No abstract

Chlorophyllase (CLH) is a common plant enzyme that catalyzes the hydrolysis of chlorophyll to form chlorophyllide, a more hydrophilic derivative. For more than a century, the biological role of CLH has been controversial, although this enzyme has been often considered to catalyze chlorophyll catabolism during stress-induced chlorophyll breakdown. In this study, we found that the absence of CLH does not affect chlorophyll breakdown in intact leaf tissue in the absence or the presence of methyljasmonate, which is known to enhance stress-induced chlorophyll breakdown. Fractionation of cellular membranes shows that Arabidopsis (Arabidopsis thaliana) CLH is located in the endoplasmic reticulum and the tonoplast of intact plant cells. These results indicate that CLH is not involved in endogenous chlorophyll catabolism. Instead, we found that CLH promotes chlorophyllide formation upon disruption of leaf cells, or when it is artificially mistargeted to the chloroplast. These results indicate that CLH is responsible for chlorophyllide formation after the collapse of cells, which led us to hypothesize that chlorophyllide formation might be a process of defense against chewing herbivores. We found that Arabidopsis leaves with genetically enhanced CLH activity exhibit toxicity when fed to Spodoptera litura larvae, an insect herbivore. In addition, purified chlorophyllide partially suppresses the growth of the larvae. Taken together, these results support the presence of a unique binary defense system against insect herbivores involving chlorophyll and CLH. Potential mechanisms of chlorophyllide action for defense are discussed.

Carbon autonomy of current-year shoots in flowering, and of current-year shoots plus 1-year-old shoots (1-year-old shoot system) in fruiting of Siberian alder (Alnus hirsuta var. sibirica) was investigated using a stable isotope of carbon, (13)C. The current-year shoot and 1-year-old shoot systems were fed (13)CO(2) and the atom% excess of (13)C in flowers and fruits was determined. The majority of photosynthate allocated to flower buds was originally assimilated in the leaves of the flowering current-year shoots. Of all the current-year shoots on fruiting 1-year-old shoots, only those nearest to the fruits allocated the assimilated photosynthate to fruit maturation. These results indicate that the current-year shoots and 1-year-old shoot systems are carbon-autonomous units for producing flowers and maturing fruits, respectively.

Current shoots, which form the crown of a tree, are specialized in various functions such as crown expansion, reproduction, and assimilation. We examined the temporal and spatial distribution of reproductive shoots in Alnus hirsuta Turcz. var. sibirica (Fischer) C.K. Schn., assessed their direct and indirect costs of reproduction, and explained their distribution in the crown as the reproductive strategy of a current shoot population. The upper and lower limits to the lengths of current shoots for reproductive growth (flower formation) were 40 and 10 cm, respectively. Reproductive 1-year-old shoots produced fewer shoots in the following year than non-reproductive 1-year-old shoots. In current shoots longer than 40 cm, the increment of reproductive output in the following year by abandonment of reproduction surpassed the decrement of reproductive output in the current year by abandonment of reproduction. This may be one reason for the upper limit of reproductive shoot length. Thus, the current shoot population of A. hirsuta var. sibirica may be divided into three functionally specialized subpopulations: reproductive, maintenance, and exploratory. This specialization is considered to be a reproductive strategy to maximize their lifetime reproductive success.Key words: current shoot population, reproductive ecology, functional specialization, cost of reproduction, Japanese alder.