Leaf Senescence Is Accompanied by an Early Disruption of the Microtubule Network in Arabidopsis

Abstract

1Z4 (A.A.) The dynamic assembly and disassembly of microtubules (MTs) is essential for cell function. Although leaf senescence is a welldocumented process, the role of the MT cytoskeleton during senescence in plants remains unknown. Here, we show that both natural leaf senescence and senescence of individually darkened Arabidopsis (Arabidopsis thaliana) leaves are accompanied by early degradation of the MT network in epidermis and mesophyll cells, whereas guard cells, which do not senesce, retain their MT network. Similarly, entirely darkened plants, which do not senesce, retain their MT network. While genes encoding the tubulin subunits and the bundling/stabilizing MT-associated proteins (MAPs) MAP65 and MAP70-1 were repressed in both natural senescence and dark-induced senescence, we found strong induction of the gene encoding the MT-destabilizing protein MAP18. However, induction of MAP18 gene expression was also observed in leaves from entirely darkened plants, showing that its expression is not sufficient to induce MT disassembly and is more likely to be part of a Ca 2+ -dependent signaling mechanism. Similarly, genes encoding the MT-severing protein katanin p60 and two of the four putative regulatory katanin p80s were repressed in the dark, but their expression did not correlate with degradation of the MT network during leaf senescence. Taken together, these results highlight the earliness of the degradation of the cortical MT array during leaf senescence and lead us to propose a model in which suppression of tubulin and MAP genes together with induction of MAP18 play key roles in MT disassembly during senescence.

Organisation and regulation of the cytoskeleton in plant programmed cell death

“…14,15 During natural and dark-induced senescence in Arabidopsis leaves, the cortical microtubule network in the epidermis and mesophyll cells gradually depolymerises (Table 1). 16 This disassembly is tightly controlled, as neighbouring stomatal cells have an intact microtubule network.…”

“…Transcription of a microtubule severing protein MAP18 increases upon activation of senescence, implicating it in driving disassembly of microtubules in this system. 16 It is unknown whether some MAPs remain associated with fragmented microtubules to facilitate dismantling of the array or whether this is a consequence of, for example altered pH and cytosolic free Ca 2 þ ([Ca 2 þ ] cyt ), which can inhibit tubulin polymerisation. The depolymerisation of microtubules during PCD in these model systems therefore appears to be controlled by a reduction of nucleation, inhibition of bundling, and increase in severing of microtubules.…”

“…12 During early differentiation in embryo suspensor, microtubules are decorated with MAP65. 12 The onset of PCD is accompanied by a reduction in MAP65 gene transcription 16 and the dissociation of MAP65 from microtubules. 12 This is followed by fragmentation and depolymerisation of the microtubule network (Figure 3b).…”

Correlations among Attributes of Senescence and Antioxidative Status of Leaf Discs during Epiphyllous Bud Differentiation in Kalanchoe pinnata Lam. (Pers.)

“…Leaf senescence in general is characterized by diminishing photosynthesis accompanied by disintegration of organelle structures, degradative loss of chlorophyll and proteins, disruption of the microtubular network, and a dramatic increase in lipid peroxidation as well as membrane leakiness (Dhindsa et al, 1981;Thompson et al, 1987;Leshem, 1988;Halliwell and Gutteridge, 2006;Schelbert et al, 2009;Keech et al, 2010). Generally, the fi rst visible sign of senescence is the onset of chloroplast degradation (Thomas and Stoddart, 1980;Smart, 1994), concurrent with a decrease of the chlorophyll content, while the catabolic products are transported to the vacuole.…”

The conserved mobility of mitochondria during leaf senescence reflects differential regulation of the cytoskeletal components inArabidopsis thaliana

“…Therefore, its understanding represents a fundamental scientific challenge, notably in the present conjuncture, when improving crop yields becomes crucial for feeding our fast growing population. The primary function of this complex and organized process is the retrieval and reallocation of (1) what are the molecular mechanisms responsible for such degradation and in a longer term (2) how do cells fulfill their metabolic functions with such a reduced MT network. To address the first question, we primarily assumed that although MT assembly is energy dependent, the lack of ATP/GTP was not responsible for the disorganization of the MT lattice as we previously showed mitochondria to provide sufficient ATP to maintain cellular functions until the end of the process of senescence.…”