Studies on the Behavior of the Senescence Signal in Anoka Soybeans

Abstract

Soybean, a monocarpic plant, has been found to undergo rapid senescence as its fruits mature. In soybeans (Glycine max[L.J Merrill) cv. Anoka, foliar senescence begins during the period of most rapid pod-fill (seed growth), and it can be eliminated by surgical removal of the seeds at an early stage of their growth. Experiments in which fruits are removed from some regions of the plant but aBowed to remain on other regions have established that the transmission of the senescence signal is limited; it affects mainly those leaves nearest to the nodes bearing the fruits. The implications of this localized signal movement are discussed.The phenomenon of monocarpy (a single fruiting phase followed by death) has been recognized for some time, and various theories have been offered to explain the mechanism by which flowering/fruiting induces the death of the entire plant (8, 9). In Anoka soybeans, senescence begins when most of the fruits on the plant are undergoing rapid pod-fill (5), and we and others (4) have found that the fruits cause the rest of the soybean plant to begin senescence which leads to death. In this paper, we will use the term "senescence signal" to refer to this, as yet, uncharacterized correlative influence of the fruits on foliar senescence without attempting to limit the notion of a signal to any one of the possible mechanisms for monocarpic senescence. Information on the behavior of this killing signal, such as degree of movement within the plant, will be important in analyzing the mechanism by which the fruits induce death. The results discussed here have been presented at meetings (10, 11 In other surgical experiments, the seeds were excised by slitting the fruit pods with a razor blade next to each seed and parallel to the main longitudinal vein of the pod but without touching the seed (see Fig. 1). The seeds were then forced out through the slits and lanolin was applied to the cuts. In shamoperated plants, the pods were treated in the same way except the seeds were not forced out.Measurement of Senescence and Fruit Development. Generally, each treatment group consisted of five plants (an average value was used in the figs.), and each experiment was repeated at least once.Senescence was measured as the percentage of total leaves on the plant greater than '/2 yellow (scored visually). This visual method of scoring leaf chlorosis has been described in detail and has been correlated with methods which use leaf absorbance of light at 660 nm or which measure amounts of extractable Chl (5).In order to describe fruit maturation quantitatively, we developed a measure of the average fruit maturity based on the proportion of total fruits in each of five fruit development stages given below.Stage 1: pod length greater than 1 cm but pod not full width and the characteristic bulges of the seed cavity not yet developed.Stage 2: pod full width but the largest seed less than 1 cm long (can be estimated by examining the fruit in front of a bright light).Stage 3: seed length greater than or equal to 1 c...

Seed Production Affects Maternal Growth and Senescence in Arabidopsis

“…Interestingly, control mechanisms exerted by developing seeds over maternal growth and life span have been observed in plants (Murneek, 1926;Molisch, 1929;Noodén et al, 2003). In legumes such as soybean (Glycine max), for example, both reproductive growth and the onset of leaf senescence are under correlative control (Noodén and Penney, 2001) by developing seeds so that plants grow and live considerably longer when developing fruits are continuously removed (Leopold et al, 1959;Lindoo and Noodén, 1977). Such a pattern is readily found in monocarpic plants (i.e.…”

“…Peas seem to be a good example of such apex senescence as DNA fragmentation (a sign of cell death) was detected in mitotically arrested apices (Noodén and Penney, 2001;Wang et al, 2007). In previous work, levels of hormones such as GA and auxin were shown to be affected during monocarpic senescence in pea shoots (Leopold et al, 1959;Lindoo and Noodén, 1977;Zhu and Davies, 1997). The role and the causality of multiple correlated events during monocarpic senescence are, however, still under debate (Noodén et al, 2003;Davies and Gan, 2012).…”

“…Mainly, two competing hypotheses have received the most attention in the past, one that invokes a simple source-sink relationship (Molisch, 1929) and one that invokes offspring-derived signals (Wilson, 1997). By far the most extensive research examining the causes of monocarpic senescence has been performed in legumes such as pea (Pisum sativum) and soybean (Leopold et al, 1959;Lindoo and Noodén, 1977;Noodén and Murray, 1982), but even in these species the debate has not been settled. A potential role of the hormones auxin and abscisic acid (ABA) in correlative control has been discussed (Tamas and Engels, 1981).…”

Distribution and Mobilization of Sulfur during Soybean Reproduction

“…The rapid mobilization of highly mobile nutrients, such as N and P, from leaves and subsequent loss of photosynthetic activity cause vegetative tissues to appear to be "self-destructing", through nutrient removal and mobilization to seed (Nooden, 1988) Senescence, although closely linked to the development of reproductive structures (Lindoo and Nooden, 1977), is actually a preprogrammed process that ultimately causes the death of an organism or any part thereof (Nooden and Leopold, 1978). In soybean, developing seeds bring about the plant's death, late in pod fill, though a senescence signal (Lindoo and Nooden, 1977). This hormonal signal is thought to be cytokinin based and is targeted primarily to leaves.…”

Transmission of the Monocarpic Senescence Signal via the Xylem in Soybean

“…Because depodding (4) and deseeding (6) extend the life of soybean plants far beyond normal, the seeds must control monocarpic senescence in soybeans. For convenience, this influence of the seeds will be termed the senescence signal (6). In the soybean, the primary target of the senescence signal is the leaves (8).…”