Stay green-A useful trait to fight against heat stress in wheat

Sundeep Kumar*, Ruchi Bansal, Amit K. Singh, Sheel Yadav and Jyoti Kumari

National Bureau of Plant Genetic Resources, Pusa, New Delhi-110012 (INDIA)

* Corresponding author: Sundeep Kumar (E-mail: Sundeep.Kumar@icar.gov.in) 

Importance of wheat as staple food is well known as being life line of 35% of the world population. But in past two-three decades, unpredictable climatic conditions have resulted in stagnation in production. Average temperature has risen significantly enforcing us to develop climate resilient high yielding varieties. To develop climate resilient varieties, we need to develop better understanding of the traits which respond to climate changes i.e., elevated temperature, drought and excess rainfall. We further require understanding how tolerance against rising temperature can be improved by exploiting key traits (Halford, 2009).

Temperatures above the optimum level have negative effect on crop growth and injure plant tissues, which we generally call ‘heat stress’ (Wahid et al, 2007). Heat stress (high ambient temperature) is a serious problem to crop production worldwide (Hall, 1992). High temperature at the time of grain filling is a major constraint for the successful wheat production in many parts of the world. Many morphological and physiological traits have been explored for their utility in breeding for heat tolerance. Stay green has been identified as one of the important traits that allow plants to retain their leaves in the active photosynthetic state when exposed to high temperature (Rosenow, 1983). Thus, by exploiting stay green trait, we can increase the productivity of wheat, even under heat stress environments. With increasing threat of global warming, traits like stay green are in demand for making use in development of heat tolerant cultivars (Kumar et al., 2013).

Senescence and stay green:

The term ‘stay green’ (SG) is given to a variant in which senescence (loss of chlorophyll) is delayed compared to a standard reference genotype. It is considered an important trait that allows plants to retain their leaves in the active photosynthetic state even under stress conditions. Early onset of senescence affects assimilation and grain filling in crop plants. The rate of senescence determines the maintenance of chlorophyll and hence photosynthesis for sink formation. Therefore, any defense mechanism that postpones the onset of senescence and keeps leaves green (active photosynthetic state) is expected to give additional yield to crop plants.

What type of stay green is beneficial to plants?

Stay green is not always useful. Stay green is of two types; (i) Functional stay green (ii) Non functional/cosmetic stay green. In case of functional stay green mutant, either the initiation of senescence is delayed (Type A) or senescence progression is slow (Type B). Thus functional stay green trait is of agronomic interest because photosynthetic activity is retained for more time as compared to standard genotype and it provides yield advantages under stress conditions. In nonfunctional/cosmetic stay green mutants, senescence occurs at normal rate and photosynthetic capacity is lost, but leaf colour is retained due to defects in chlorophyll degradation pathway (Thomas and Howarth 2000).

Leaf senescence is induced by a number of environmental and developmental factors and the duration of the leaves to be green is determined by the genetic background (Wingler et al., 1998). High intrinsic chlorophyll concentration has also been associated with improved stay green in sorghum and reported to reduce post-anthesis drought induced senescence (Karen et al., 2007). The luxury type of stay green is harmful to the plants as it leads to growth of vegetative parts of the plant and does not contribute towards grain yield. On the other hand, functional stay green is useful as photosynthetic products moves from source (leaf) to sink (grain) under stress condition. 

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. Stay-green variants reveal how autumnal senescence and dormancy are coordinated in trees. The stay-green phenotype can be the result of alterations in hormone metabolism and signalling, particularly affecting networks involving cytokinins and ethylene. Members of the WRKY and NAC families, and an ever-expanding cast of additional senescence-associated transcription factors, are identifiable by mutations that result in stay-green. Empirical selection for functional stay-green has contributed to increasing crop yields, particularly where it is part of a strategy that also targets other traits such as sink capacity and environmental sensitivity and is associated with appropriate crop management methodology. The onset and progress of senescence are phenological metrics that show climate change sensitivity, indicating that understanding stay-green can contribute to the design of appropriate crop types for future environments. The stay-green phenotype observed after flowering is a consequence of physiological processes initiated earlier in crop growth. In general, stay-green is largely a constitutive trait in that the plants are equipped for the challenge of drought/heat before the onset of the stress e.g., reduced tillering regardless of water regime (Borrell et al., 2000), although there are some adaptive components. However, a post-flowering drought/heat is required for expression of the trait, that is, the stay-green phenotype will emerge when water conserved before flowering is utilized by the crop after flowering under terminal drought/heat conditions.

The stay-green trait has been reported to influence grain yield of different crops especially under drought conditions. Positive correlation of stay green trait with high grain yield has been found in sorghum (Rosenow, 1983; Victor et al., 1989; Evangelista and Tangonan, 1990), soybean (Phillips et al., 1984) and maize (Duvick, 1984; Russel, 1986; Ceppi et al., 1987). Stay green trait in wheat has been associated with increase in leaf area, rate and duration of grain filling and photosynthetic competence (Spano et al., 2003).

