Abstract:
Ectopic cystatin expression has long been used in plant pest management but the
cysteine protease targets of these inhibitors might also have important functions in the
control of plant lifespan and stress tolerance that remain poorly characterised. We
therefore characterised the effects of expression of the rice cystatin, oryzacystatin-I
(OCI), on the growth, development and stress tolerance of crop (soybean) and model
(Arabidopsis thaliana) plants. Ectopic OCI expression in soybean enhanced shoot
branching and leaf chlorophyll accumulation at later stages of vegetative development
significantly and enhanced seed protein contents and decreased the abundance of
mRNAs encoding strigolactone synthesis enzymes. The OCI expressing A. thaliana
showed a slow growth phenotype, with increased leaf numbers and enhanced shoot
branching at flowering. The OCI-dependent inhibition of cysteine proteases enhanced
drought tolerance in soybean and A. thaliana, photosynthetic CO2 assimilation being
much less sensitive to drought-induced inhibition in the OCI expressing soybean lines.
Ectopic OCI expression or treatment with the cysteine protease inhibitor E64 increased
lateral root densities in A. thaliana. E64 treatment also increased lateral root densities in
the max2-1 mutants that are defective in strigolactone signalling, but not in the max3-9
mutants that are defective in strigolactone synthesis. Taken together, these data provide
evidence that OCI-inhibited cysteine proteases participate in the control of growth and
stress tolerance through effects on strigolactones. We conclude that cysteine proteases
are an important target for manipulation not only to control plant growth, development
and stress tolerance, but also seed quality traits.
