Arabidopsis is a monocarpic plant with a short life cycle that can be divided into four major stages: seedling development, vegetative development (or leaf rosette development), and partly overlapping flowering and rosette senescence (Boyes et al., 2001). The lifespan of Arabidopsis extends from radicle emergence until complete senescence of the rosette and cessation of flowering. We found that decreasing the light intensity from 150 μE m−2 s−1 [standard conditions recommended for Arabidopsis (Hennig, 2010); hereafter referred to as normal light (NL)] to 100 μE m−2 s−1 (low light; LL) under constant photoperiod (16 h) delayed most of the developmental transitions in wild-type Columbia (WT) plants (Table S1) resulting in extension of the individual stages of life cycle (Fig. 1A, Table S2) and cumulative increase in the mean lifespan up to ~25% (Fig. 1A). To confirm that LL treatment mimics caloric restriction (CR), we measured photosynthetic capacity (SPAD values proportional to chlorophyll content) and glucose content in the rosette leaves at the onset of flowering. As expected, lowering light intensity was accompanied by decrease in both SPAD values and glucose content (Fig. S1).
It has recently been shown in yeast and animal models that CR promotes longevity via activation of autophagy (Madeo et al., 2010; Rubinsztein et al., 2011). To address possible roles of autophagy in LL-induced lifespan extension in Arabidopsis, we analyzed two autophagy markers, the degradation of the autophagic adapter protein neighbor of BRCA1 gene product (NBR1) by Western blots (Svenning et al., 2011; Klionsky et al., 2012) and the expression of ATG8a by real-time quantitative PCR (Thompson et al., 2005) in the leaves of NL- and LL-grown WT plants at the onset of flowering and 10 days later. Under LL, degradation of NBR1 and upregulation of ATG8a over a 10-day period were, respectively, ~3.5 and ~2 times higher, as compared with NL-grown plants (Fig. 2), indicating that LL stimulates autophagy.
Next, we investigated whether autophagy is required for lifespan extension in Arabidopsis using knockout lines atg5-1 (Thompson et al., 2005) and atg7-2 (Hofius et al., 2009). It has previously been shown that autophagy-deficient mutants of Arabidopsis have enhanced expression of ATG8 genes, especially under conditions inducing autophagy (Thompson et al., 2005). Consistent with these data, knockout of either ATG5 or ATG7 led to upregulation of ATG8a in response to the LL conditions to the levels higher than in WT and complementation lines (Fig. 2B). At the same time, both atg5 and atg7 plants grown under LL revealed strong accumulation of NBR1, indicative of impaired autophagic flux (Fig. 2A). ATG knockout plants have previously been shown to exhibit an early-senescence phenotype even under favorable growth conditions (Liu & Bassham, 2012). Apart from the early onset and completion of rosette senescence, faster completion of flowering and accelerated silique shattering (Table S1), we found that the lengths of all major stages of the life cycle in atg5 and atg7 plants were reduced regardless of the light intensity (Fig. 1A, Table S2). As a result, the mean lifespan of atg5 and atg7 mutants was decreased by ~22% and ~12%, respectively, under NL, and by ~32% and ~22%, respectively, under LL. Collectively, these data demonstrate that autophagy attenuates transition through the successive stages of Arabidopsis life cycle and therefore is the prerequisite for longevity.
We have noticed that both atg5 and atg7 plants retained the ability to initiate CR-like response (i.e., decrease in both photosynthetic capacity and glucose content) to LL (Fig. S1). Therefore, we assessed whether these plants were able to sense CR and extend their lifespan similar to WT. The survival data show that the LL-induced increase in plant longevity was dramatically reduced (atg5) or completely abrogated (atg7) in the autophagic mutants, as compared with WT and complementation lines (Fig. 1). Thus, our findings reveal that intact autophagy machinery confers lifespan extension in Arabidopsis under the CR conditions.
Restricting uptake of calories has long been known as a simple and potent method for increasing longevity in heterotrophs (Mair & Dillin, 2008), and has recently been linked to the activation of autophagy (Madeo et al., 2010; Rubinsztein et al., 2011). Our work provides the first experimental setting for studying mechanisms increasing plant longevity under the CR and establishes autophagy as one of the crucial components. The plant autophagic machinery, and especially induction phase of plant autophagy, remains poorly understood, hampering our mechanistic insight into how autophagy is activated under the LL conditions. Once activated, autophagy can act in a number of pathways to confer cytoprotection (e.g., elimination of oxidized proteins and defective/old organelles) and sustain plant development (e.g., remobilization of nutrients) (Liu & Bassham, 2012), which collectively should lead to the lifespan extension.
Being an ephemeral monocarpic plant, Arabidopsis literally reproduces itself to death providing a tractable model to investigate lifespan regulation. Considering a central role of autophagy in the longevity of such evolutionary distant organisms as yeast, Arabidopsis, and mammals, autophagy is likely to play similarly important role in the longevity of polycarpic plants. Therefore, genetic or pharmacological manipulation of autophagy might open new venues for crop and tree production.