Differentiation of Mesophyll and Paraveinal Mesophyll in Soybean Leaf

Abstract

The paraveinal mesophyll (PVM) in trifoliolate soybean leaves is a one-cell-thick reticulum extending between the vascular bundles at phloem level. This study describes various features of PVM differentiation in relation to other tissues during leaf ontogeny and also gives quantitative data on cell, vacuole, cytoplasm, and nucleus compartment size in mature leaves. The mitotic index and cell expansion dynamics during leaf development showed that the PVM stopped dividing and began differentiating 4-5 d before the palisade mesophyll. The PVM vacuole begins expansion while the other mesophyll layers still show cell division, reaching 80% of the mature cell volume vs. 30% for mesophyll vacuoles. Almost 40% of the mesophyll vacuolar volume in mature leaves is in PVM. PVM nuclei are twice the diameter of mesophyll nuclei and incorporate higher levels of 3H-thymidine, an indication of greater turnover or perhaps gene amplification or endoreduplication. This temporal quantification of mitotic activity and cell and vacuole expansion rates is important in understanding how the anatomical and physiological differentiation seen in soybean leaf is generated. We show that the large cell and vacuole size of the PVM and its netlike planar extension are a consequence of a combination of early cessation of mitosis, persistant cell expansion, and proportionally greater vacuole to cytoplasm growth relative to surrounding mesophyll cells. An intriguing variation in anatomy occurs in the unifoliolate primary leaf, which has two layers of PVM cells instead of the one layer found in trifoliolate leaves. It was determined that the first layer of PVM begins to differentiate before the second layer is created. The second layer of PVM is derived from an asymmetric division of a second layer of palisade mesophyll initials Both PVM layers then show dynamics of differentiation relative to surrounding mesophyll that is similar to that described for trifoliolate leaves. The mechanism by which two layers of PVM are formed in primary leaves does not represent a simple modification of the trifoliolate leaf ontogeny or anatomy, such as division of an existing PVM initial layer as we hypothesized. PVM formation in the two leaf types may provide a good system for elucidating factors controlling programmed development of tissue types and tissue quantity.