The nitric oxide (NO)-deficient mutant nos1/noa1 exhibited an early leaf senescence phenotype. ETHYLENE INSENSITIVE 2 (EIN2) was previously reported to function as a positive regulator of ethylene-induced senescence. The aim of this study was to address the question of how NO interacts with ethylene to regulate leaf senescence by characterizing the double mutant ein2-1 nos1/noa1 (Arabidopsis thaliana). Double mutant analysis revealed that the nos1/noa1-mediated, dark-induced early senescence phenotype was suppressed by mutations in EIN2, suggesting that EIN2 is involved in nitric oxide signaling in the regulation of leaf senescence. The results showed that chlorophyll degradation in the double mutant leaves was significantly delayed. In addition, nos1/noa1-mediated impairment in photochemical efficiency and integrity of thylakoid membranes was reverted by EIN2 mutations. The rapid upregulation of the known senescence marker genes in the nos1/noa1 mutant was severely inhibited in the double mutant during leaf senescence. Interestingly, the response of dark-grown nos1/noa1 mutant seedlings to ethylene was similar to that of wild type seedlings. Taken together, our findings suggest that EIN2 is involved in the regulation of early leaf senescence caused by NO deficiency, but NO deficiency caused by NOS1/NOA1 mutations does not affect ethylene signaling.

Niu YH, Guo FQ (2012) Nitric oxide regulates dark-induced leaf senescence through EIN2 in Arabidopsis. J. Integr. Plant Biol. 54(8), 516–525.

Plant leaf senescence has been recognized as the last phase of plant development, a highly ordered process regulated by genes known as senescence associated genes (SAGs). However, the function of most of SAGs in regulating leaf senescence as well as regulators of those functionally known SAGs are still unclear. We have previously developed a curated database of genes potentially associated with leaf senescence, the Leaf Senescence Database (LSD). In this study, we built gene networks to identify common regulators of leaf senescence in Arabidopsis thaliana using promoting or delaying senescence genes in LSD. Our results demonstrated that plant hormones cytokinin, auxin, nitric oxide as well as small molecules, such as Ca²⁺, delay leaf senescence. By contrast, ethylene, ABA, SA and JA as well as small molecules, such as oxygen, promote leaf senescence, altogether supporting the idea that phytohormones play a critical role in regulating leaf senescence. Functional analysis of candidate SAGs in LSD revealed that a WRKY transcription factor WRKY75 and a Cys2/His2–type transcription factor AZF2 are positive regulators of leaf senescence and loss-of-function of WRKY75 or AZF2 delayed leaf senescence. We also found that silencing of a protein phosphatase, AtMKP2, promoted early senescence. Collectively, LSD can serve as a comprehensive resource for systematic study of the molecular mechanism of leaf senescence as well as offer candidate genes for functional analyses.

Li Z, Peng J, Wen X, Guo H (2012) Gene network analysis and functional studies of senescence-associated genes reveal novel regulators of Arabidopsis leaf senescence. J. Integr. Plant Biol. 54(8), 526–539.

In order to analyze the signaling function of hydrogen peroxide (H₂O₂) production in senescence in more detail, we manipulated intracellular H₂O₂ levels in Arabidopsis thaliala (L.) Heynh by using the hydrogen-peroxide-sensitive part of the Escherichia coli transcription regulator OxyR, which was directed to the cytoplasm as well as into the peroxisomes. H₂O₂ levels were lowered and senescence was delayed in both transgenic lines, but OxyR was found to be more effective in the cytoplasm. To transfer this knowledge to crop plants, we analyzed oilseed rape plants Brassica napus L. cv. Mozart for H₂O₂ and its scavenging enzymes catalase (CAT) and ascorbate peroxidase (APX) during leaf and plant development. H₂O₂ levels were found to increase during bolting and flowering time, but no increase could be observed in the very late stages of senescence. With increasing H₂O₂ levels, CAT and APX activities declined, so it is likely that similar mechanisms are used in oilseed rape and Arabidopsis to control H₂O₂ levels. Under elevated CO₂ conditions, oilseed rape senescence was accelerated and coincided with an earlier increase in H₂O₂ levels, indicating that H₂O₂ may be one of the signals to inducing senescence in a broader range of Brassicaceae.

Increasing photosynthetic capacity by extending canopy longevity during grain filling using slow senescing stay-green genotypes is a possible means to improve yield in wheat. Ethyl methanesulfonate (EMS) mutated wheat lines (Triticum aestivum L. cv. Paragon) were screened for fast and slow canopy senescence to investigate the impact on yield and nitrogen partitioning. Stay-green and fast-senescing lines with similar anthesis dates were characterised in detail. Delayed senescence was only apparent at higher nitrogen supply with low nitrogen supply enhancing the rate of senescence in all lines. In the stay-green line 3 (SG3), on a whole plant basis, tiller and seed number increased whilst thousand grain weight (TGW) decreased; although a greater N uptake was observed in the main tiller, yield was not affected. In fast-senescing line 2 (FS2), yield decreased, principally as a result of decreased TGW. Analysis of N-partitioning in the main stem indicated that although the slow-senescing line had lower biomass and consequently less nitrogen in all plant parts, the proportion of biomass and nitrogen in the flag leaf was greater at anthesis compared to the other lines; this contributed to the grain N and yield of the slow-senescing line at maturity in both the main tiller and in the whole plant. A field trial confirmed senescence patterns of the two lines, and the negative impact on yield for FS2 and a positive impact for SG3 at low N only. The lack of increased yield in the slow-senescing line was likely due to decreased biomass and additionally a possible sink limitation.

