Cotton-picking thrips turn over a new leaf
One of the hazards of being rooted to the spot is the inability to run away. Thus during the course of evolution, mechanisms have developed for defence against herbivores and for recovery from herbivory. It is the latter that interests
Lei and Wilson, Cotton Research Unit in Narrabi, New South Wales, Australia (pp. 179–186). Early in the year, cotton plants may be infested by thrips, which can reduce leaf area by up to 50 %. However, as the thrip population declines later in the year, the plants recover their leaf area and the authors have investigated how they are able to do this. One of the sites of thrip attack is the shoot meristem where they feed on emerging leaf primordia. However, the primordia are not destroyed totally but instead grow out into damaged and stunted leaves with much-reduced cell numbers, and consequently much reduced area. But how do the plants recover from this? Several hypotheses had been previously suggested including the idea that the damaged leaves have a much higher photosynthetic rate than undamaged leaves, thus producing enough photosynthate to drive the recovery of leaf area. However, the authors’ comparison of thrip-damaged and control leaves showed that this, and other essentially physiological mechanisms were not the basis for leaf area recovery. Instead, they propose a mechanism that involves the acceleration of leaf emergence and enlargement from later nodes. In support of this, they report that in the later stages of recovery, leaf emergence in recovering plants was two nodes more advanced than in control plants. They suggest that this happens because thrip-damaged early leaves, with their reduced cell numbers, complete expansion earlier, thus releasing resources for development of later leaves. This of course raises another question: are the extra resources themselves the direct signal for accelerated leaf ontogeny or is there a more complex signalling pathway?
j.a.bryant{at}exeter.ac.uk
