We aim to understand the pathway of sucrose metabolism in the non-photosynthetic cells of plants. Non-photosynthetic cells are dependent for metabolism and growth on carbon from imported sucrose. Only two classes of enzymes in plants can catalyse the entry of carbon from sucrose into cellular metabolism: sucrose synthases and invertases. Most of the biomass on the planet – and almost all of the material we harvest as food crops – consists of carbon metabolised via these two enzymes. It has been generally accepted that sucrose synthases are central, “housekeeping” enzymes, whereas invertases have a variety of specific roles in particular conditions and organs.
We have discovered that this perception is incorrect (Barratt et al., 2009). Using knockout mutants of Arabidopsis we have demonstrated first that sucrose synthase (SUS) is largely dispensable except in specific developmental and environmental contexts, and second that specific isoforms of neutral, cytosolic invertase (CINV) are essential for normal growth and development.
Wild-type (left) and cinv1/cinv2 mutant (right) plants of the same age. The cinv1/cinv2 mutant carries T-DNA insertions in genes encoding two isoforms of invertase located in the cytosol.
In further research on the roles of SUS, we found that two of the six isoforms (SUS5 and SUS6) are confined to the phloem. We believe that they supply UDPglucose to an isoform of callose synthase, CSL7, that catalyses the synthesis of the callose lining of the pores of the sieve plates between the sieve tube cells of the phloem. Mutants lacking CSL7 are defective in inflorescence growth. This appears to be because the sieve plate pores collapse, preventing the movement of sucrose from the leaves to the inflorescence and leading to sugar starvation and slow growth. We propose that the callose linings hold the pores open to allow mass flow of material through the phloem.
Left, normal sieve plate pores in the stem, with callose lining (white layer). Right, sieve plate pores in csl7 mutant, with no callose lining.
In further research on the roles of two neutral invertases necessary for normal growth of the plant, we collaborate with Trevor Wang (JIC https://www.jic.ac.uk/profile/trevor-wang.asp) who has discovered an equivalent mutant in the model legume Lotus japonicus. In a BBSRC funded project we are investigating whether the invertases are required because they are the main means by which sucrose can enter metabolism, or whether they have separate, unique functions that link carbon availability to growth and development.
Barratt DHP, Derbyshire P, Findlay K, Pike M, Wellner M, Lunn J, Feil R, Simpson C, Maule A, Smith AM (2009) Normal growth of Arabidopsis plants requires cytosolic invertase but not sucrose synthase. Proc Natl Acad Sci USA 106, 13124-13129
Barratt DHP, Kölling K, Graf A, Pike M, Calder G, Findlay K, Zeeman SC, Smith AM (2011) Callose synthase GSL7 is necessary for normal phloem transport and inflorescence growth in Arabidopsis. Plant Physiol 155, 328-341
Smith AM, Kruger NJ, Lunn JE (2012) Source of sugar nucleotides for starch and cellulose synthesis. Proc Natl Acad Sci SA 109, E776