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68 CHAPTER 4

Early experiments on the mechanism of

apical growth in fungi

Robertson (1958, 1959) did many of the key early

experiments on apical growth of fungi, using

extremely simple methods coupled with truly remark-

able insight. He grew colonies of Fusarium oxysporum

or N. crassaon nutrient-rich agar, flooded them with

water and then observed the behavior of the tips. As

shown in Fig. 4.3, roughly half of the tips stopped

growing immediately and then resumed growth

within a minute, but from a narrower apex than

before. The other tips stopped growing for several

minutes, swelled into a diamond shape during this

time, and eventually regrew by producing one or

more narrow tips just behind the original apex. He then

repeated the experiments, again flooding the colonies

with water but replacing this within 40 seconds by a

solution of the same osmotic potential as the original

agar (an isotonic solution). This caused all the tips to

stop for several minutes, but they swelled during this

time and eventually regrew from narrow subapical

branches (Fig. 4.3).

To interpret these findings, Robertson hypothesized

that the normal pattern of apical growth involves two

independent processes: (i) continuous extension of a

plastic, deformable tip and (ii) rigidification of the

wall behind the extending tip. He envisaged these

two processes as occurring at the same rate, but with

rigidification always slightly behind the tip, like two

cars travelling along two lanes of a motorway at

exactly the same speed but one is always slightly

behind the other. Then, if extension growth is halted

by an osmotic shock (adding water) the process of

rigidification will continue and tend to “overtake” the

Fig. 4.1(a –i) Sequence of frames from a videotape of Neurospora crassagrowing over a 1-hour period beneath a
coverslip on an agar plate. (Frames (a–c) are shown at higher magnification than the other frames.)

Fig. 4.2Incorporation of radiolabeled wall precursors

during a brief (5-minute) exposure.