their differential phenotypic characteristics when crossed with such mutants. Altogether, the results indicated that Kinesin 13B is essential for the homeodomain-leucine zipper transcription factors I to exert their functions, probably via regulation of the intracellular distribution of these transcription factors by the motor protein.