Discussion

The cells of potato tuber contain a large number of amyloplasts，whose function is to accumulate and store starch in the form of reserve starch granules.It is of interest that upon exposure to light，the amyloplasts are transformed into chloroplasts，which have a different function.The changes in ultrastructure of potato tuber amyloplasts during greening were investigated by electron microscope.It was shown that the main developmental features were elongation of vesicles into thylakoids，the differentiation of grana and the appearance of ribosomes in the stroma［3］，although the ultrastructure is generally less well developed compared to that for normal leaves.On the other hand，there is very little information on biochemical alterations in potato tuber during greening.To confirm the transformation of amyloplasts into chloroplasts，this study provides some biochemical evidence：① the synthesis of photosynthetic pigments；②the synthesis of RuBPCase and other proteins；③ Hill reaction activity.

Figure 4　SDS-PAGE of light-（1）and dark-（d）initiated polypeptides synthesized in chloroplasts isolated from light-treated potato tuber discs.（R）photograph of stained gel.（L）fluorograph of（R）.Arrows indicate the mol wt of light-stimulated polypeptides and band corresponding to LS.Althhough the dark-incubated sample was more concentrated than the light-incubated sample（R），the fluorograph（L）indicates there was more incorporation of［35 S］methionine into the light-induced polypeptides than into the dark-initiated polypeptides，demonstrating that more polypeptides were synthesized in the light than in the dark.The samples were incubated with［35S］methionine for 120 min

Before illumination the potato tubers contain no Chl or Pchl，but do contain carotenoid which increases during greening［1］.Very low light intensity（400 lux）was required to initiate greening.The light may penetrate into the potato tuber discs and evoke the development of chloroplasts from amyloplasts.During the development of chloroplasts from amyloplasts，assembly of Chl into the newly synthesized membranes occurs.Cold storage may cause the breakage of membranes［13］；however，some studies suggested that amyloplast membranes remain intact in cold storage［14］.It was frequently observed that the chloroplasts were formed as streaks in the potato tuber discs，suggesting a special structure is linked to the chloroplast development.(https://www.daowen.com)

The Schlieren pattern of the extracts from light-induced potato tuber tissue as well as its immunoprecipitation reaction with antiserum to RuBPCase demonstrated that RuBPCase，an important enzyme in the photosynthetic carbon cycle，was de novo synthesized during greening.This result was further confirmed by the active light-dependent protein synthesis in the isolated chloroplasts.The biosynthesis of RuBPCase during greening suggests the involvement and operation of CO2 assimilation in the chloroplasts.Furthermore，the photosynthetic function of electron transport and O2 evolution in isolated chloroplasts from potato tuber was shown by the high Hill reaction activity.

The transformation of amyloplasts into chloroplasts is absolutely light-dependent.The fact that red light did not stimulate this transformation indicates that phytochrome may not participate in this regulation.This coincides with the result that potato tuber does not have phytochrome［7］.What is the photoreceptor in this light-induced organelle transformation？How does light turn on the genes for the development of chloroplasts，and turn off the genes for the development of amyloplasts？Undoubtedly，this system provides an attractive and challenging model for investigations into the molecular mechanisms underlying the photoregulation of development and gene expression.