**7. Conclusion**

frequency from small *Festuca* callus explants was also enhanced in dark as compared with light situation. Acetosyringone in cocultivation media greatly improved *Agrobacterium* growth, while acetosyringone-free MS media delayed *Agrobacterium* growth and subsequently, most probably, could decrease *Agrobacterium*-mediated transformation efficiencies. The heterolo‐ gous transformation of *P5CS* in *F. arundinacea* background was confirmed by PCR and transient *GUS* assay, which most probably can improve tall fescue tolerance more to drought stress.

During senescence, plant-recycled valuable nutrient components from the leaves and leaf chlorophyll (Chl) are usually converted to a colorless product through six known Chl catab‐ olism enzymes (CCEs) and a metal-chelating substance (MCS) [70]. First, Chl *b* reductase (CBR) reduces 7-formyl group of Chl *b* to a hydroxymethyl group. There are two CBR isoforms which are encoded by *NON-YELLOW COLORING 1* (*NYC1*) and *NYC-LIKE* (*NOL*) genes. Another key regulator of Chl degradation is hydroxymethyl Chl reductase (HCAR), which reduces Chl *b* to Chl *a* [71, 72]. HCAR has recently been identified in *Arabidopsis*. Then, the Mg+ atom of Chl *a* is removed by MCS, which is then called pheophytin *a* (Phein *a*). Pheophytinase (PPH) produces Pheophorbide *a* (Pheide *a*) by catalyzing Phein *a*. Subsequently, the chlorin macro‐ cycle of Pheide *a* is oxygenolytically opened by Pheide *a* oxygenase (PAO) [73], and red Chl catabolite (RCC) which is the product of this reaction is reduced to a nonphototoxic primary fluorescent Chl catabolite (*p*FCC) by RCC reductase (RCCR). Besides CCEs and MCS, STAY-GREEN1 (SGR1) or nonyellowing mutant can cause a stay-green phenotype in many plant species, such as *Arabidopsis* and rice [74]. The *NYE1* (nonyellowing) gene in *Arabidopsis* has been identified by positional cloning. NYE1 is now widely referred as SGR (Stay GReen) [75]. The *nye1-1* mutant could retain 50% chlorophyll at the end of 6-day dark incubation, whereas the wild type (*Columbia-0*) has resulted in the degradation of chlorophyll to less than 10% [75]. In addition to that, qPCR result has outlined *AtNYE1* as an extremely induced gene by

In *Festuca arundinacea*, this gene is called NONYELLOWING PROTEIN1 [nonyellowing gene (NYE)], which has 278 amino acids [76]. *FaNYE1* or *SGR1* has a high similarity to *Arabidopsis NYE1*, either by sequences or by function. *FaNYE* has been identified recently by RACE-PCR [76]. Overexpression of *AtNYE1* results in pale-yellow leaves to even albino seedlings [74]. Degreening phenotype in tall fescue occur during severe stress conditions and harsh seasons. BLAST analysis revealed that *FaNYE* has 89% sequence similarity to *Triticum urartu* Tumanian ex Gandilyan and 83% sequence similarity to *Hordeum vulgare* (NCBI). The negative correlation of *FaNYE* transcript with chlorophyll content has been previously addressed in *F. arundina‐ cea* affected by dark treatment and natural senescence, which in 9 days' dark incubation augmented *FaNYE* transcript level just about 52-fold. Furthermore, overexpressing *FaNYE* ORF in *Col-0* background leads to accelerated senescence [76]. Leaf chlorophyll concentration of the mutant diminished very slowly in rice *sgr* mutant, but steeply decreased in wild type

**6. NYE/SGR protein role during drought-mediated chlorophyll**

**degradation**

72 Water Stress in Plants

senescence signals [75].

The kinetic results of this research noticeably suggest that proline, SOD, and APX probably can be key components to protect tall fescue cells from severe drought stress. Furthermore, the aforementioned components can be key targets for genetic manipulation of tall fescue, so as to maintain its cell homeostasis against drought stress. In addition, proline and SOD can be considered as the main physiological indicators for the assessment of adaptability to environ‐ ment conditions. Also, targeting SGR in leaves of tall fescue can open a new window for future research on leaf senescence.
