Pectin cell wall remodeling through PLL12 and callose deposition through polar CALS7 are necessary for long-distance phloem transport

Abstract

In plants, the phloem distributes photosynthetic products for metabolism and storage over long distances. It relies on specialized cells, the sieve elements, which are enucleated and interconnected through large so-called sieve pores in their adjoining cell walls. Reverse genetics identified PECTATE LYASE LIKE 12 (PLL12) as critical for plant growth and development. Using genetic complementation, we establish that PLL12 enzyme activity is required exclusively late during cell differentiation. Physiological assays suggested a role specifically in phloem long distance transport while short distance symplastic transport does not require PLL12. Exploiting CALLOSE SYNTHASE 7 (CALS7) as a polar fluorescent marker, we identified structural defects in sieve pores in pll12. Due to the serial arrangement of sieve elements in the phloem, such defects should affect a plants ability specifically for symplastic transport over long distances.