Aging is a universal property of multicellular organisms. Although some tree species can live for centuries or millennia, the molecular and metabolic mechanisms underlying their longevity are unclear. To address this, we investigated...
moreAging is a universal property of multicellular organisms. Although some tree species can live for centuries or millennia, the molecular and metabolic mechanisms underlying their longevity are unclear. To address this, we investigated age-related changes in the vascular cambium from 15-to 667-y-old Ginkgo biloba trees. The ring width decreased sharply during the first 100 to 200 y, with only a slight change after 200 y of age, accompanied by decreasing numbers of cambial cell layers. In contrast, average basal area increment (BAI) continuously increased with aging, showing that the lateral meri-stem can retain indeterminacy in old trees. The indole-3-acetic acid (IAA) concentration in cambial cells decreased with age, whereas the content of abscisic acid (ABA) increased significantly. In addition, cell division-, cell expansion-, and differentiation-related genes exhibited significantly lower expression in old trees, especially miR166 and HD-ZIP III interaction networks involved in cambial activity. Disease resistance-associated genes retained high expression in old trees, along with genes associated with synthesis of preformed protective secondary metabolites. Comprehensive evaluation of the expression of genes related to autophagy, senescence, and age-related miRNAs, together with analysis of leaf photosynthetic efficiencies and seed germination rates, demonstrated that the old trees are still in a healthy, mature state, and senescence is not manifested at the whole-plant level. Taken together, our results reveal that long-lived trees have evolved compensatory mechanisms to maintain a balance between growth and aging processes. This involves continued cambial divisions, high expression of resistance-associated genes, and continued synthetic capacity of preformed protective secondary metabolites. aging | cambium | Ginkgo biloba | old trees | senescence A ging occurs in most multicellular organisms, and in yeast and animal cells is frequently accompanied by telomere attrition, epigenetic alterations, loss of proteostasis, and somatic mutations. However, in plants, aging is complex and multifac-torial and is regulated by both genetic and environmental factors (1). Aging is associated with deterioration of growth and differentiation as well as with maturity, whereas senescence, which ends in death, is the last developmental stage (1, 2). Programmed cell death and leaf senescence at the cellular and organ/tissue levels have been extensively studied (3, 4). However, due to the complex life cycles of plants, evolutionary theories of aging have somewhat been neglected in the plant kingdom, and thus the mechanisms underlying aging at the whole-plant level remain enigmatic. In animal cells, the age-related decline in cell/tissue function usually correlates with a reduction in stem cell activity. Similarly, plant meristems are critical for many aspects of growth and development. Almost all postembryonic production of plant tissues is the result of cell proliferation and differentiation from meristems. Maintenance of meristem activity results in some woody and herbaceous perennials living for many years. In woody plants, the apical meristem in the tree top is usually damaged by natural stresses (e.g., freezing injury, lightning strikes, or fracture by wind) in old trees. However, the vascular cambium (VC) meristem, a continuous cylinder of meristematic cells in the stem, is viable throughout the lifespan of the tree, producing secondary xylem to the inside and secondary phloem to the outside (5). Compared with young trees, old trees are characterized by a later onset of xylogenesis, a shorter growing season, and a lower growth rate, resulting in a smaller number of xylem cells (6), suggesting that cambial meristem activity is related to age in woody plants. Nevertheless, how aging is manifested in cambial meristems of long-lived trees remains unknown. Due to their large size, relatively slow growth rate, and long generation time, classical genetic screening of long-lived trees is difficult. The development of sequencing technologies, including RNA, sRNA, and degradome sequencing, has now made it possible to test the involvement of thousands of genes in a biological Significance There is considerable interest in how ancient trees maintain their longevity. Ginkgo biloba is the only living species in the division Ginkgophyta, and specimens can live for over 1,000 y. Here, we show that trees up to 600 y of age display similar leaf areas, leaf photosynthetic efficiencies, and seed germination rates. Transcriptomic analysis indicates that the vascular cam-bium of the oldest trees, although undergoing less xylem generation, exhibits no evidence of senescence; rather, extensive expression of genes associated with preformed and in-ducible defenses likely contributes to the remarkable longevity of this species.
