In plants, both endogenous mechanisms and environmental signals regulate developmental transitions such as seed germination, induction of flowering, leaf senescence and shedding of senescent organs. Auxin response factors (ARFs) are transcription factors that mediate responses to the plant hormone auxin. We have examined Arabidopsis lines carrying T-DNA insertions in AUXIN RESPONSE FACTOR1 (ARF1) and ARF2 genes. We found that ARF2 promotes transitions between multiple stages of Arabidopsis development. arf2 mutant plants exhibited delays in several processes related to plant aging, including initiation of flowering, rosette leaf senescence, floral organ abscission and silique ripening. ARF2 expression was induced in senescing leaves. ARF2 regulated leaf senescence and floral organ abscission independently of the ethylene and cytokinin response pathways. arf1 mutations enhanced many arf2 phenotypes, indicating that ARF1 acts in a partially redundant manner with ARF2. However, unlike arf2 mutations, an arf1 mutation increased transcription of Aux/IAA genes in Arabidopsis flowers, supporting previous biochemical studies that indicated that ARF1 is a transcriptional repressor. Two other ARF genes, NPH4/ARF7 and ARF19, were also induced by senescence, and mutations in these genes enhanced arf2 phenotypes. NPH4/ARF7 and ARF19 function as transcriptional activators, suggesting that auxin may control senescence in part by activating gene expression.

/pdf/auxin-response-factor1-and-auxin-response-factor2-regulate-23y4rtf6h8.pdf

AUXIN RESPONSE FACTOR1 and AUXIN RESPONSE FACTOR2 regulate senescence and floral organ abscission in Arabidopsis thaliana.

A comparative transcriptome analysis for successive stages of Arabidopsis (Arabidopsis thaliana) developmental leaf senescence (NS), darkening-induced senescence of individual leaves attached to the plant (DIS), and senescence in dark-incubated detached leaves (DET) revealed many novel senescence-associated genes with distinct expression profiles. The three senescence processes share a high number of regulated genes, although the overall number of regulated genes during DIS and DET is about 2 times lower than during NS. Consequently, the number of NS-specific genes is much higher than the number of DIS- or DET-specific genes. The expression profiles of transporters (TPs), receptor-like kinases, autophagy genes, and hormone pathways were analyzed in detail. The Arabidopsis TPs and other integral membrane proteins were systematically reclassified based on the Transporter Classification system. Coordinate activation or inactivation of several genes is observed in some TP families in all three or only in individual senescence types, indicating differences in the genetic programs for remobilization of catabolites. Characteristic senescence type-specific differences were also apparent in the expression profiles of (putative) signaling kinases. For eight hormones, the expression of biosynthesis, metabolism, signaling, and (partially) response genes was investigated. In most pathways, novel senescence-associated genes were identified. The expression profiles of hormone homeostasis and signaling genes reveal additional players in the senescence regulatory network.

Transcription Analysis of Arabidopsis Membrane Transporters and Hormone Pathways during Developmental and Induced Leaf Senescence

Elucidating mechanisms underlying organ abscission

: It is shown that auxin enhanced ethylene evolution occurs in a variety of plant material, and that ethylene evolution can be constrolled by endogenous as well as exogenous levels of auxin The importance of auxin regulated ethylene evolution on a physiological process is demonstrated by investigating the abscission of bean petiole explants

Regulation of Ethylene Evolution and Leaf Abscission by Auxin.

The Arabidopsis thaliana YUCCA family of flavin monooxygenase proteins catalyses a rate-limiting step in de novo auxin biosynthesis. A YUCCA6 activation mutant, yuc6-1D, has been shown to contain an elevated free IAA level and to display typical high-auxin phenotypes. It is reported here that Arabidopsis plants over-expressing YUCCA6, such as the yuc6-1D activation mutant and 35S:YUC6 transgenic plants, displayed dramatic longevity. In addition, plants over-expressing YUCCA6 exhibited classical, delayed dark-induced and hormone-induced senescence in assays using detached rosette leaves. However, plants over-expressing an allele of YUCCA6, that carries mutations in the NADPH cofactor binding site, exhibited neither delayed leaf senescence phenotypes nor phenotypes typical of auxin overproduction. When the level of free IAA was reduced in yuc6-1D by conjugation to lysine, yuc6-1D leaves senesced at a rate similar to the wild-type leaves. Dark-induced senescence in detached leaves was accompanied by a decrease in their free IAA content, by the reduced expression of auxin biosynthesis enzymes such as YUCCA1 and YUCCA6 that increase cellular free IAA levels, and by the increased expression of auxin-conjugating enzymes encoded by the GH3 genes that reduce the cellular free auxin levels. Reduced transcript abundances of SAG12, NAC1, and NAC6 during senescence in yuc6-1D compared with the wild type suggested that auxin delays senescence by directly or indirectly regulating the expression of senescence-associated genes.

YUCCA6 over-expression demonstrates auxin function in delaying leaf senescence in Arabidopsis thaliana

eourtesy of Sharp and Dohme and Burroughs, Welcome Co. 6. Obtained through the courtesy of E. Katchalski. 7. G. Ehrlich and G. B. B. M. Sutherland, J. Am. Chem. Soc. 76, 5268 (1954); E. R. Blount and H. Lenormant, J. Opt. Soc. Amer. 43, 1093 (1953). 8. R. H. Maybury, S. Gordon, J. J. Katz, J. Chem. Phys., in press. 9. R. H. Maybury, J. J. Katz, S. Gordon, Rei1. Sci. Instr. 25, 1133 (1954). 10. E. J. Ambrose and A. Elliott, Proc. Roy. Soc. London A208, 75 (1951). 11. F. W. Putnam, in H. Neurath and K. Bailey, The Proteins, vol. I, pt. B (Academic Press, New York, 1953), p. 825.

https://science.sciencemag.org/content/sci/121/3148/644.full.pdf

Auxin Gradient Theory of Abscission Regulation

https://science.sciencemag.org/content/sci/114/2974/688.full.pdf

Acceleration and retardation of abscission by indoleacetic acid.

/pdf/a-method-for-the-study-of-foliar-abscission-in-vitro-3q3ygq89pv.pdf

A method for the study of foliar abscission in vitro.

Some effects of water and oxygen on abscission in vitro.

Publisher Summary Defoliation is accelerated leaf fall and requires the functioning of living tissues at the leaf base. It is affected by many factors, internal and external. The principal internal factor is the hormone, auxin, the gradient of which across the leaf base regulates leaf fall. The principal external factors are temperature and moisture; defoliation is favoured by relatively high temperature and moderate leaf moisture. Defoliants appear to act through a mild general injury to both blade and petiole; they have no specific biochemical action. Defoliation of cotton is an established practice in many regions and under some conditions but is not necessary in other regions or under other conditions. Successful cotton defoliation requires careful attention in growing the crop and in applying the defoliant. Chemical defoliation and desiccation have been recently discovered to be valuable aids in the harvest of crops. In 1955, approximately 2,968,973 acres of cotton were treated with defoliants and desiccants. This chapter discusses such harvest-aid practices and the basic factors, which underlie these practices.

Defoliation and Desiccation: Harvest-Aid Practices

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