How useful stay green can be to provide tolerance against heat stress?

Stay green is a useful trait having potential to significantly enhance the radiation use efficiency by improving the longevity of photosynthetic machinery of plants. Stay green promotes development of the economic product especially in crops where grain is the final yield. It provides additional yield (up to 20%) to the plants. Loss of chlorophyll during grain filling toll significant yield reduction as leaves-active photosynthetic part of the plant does not contribute their role in photosynthesis thus no food synthesis and no transmission of food from leaves to grains. According to Thorne (1982), during grain maturation in wheat, green and viable leaves significantly contribute photosynthates to developing grain. Since there is a strong association between the duration of photosynthetically active leaf area and grain yield (Rawson et al., 1983), selection for stay green is expected to have a significant implication in productivity of wheat particularly under harsh environments (Reynolds et al., 1999). Role of stay green has been established in promoting tolerance to abiotic stresses such as heat and drought (Viswanathan and Bindinger, 2002).

The protection of the photosynthetic apparatus of chloroplasts, such as the maintenance of photosystem II (PSII) and control of content of reactive oxygen species, was also indicated as a major contribution in slowing the degeneration of tissues in genotypes wheat with functional stay-green character (Luo et al., 2006). In Arabidopsis, the cosmetic stay-green mutant showed greater ability to control the redox potential and better maintenance of photosynthetic structures, even showing a reduction of some photosynthetic parameters such as ability to assimilate CO₂ (Grassl et al., 2012).

Usefulness of stay green trait in different crops:

The usefulness of stay green trait in the genotypes is to maintain the green coloration even under high temperature (>30^oC) and are assumed to maintain lower canopy temperatures, which might be a desirable trait for developing heat tolerant wheat varieties (Kumari et al., 2006). Stay green duration of flag leaves and harvest index showed positive correlation with water use efficiency during grain formation of wheat (Gorny and Garczynski, 2002). Stay green has been found useful in other cereal crops like sorghum, pearl millet, maize etc for heat stress tolerance. In sorghum, stay-green genotypes are reported to not only remain green but also contain significantly more carbohydrate in the stem at all maturity stages than go-brown types and have a higher grain weight (Mc Bee et al., 1983).

Under water limited conditions, stay-green genotypes in case of sorghum retain more green leaf are than do genotypes not possessing this trait, and they also continue to fill grain normally under abiotic stress conditions (Rosenow, 1983). Moreover, there is a positive association between stay-green and grain yield under water-limited environments (Borrell and Douglas, 1996). The beneficial effect of stay-green trait towards grain yield is also reported in soybean (Philips et al., 1984), maize (Duvick, 1984; Russel, 1986; Ceppi et al., 1987) and sunflower (Cuckadar-Olmedo and Miller, 1997). Stay-green trait is also known to reduce lodging (Duncan et al., 1981) and there is good association with resistance to stem rots as well (Rosenow, 1983; Evangelista and Tangonan, 1990) suggesting that stay-green leaves remain photosynthetically active. Stay green trait is considered a useful trait in many crops for stress tolerance and for achieving yield gains. Stay green has shown association with other desirable traits i.e., tolerance to abiotic (Kassahun et al., 2010) and biotic (Joshi et al., 2007) stresses, greater number of fertile tillers (Ahlawat et al., 2008), higher number of grains per ear (Luche et al., 2013), higher industrial quality (Silva et al., 2004) have been observed. However, how useful it would be and maintenance of stay green during grain filling is more important and deciding factor. Information about stay green trait is limited in number in wheat. Despite the importance of stay-green trait, this trait has remained ignored in important cereal crop like wheat. The only report by Silva et al. (2000) only throws some light on its genetic basis in one cross investigated by them. In India, variability for stay green trait among wheat genotypes was observed by Joshi et al. (2007).

Use of molecular markers

Due to importance of this trait in heat stress tolerance, more emphasis has been given in last two decades to dissect the mechanism of stay green at molecular level. The mechanism of senescence may get affected by number of candidate genes which are expected to have differential expression in stay green and non-stay green genotypes. In routine experiments, it has been seen that some genotypes tend to remain green even after physiological maturity. This suggests that there could be substantial variability in wheat for this trait which has remained unexploited so far. The enhancement of stay green efficiency is expected to be much higher if information is generated about the presence of stay green genes/QTLs in the promising genotypes with the help of linked SSR markers.

