Stay Green: A Better Subvention For Crop Improvement

2017-08-11 08:56:17



The diversity of plant life is manifested, at least in part, by thevaried pigment compositions that occur between species andwithin species at different stages of the life cycle. Plants control the abundance of pigments within their cells depending on the developmental stage of particular tissues or organs and in response to environmental cues. Chlorophyll is degraded in plants as a result of natural or induced senescence, at the onset of fruit ripening, and in response to pathogen infection.The pigmentcomposition of plants is regulated by genetic and environmentalstimuli and is the result of a balance between de novo synthesis anddegradation. Pigment changes occur during a variety of processesincluding energy acquisition, photo-protection, nutrient recycling,floral development and fruit ripening. For example, the transitionfrom a skoto-morphogenic (dark) to photo-morphogenic (light)growth occurs in germinating seedlings as they emerge from thesoil and is accompanied by the synthesis of chlorophyll and carotenoids as the proplastid transitions into the chloroplast. Asthe plant ages and senescence is initiated, the chloroplasttransitions to a gerontoplast, the thylakoid membranes are brokendown, chlorophyll is degraded and nutrients are released andrecycled to the rest of the plant or the developing seeds.

 When plant attains senescence stage but still chlorophyll breakdown has not shown it is nothing but a stay green trait.In other words, stay-green results when the plant’s normal process of senescence is disrupted. The accumulation of the brightly colored pigments serves as a signal that facilitates plant reproduction. Determinant crops show one direction growth/reproduction where in indeterminant repeated cycles of reproduction makes several chlorophyll to other pigment conversions. Within the myriad of biochemical reactions that constitute compositional changes in plant pigments, the degradation of chlorophyll is a fundamental process that is of central importance to plant metabolism. Stay green is an effective strategy for increasing of crop yield, particularly under waterlimited conditions. Study of genotypic variance of stay green is very useful for breeding program of stay green character.The molecular knowledge paved a way to know the identity of the long sought stay-green (SGR/SID) genes.

Mechanism of stay green:

1. The extended foliar greenness during grain filling under post anthesis drought is the stay green phenomena. It is the heritable delayed foliar senescence character in model and crop plant species and genotypes possessing this trait maintain more photo-synthetically active leaves than genotypes not possessing this trait. It is required especially in a drought environmental condition, to keep greenness of leaves alive for longer period of time, especially during the grain filling stage, to maintain or increase higher grain yield.

2. Some forest plants adapt to shade by mixotrophy, i.e., they obtain carbon both from photosynthesis and from their root mycorrhizal fungi. It is observed that the plants which obtain their carbon source both from aerial parts and carbon fixing root organisms show prolonged vegetation. The adaptations for this evolutionary transition, and the reasons why it has happened a limited number of times, remain In orchids.

3. Functional and cosmetic stay green: Stay-greens have been broadly divided into cosmetic, where the primary lesion is confined to pigment catabolism, and functional, in which the entire senescence syndrome, of which chlorophyll catabolism is only one component, is delayed or slowed down, or both. In functional stay-greens, the transition from the carbon capture period to the nitrogen mobilization (senescence) phase of canopy development is delayed and the senescence syndrome proceeds slowly.It is considered a beneficial trait that can increase grain yield in cereal crops. E.g. japonicarice ‘SNU-SG1’ had a good seedsetting rate and grain yield. Thus, functional stay-green mutants could be affected in timing of senescence initiation or speed of senescence progression. These mutants delay the senescence initiation or progression (cosmetic) might have an advantageous effect on yield. A famous cosmetic stay-green mutant is the green cotyledon mutant, one of seven Pisumsativum (pea) varieties used by Gregor Mendel to establish the laws of genetics.

4.  Relation of N2 with stay green traits: Chlorophyll pigment is consist of N2, Mg+2 and others constituent. Stay-green can be viewed as a   consequence of the balance between N demand by the grain and N supplyduring grain filling stage. While N2 is the constituent of most of the proteins (rubisco involved in photosynthesis). Yield and composition in high-carbon (C) crops such as cereals, and in high-nitrogen (N) species such as legumes, reflect the source–sink relationship with canopy C capture and N remobilization.The plants with higher CO2 utilization efficiency are found with better stay green traits.

5. Some of the plant traits responsible for stay green are; protochlorophyllideoxidoreductase, an enzyme helps plants to yield chlorophyll independent of light carbon capture, nitrogen remobilization, and stay-green-C and N export cease in the terminal phase of leaf death, hormones, transcription factors, expression levels of Reactive Oxygen Species (ROS)  WRKY  and NAC genes.

Five ways to staygreen

1. Type Astay green trait occurs when the leaves and stems maintain their photosynthetic area active for a longer period of time, experiencing a delay in plant senescence.