Derkx AP, Orford S, Griffiths S, Foulkes MJ, Hawkesford MJ (2012) Identification of differentially senescing mutants of wheat and impacts on yield, biomass and nitrogen partitioning. J. Integr. Plant Biol. 54(8): 555–566.

Leaf senescence is induced or accelerated when leaves are detached. However, the senescence process and expression pattern of senescence-associated genes (SAGs) when leaves are detached are not clearly understood. To detect senescence-associated physiological changes and SAG expression, wheat (Triticum aestivum L.) leaves were detached and treated with light, darkness, low temperature (4 °C), jasmonic acid (JA), abscisic acid (ABA), and salicylic acid (SA). The leaf phenotypes, chlorophyll content, delayed fluorescence (DF), and expression levels of two SAGs, namely, TaSAG3 and TaSAG5, were analyzed. Under these different treatments, the detached leaves turned yellow with different patterns and varying chlorophyll content. DF significantly decreased after the dark, ABA, JA and SA treatments. TaSAG3 and TaSAG5, which are expressed in natural senescent leaves, showed different expression patterns under various treatments. However, both TaSAG3 and TaSAG5 were upregulated after leaf detachment. Our results revealed senescence-associated physiological changes and molecular differences in leaves, which induced leaf senescence during different stress treatments.

Zhao L, Zhang H, Zhang B, Bai X, Zhou C (2012) Physiological and molecular changes of detached wheat leaves in responding to various treatments. J. Integr. Plant Biol. 54(8), 567–576.

Cotton (Gossypium hirsutum L.) premature leaf senescence-resistant inbred XLZ33 and senescence-susceptible inbred lines XLZ13 were selected and crossed to produce F₁, F₁-reciprocal, F₂ and BC₁ generations for evaluation of leaf senescence process and inheritance. The results showed that leaf senescence processes for XLZ13 and XLZ33 were obviously different and leaf senescence traits could be distinguished between the two parents at particular periods of cotton growth. Inheritance anlysis for the cotton premature leaf senescence resistant trait further showed that the segregation in the F₂ fit a 3:1 ratio inheritance pattern, with resistance being dominant. The backcross of F₁ to the susceptible parent produced a 1:1 ratio, confirming that cotton premature leaf senescence resistant trait was from a single gene. The single dominant gene controlling cotton premature leaf senescence resistance in XLZ33 was named as cotton premature leaf senescence resistance 1, with the symbol cpls^r1.

Leaf senescence is a complex phenomenon occurring in all plant species, but it is still poorly understood in plants grown in Mediterranean field conditions and well-adapted to harsh climatic conditions. To better understand the physiological processes underlying leaf senescence in mastic trees (Pistacia lentiscus L.), we evaluated leaf growth, water and N content, photosystem II (PSII) photochemistry, lipid peroxidation and levels of photosynthetic pigments, antioxidants, abscisic acid, and salicylic acid and jasmonic acid during the complete leaf lifespan, from early expansion to late senescence in relation to natural climatic conditions in the field. While mature leaves suffered from water and N deficit during late spring and summer, both young (emerging) and old (senescing) leaves were most sensitive to photo-oxidative stress, as indicated by reductions in the F_v/F_m ratio and enhanced lipid peroxidation during late autumn and winter. Reductions in the F_v/F_m ratio were associated with low α-tocopherol (vitamin E) levels, while very old, senescing leaves additionally showed severe anthocyanin losses. We have concluded that both young (emerging) and old (senescing) leaves suffer oxidative stress in mastic trees, which may be linked in part to suboptimal temperatures during late autumn and winter as well as to low vitamin E levels.

Juvany M, M¨ uller M, Munn´e-Bosch S (2012) Leaves of field-grown mastic trees suffer oxidative stress at the two extremes of their lifespan. J. Integr. Plant Biol. 54(8), 584–594.

Annual plants usually flower and set seed once before senescence results in the death of the whole plant (monocarpic senescence). Leaf senescence also occurs in polycarpic perennials; even in “evergreen” species individual leaves senesce. In the annual model Arabidopsis thaliana sugars accumulate in the senescent leaves and senescence is accelerated by high sugar availability. Similar to A. thaliana, sugar contents increased with leaf age in the perennial Arabis alpina grown under warm conditions (22 °C day/18 night). At 5 °C, sugar contents in non-senescent leaves were higher than at a warm temperature, but dependent on the accession, either sugars did not accumulate or their contents decreased in old leaves. In A. alpina plants grown in their natural habitat in the Alps, sugar contents declined with leaf age. Growth at a cold temperature slightly delayed senescence in A. alpina. In both warm and cold conditions, an external glucose supply accelerated senescence, but natural variation was found in this response. In conclusion, sugar accumulation under warm conditions could accelerate leaf senescence in A. alpina plants, but genotype-specific responses and interactions with growth temperature are likely to influence senescence under natural conditions.

Wingler A, Stangberg EJ, Saxena T, Mistry R (2012) Interactions between temperature and sugars in the regulation of leaf senescence in the perennial herb Arabis alpina L. J. Integr. Plant Biol. 54(8), 595–605.