EVIDENCE FOR A REQUIREMENT FOR PROTEIN SYNTHESIS FOR AUXIN-INDUCED CELL ENLARGEMENT.

The basic problem of how auxin causes cell enlargement in higher plants has been studied for years, but with little success. That the growth in size is made possible by an increasing cell-wall plasticity seems fairly clear, but the metabolic events which intervene remain obscure. There is extensive evidence both for and against a methylation process,1 which modifies the physical properties of the cellwall pectin, and there are several phenomena which may or may not be directly related, such as the acceleration of protoplasmic streaming and the increase in respiratory rate. This latter, though in general it shows excellent correlation with growth, varies greatly in magnitude in different experimental objects, but the fact that growth is inhibited by anaerobiosis, by fluoride, CO in darkness, 2,4-dinitrophenol, iodoacetate, and arsenite certainly indicates participation of an oxidative source of energy.2 However, the fact that cell enlargement is much more sensitive to some of the inhibitors than is respiration rate does suggest that the connection may be only indirect. Recently, Ray and Ruesink3 demonstrated a lag of several minutes (at 250C) before auxin increases the growth rate of coleoptile sections. Since the processes that occur during the lag have a Qio of about 2, this bears out the idea that one or more metabolic processes intervene. Among the processes which have from time to time been implicated in cell enlargement is protein synthesis. In some tissues (e.g., artichoke slices4) the formation of total protein has been shown to be greatly promoted by auxin; in others, (e.g., potato slices,4 and pea stem,5 soybean hypocotyl,6 or Avena coleoptile sections7) there is a much smaller promotion which may not always occur. In the corn coleoptile a brief study of the incorporation of C'4-leucine appears to show no promotion at all,8 although the actual elongation in those experiments was rather limited. A possible interpretation of the scanty published data is that auxin may promote synthesis or turnover of a protein which in some tissue comprises only a small part of the total protein content, while in others it makes up a more considerable fraction. This paper presents an examination of such a possibility, and offers evidence that synthesis of some protein is indeed among the processes required for cell enlargement. Instead of measuring total protein, we have followed incorporation of L-leucine1Cl4 into suspendible protein and into a readily centrifuged protein preparation ("heavy fraction") which is considered to be in good part cell wall. Much of the work rests on the use of selective metabolic inhibitors which are known to act on various phases of protein synthesis. A few such inhibitors have been used earlier. Long ago, Bonner7 showed that the amino acid analogue canavanine inhibited auxin-induced elongation of coleoptile sections, and his data also show an inhibition exerted by hydroxyproline, an analogue of proline.9 Later, Schrank found ethionine to have a similar effect.10 The specificity of canavanine and ethionine is unfortunately limited by the par-