2.  Type B, senescence occurs in the normal period of plant development, but it occurs relatively slowly.

3.  Type C, also known as cosmetic stay-green, there is an accumulation of pigments on the surface of the organ, giving the impression that there is a reduction of senescence.However, the rate of degradation of protein and chlorophyll occurs normally below the green Surface. Type C stay-green mutants undergo senescence at a normal rate, including a decline in photosynthetic capacity, but maintain chlorophyll levels. Thus, these mutants arise due to specific defects in the chlorophyll degradation pathway. The conversion of pheophorbide A into red chlorophyll catabolite is the point at which the typical green color associated with chlorophyll is lost.

4.  Type D is recurrent in the herbaria and freezing of vegetables, in which the green color is maintained with leaf death via freezing, boiling or drying.

5.  Type E is described as the one with the Highest content of chlorophyll in photosynthetic tissues, and that increased concentration results in delay in yellowing of leaves and stems (similar to type A) and maintenance of green tissue, even with the reduced ability of fixing carbon dioxide. 


Significance of stay green traits

1.  With agronomic traits: higher Nitrogen uptake during grain filling, drought and lodging resistance, enhancingthe fodderimprovement and silage making quality are most common features. In sorghum and maize non stay-green genotypes start losing the green color of their leaves approximately 30 days after anthesis, while the stay-green ones do not show similar process until physiological maturity. Vegetative growth and the reproductive phase in wheat differ in their sensitivity to temperature heat stress reduces photosynthesis through disruptions in the structure and function of chloroplasts, and reductions in chlorophyll content, therefore, can be used as stress indicator for screening (observed in wheat). The association between stay-green and desirable traits such as greater number of fertile tillers, higher number of grains per ear, higher industrial quality, basal metabolism, stomatal regulation, higher yield, tolerance to abiotic and biotic stresses have been reported

2.  In horticultural crop it increases market value: Several approaches have been utilized to manipulate hormone level or responses in transgenic plants to reduce the incidence of postharvest yellowing.  It extends shelf-life and helps in long term transportation. Eg. Intomato. The stay-green variants reveal how autumnal senescence and dormancy are coordinated in trees. These phenotypes can be the result of alterations in hormone metabolism and signaling, particularly affecting networks involving cytokinins and ethylene.

3. Genetics Originally, absence of SGR was considered to inhibit Chlorophyllbreakdown, but the SGR is not directly involved in a Chlcatabolic step; instead, it is required for the dismantling of photosynthetic Chl–protein complexes, thus allowing Chl-breakdown enzymes to access their substrate. Highlighting the importance of genetic control of stay-green character expression in major crops, Australian scientists from the universityof Queensland applied for a patent on the plant transformation through STGXgenes that control expression of stay-green, order to obtain drought tolerance.Several stay-green QTL (Quantitative trait loci) have been identify in several crops like sorghum, wheat, rice etc.  By hybridization program, it is enable to transfer identified stay-green QTL to the genotype having good agronomic background. Inheritancecharacter is governed by a single gene having two alleles, which has a partially dominant gene action, with great participation of additivity and high heritability.

 Some strategies to identify the stay green genes: (i) Classical mapping, genetic analysis followed by direct gene identification through sequencing of the mapped chromosomal region and verification through mutant complementation(ii) A combination of genetic analysis and usage of publicly available genome and transcriptome resources. ‘stay-green’ (SGR) maize hybrids have been selected because of their greater tolerance to post-silking environmental stresses (which include the ‘stay-green’ trait was only exhibited in the presence of adequate N.These traits in plants can be improved by selection, hybridization, mutation and polyploidy breeding. Growth promoter induction and spraying mutagens are the measures only for one generation.Mutants found for stay green are SGR,NYE 1, SID with stay green proteininvolved in stabilizing catabolic complex. Genes PPH, CRN 1 AND NCY 3 withphaeophytase protein is responsible for dehydrating phaeophytin.

4.   Proteomics: Stay-green genes encode members of a new family of chloroplast-located proteins, which are likely to function in dismantling of photosynthetic chlorophyll–apoprotein complexes. Their activity is considered as a prerequisite for both chlorophyll and apoprotein degradation during senescence.

5.  Several traits, such as increased water and chlorophyll content in leaves at physiological maturity, water and sucrose content in stalks at grain fill, water content in husks and cobs, and higher grain protein content are tightly connected with delayed senescence. Stay-green is an indicator of good plant health later in the season, reduced progressive senescence, tolerance to post-flowering drought and stalk lodging.

 Understanding the origins and implications of stay-green followed from the discovery of the pathway of chlorophyllcatabolism and associated genes, growing awareness of the functional significance ofthe photosynthetic and nitrogen remobilization phases ofleaf development ofthe role of leaf senescence in stress responses, identification of system-wide regulators of thetiming and rate of the senescence syndrome should be highly prioritized. In future stay green could be a major trait in integrated farming system where it may play a greater role in crop improvement, crop market value, dairy and other agro-based industries.