The inheritance of the stay-green character in populations derived from the combination of stay-green and synchronized senescence wheat lines were reported (Silva et al., 2000). The trait is governed by a single gene having two alleles, which has a partially dominant gene action, with great participation of additivity. The stay-green character has high heritability values varying from 0.75 to 0.80, being controlled by four genes that are segregated independently, with strong contribution of additive effects (Joshi et al., 2007). Quantitative trait loci studies show that functional stay-green is a valuable trait for improving crop stress tolerance in cereals. Two QTLs i.e., Gwm1037 (QSg.bhu-3B) and Gwm691 (QSg.bhu-1A) for stay green trait has been reported by Kumar et al. (2010). Four QTLs in rice (TCS4, Csfl6, Csfl9/Tcs9 and Csfl12) were found. Moreover, the stay-green QTLs Csfl6 and Tcs9 were identified in the same position that two QTL for grain yield (Yld6 and Yld9), strengthening the link between high productivity and presence of stay-green character (Fu et al., 2011). In barley, nine QTLs associated with stay-green were identified (Emebiri, 2013), and four QTLs (SZtgb, STG1, Stg3 and Stg4) were identified in sorghum (Kassahun et al., 2010). In addition, markers for other heat tolerance traits are also available that can also be used in marker assisted breeding and selection and may be useful in identifying promising wheat lines from the gene pool available in National Gene bank and other available genetic stocks including wild types and land races.

Future prospects

The understanding of the physiological mechanisms associated with stay green habit and photosynthetic efficiency in several crops may be the key to break the plateau of productivity associated with adaptation to unfavorable environmental conditions particularly heat and drought stresses. There is need to explore this trait extensively in breeding programs in different crops to harness more advantages of this trait like genetic progress in grain yield, quality, disease resistance and tolerance to abiotic stresses. This trait in combination with other useful traits may provide the solution against the burning problems like heat and drought stresses. Information about QTLs for stay green trait in major cereal crops would also provide opportunities to use this trait in breeding programs.

References

Ahlawat S, Chhabra AK, Behl RK, Bisht SS (2008) Genotypic divergence analysis for stay green characters in Wheat (Triticum aestivum L. em. Thell). South Pacific Journal of Natural Sciences, 26:73-81.

Borrell AK, Douglas ACL (1996) Maintaining green leaf area in grain sorghum increases yield in a water limited environment. In: Foale MA, Henzell RG, Kneipp JF, eds. Proceedings of the third Australian sorghum conference. Melbourne: Australian Institute of Agricultural Science, Occasional Publication No. 93. 

Borrell AK, Hammer GL, Douglas ACL (2000) Does maintaining green leaf area in sorghum improve yield under drought? I. Leaf growth and senescence. Crop Science 2000;40:1026-1037.

Ceppi D, Sala M, Gentinetta E, Verderio A, Motto M (1987). Genotype-dependent leaf senescence in maize, Plant Physiol 85: 720–725.

Cukadar-Olmedo B, Miller JF (1997) Combining ability of the stay green trait and seed moisture content in sunflower. Crop Science 37: 150-153.

Duncan RR, Bockholt AJ, Miller FR (1981) Descriptive comparison of senescent and non-senescent sorghum genotypes. Agronomy Journal 73: 849-853.

Duvick DN (1984) Genetic contribution to yield gains of US hybrid maize 1930-1980. In: Fehr WR (ed) Genetic contribution to yield gains of five major crop plants, CSSA special publication 7. Crop Science Society of America, Madison, Pp 15-45.

Evangelista CC, Tangonan NG (1990) Reaction of 31 nonsenescent sorghum genotypes to stalk rot complex in Southern Philippines. Trop Pest Manag 36: 214–215.

Emebiri LC (2013) QTL dissection of the loss of green colour during post-anthesis grain maturation in two-rowed barley. Theoretical and Applied Genetics, 126:1873-1884.

Fu JD, Yan YF, Kim MY, Lee SH, Lee BW (2011) Population-specific quantitative trait loci mapping for functional stay-green trait in rice (Oryza sativa L.). Genome, 5:235-243.

Gorny AG, Garczynski S (2002) Genotypic and nutritional dependent variation in water use efficiency and photosynthetic activity of leaves in winter wheat. J Appl Genet 43: 145-160.

Grassl J, Pružinská A, Hörtensteiner S, Taylor NL, Millar AH (2012) Early events in plastid protein degradation in stay-green arabidopsis reveal differential regulation beyond the retention of LHCII and chlorophyll. Journal of Proteome Research, 11: 5443-5452.

Halford NG (2009) New insights on the effects of heat stress on crops. J. Exper. Bot. 60: 4215-4216.

Hall AE (1992) Breeding for heat tolerance. Plant Breeding Rev10: 129–167.

Joshi AK, Kumari M, Singh VP, Reddy CM, Kumar S, Rane J, Chand R (2007) Stay green trait: variation, inheritance and its association with spot blotch resistance in spring wheat (Triticum aestivum L.). Euphytica (2007) 153: 59-71.

Karen H, Subudhi PK, Borrell A, Jordan D, Rosenow D, Nguyen H, Klein P, Klein R, Mullet J (2007) Sorghum stay green QTL individually reduce post-flowering drought-induced leaf senescence. J Exp Bot 58:327–338. 

Kassahun B, Bidinger FR, Hash CT, Kuruvinashetti MS (2010) Stay-green expression in early generation sorghum [Sorghum bicolor (L.) Moench] QTL introgression lines. Euphytica, 172:351-362.

Kumar S, Kumari P, Kumar U, Grover M, Singh AK, Singh R, Sengar RS (2013) Molecular approaches for designing heat tolerant wheat. J. Plant Biochem. Biotechnol. 22(4):359-371.

Kumar U, Joshi AK, Kumari M, Paliwal R, Kumar S, Röder MS (2010) Identification of QTLs for stay green trait in wheat (Triticum aestivum L.) in the ‘Chirya 3’ x ‘Sonalika’ population. Euphytica 174: 437-445.

Kumari M, Singh VP, Tripathi R, Joshi AK (2006) Variation for stay green trait and its association with canopy temperature depression and yield traits under terminal heat stress in wheat. In: Proceedings of the 7th international wheat conference, Mar del Plata, Argentina, 27 November to 2nd December 2005.

Luche HS, Silva JAG Nörnberg R, Silveira FS, (Baretta D, Groli EL, Maia LC, Oliveira AC (2013) Desempenho per se e parâmetros genéticos de linhagens de trigo com expressão do caráter “stay-green”. Pesquisa Agropecuária Brasileira, 48(2):167-173.

Luo PG, Ren ZL, Wu XH, Zhang HY, Zhang HQ, Feng JA (2006) Structural and biochemical mechanism responsible for the stay-green phenotype in common wheat. Chinese Science Bulletin, 51:2595–2603.

Mc Bee GG, Waskom RM, Miller FR, Creeman RA (1983) Effects of senescence and non-senescence carbohydrates in sorghum during later kernel maturity stages. Crop Sci 23: 373-376.

Phillips DA, Pierce RO, Edie SA, Foster KW, Knowles PF (1984) Delayed leaf senescence in soybean. Crop Sci 24: 518-522.

Rawson HM, Hindmarsh JH, Fischer RA, Stockman YM (1983) Changes in leaf photosynthesis with plant ontogeny and relationships with yield per ear in wheat cultivars and 120 progeny. Aust J Plant Physiol 10: 503-514.

Reynolds MP, Rajaram S, Sayre KD (1999) Physiological and genetic changes of irrigated wheat in the postgreen revolution period and approaches for meeting projected global demand. Crop Sci 39: 1611-1621.

Rosenow DT (1983) Breeding for resistance to root and stalk rots in Texas. Sorghum root and stalk rots, a critical review. ICRISTAT, Patancheru, AP, India. Pp. 209-217.

Russel WA (1986) Contribution of breeding maize improvement in the United States, 1920s-1980s. Iowa State J Res 61: 5-34.

Silva SA, Carvallo FIF, Caetano VR, Oliveira AC, Coimbra JLM, Vasconcellos NJS, Lorencetti C (2000) Genetic basis of stay green trait. Journal of New Seeds 2(2): 55-68.

Silva SA, Carvalho FIF, Nedel JL, Vasconcellos NJS, Cruz PJ, Simioni D, Silva JAG (2004). Composição de subunidades de gluteninas de alto massa molecular (HMW) em trigos portadores do caráter “stay-green”. Ciência Rural, 34(3):679-683.

Spano G, Di Fonzo N, Perrotta C, Platani C, Ronga G, Lawlor DW, Napier JA, Shewry PR (2003) Physiological characterization of ‘stay green’ mutants in durum wheat. J Exp Bot 54: 1415-1420.

Thomas H, Howarth CJ (2000) Five ways to stay green. Journal of Experimental Botany 51: 329-337.

Thorne GN (1982) Distribution between parts of the main shoot and the tillers of photosynthate produced before and after anthesis in the top three leaves of the main shoot of Hobbit and Maris Huntsman winter wheat. Ann Appl Biol 101:553–559

Vietor DM, Cralle HT, Miller FR (1989) Partitioning of 14C-photosynthate and biomass in relation to senescence characteristics of sorghum. Crop Science, 29:1049-1053

Viswanathan, M., Bindinger, F.R. (2002) Evaluation of stay green sorghum Genotypes at ICRISAT. Crop Science 42: 965-974.

Wahid A, Gelani S, Ashraf M, Foolad MR (2007) Heat tolerance in plants: an overview. Environ. Exp. Bot. 61: 199-223.

Wingler A, von Schaewen A, Leegood RC, Lea PJ, Quick WP (1998) Regulation of leaf senescence by cytokinin, sugars, and light. Plant Physiol 116:329–335.